Fulvio Melia

Interests

Research

Theoretical Astrophysics in general, with a focus on general relativity, black-hole physics and cosmology.

Teaching

Stimulating excitement towards fundamental physics at the undergraduate level;Preparing motivated graduate students for frontline research in theoretical astrophysics and cosmology; andsharing the breathtaking discoveries in Astronomy and Astrophysics with the general public

Courses

Scholarly Contributions

Books

• Melia, F. (2020). The Cosmic Spacetime. Oxford, England: Routledge, Taylor & Francis.
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  The growth of cosmology into a precision science represents one of the most remarkable stories of the past century. Much has been written chronicling this development, but rarely has any of it focused on the most critical element of this work–the cosmic spacetime itself.Addressing this lacuna is the principal focus of this book, documenting the growing body of evidence compelling us–not only to use this famous solution to Einstein's equations in order to refine the current paradigm, but–to probe its foundation at a much deeper level. Its excursion from the smallest to largest possible scales insightfully reveals an emerging link between the Universe we behold and the established tenets of our most fundamental physical theories. Key Features:Uncovers the critical link between the Local Flatness Theorem in general relativity and the symmetries informing the spacetime's metric coefficientsDevelops a physical explanation for some of the most unpalatable coincidences in cosmologyProvides a sober assessment of the horizon problems precluding our full understanding of the early UniverseReveals a possible explanation for the origin of rest-mass energy in Einstein's theoryIn spite of its technical layout, this book does not shy away from introducing the principal players who have made the most enduring contributions to this field. Anyone with a graduate level foundation in physics and astronomy will be able to easily follow its contents.
• Melia, F. (2009). High-energy astrophysics. Princeton University Press.

Chapters

• Melia, F. (2009). Accretion onto the supermassive black hole at the centre of our galaxy. In In Celebration of K C Hines(pp 209-229). World Scientific Publishing Co. doi:10.1142/9789814293662_0013
• Stolovy, S., Melia, F., Mccarthy, D. W., Mccarthy, D. W., Yusef-zadeh, F., & Stolovy, S. R. (2007). Chapter 64. Near-Infrared Flux Limits for Sgr A* Based on NICMOS Data. In Supplement: Proceedings of the Galactic Center Workshop 2002 – The central 300 parsecs of the Milky Way (Hardcover, ISBN 3-527-40466-X)(p. 1). doi:10.1002/9783527617982.CH64
• Melia, F. (2003). Formation of supermassive black holes. In The Edge of Infinity. Cambridge University Press. doi:10.1017/CBO9780511536366.005
• Fromerth, M. J., & Melia, F. (2001). Formation of Broad Line Clouds from Turbulent Shocks in the Accretion Flows of Active Galactic Nuclei. In Black Holes in Binaries and Galactic Nuclei: Diagnostics, Demography and Formation(pp 234-235). Springer-Verlag. doi:10.1007/10720995_52
• Melia, F. (1994). An Accreting Blackhole Model for SGR A. In The Nuclei of Normal Galaxies(pp 441-448). Springer Netherlands. doi:10.1007/978-94-011-0752-5_53
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  The presence of a strong circumnuclear wind (with velocity νgw ≈ 500– 700 km s-1 and mass-loss rate ≈ 3 — 4 × 10-3 M ⊙ yr-1) in the vicinity of IRS 16 and Sgr A* is suggested by the observation of broad He I, Brα, and Brγ emission lines from this region. Sgr A* may be accreting a portion of this wind incident within the massdependent accretion radius 2GM/υ 2 gw, corresponding to ≈ 1 × 1022(M/106 M ⊙)2 g s-1 for the observed conditions in the nucleus. We here summarize the results of detailed calculations for the physical state of the infalling plasma and the corresponding spectrum derived from bremsstrahlung and magnetic bremsstrahlung emission from this gas as it descends toward the event horizon. The spectrum is tightly constrained across more than 12 decades in frequency, and the most likely value of the mass inferred from this modelling appears to he in the range ≈ 1 – 2 × 106 M ⊙. In addition, it appears that a slight excess of angular momentum in the accreting gas may be necessary in order to account for the IR luminosity from this source. We present some preliminary results of ongoing multidimensional hydrodynamic simulations that suggest the likely origin of this excess and the intensity fluctuations observed at radio wavelengths.

Journals/Publications

• Melia, F. (2023). The Cosmic Timeline Implied by the JWST High-redshift Galaxies. Monthly Notices of the Royal Astronomical Society Letters, Accepted manuscript, 11. doi:10.1093/mnrasl/slad025
• Melia, F., Wei, J., & Wu, X. (2023). Model selection using time-delay lenses. Monthly Notices of the Royal Astronomical Society, 519, 2528–2534. doi:10.1093/mnras/stac3682
• Melia, F. (2022). A Candid Assessment of Standard Cosmology. Publications of the Astronomical Society of the Pacific, 134, 121001 (24 pp). doi:10.1088/1538-3873/aca51f
• Melia, F. (2022). A Measurement of the Cosmic Expansion Within our Lifetime. European Journal of Physics, 43, 035601 (12pp). doi:10.1088/1361-6404/ac4646
• Melia, F. (2022). A Population III–Generated Dust Screen at z ∼ 16. The Astrophysical Journal, 941, 178 (20 pp). doi:10.3847/1538-4357/aca412
• Melia, F. (2022). Initial Energy of a Spatially Flat Universe -- a Hint of its Possible Origin. Astronomische Nachrichten, 343, e20224010 (9pp). doi:10.1002/asna.20224010
• Melia, F. (2022). The Electroweak Horizon Problem. Physics of the Dark Universe, 37, 101057 (10 pp). doi:10.1016/j.dark.2022.101057
• Melia, F. (2022). The Friedmann-Lemaitre-Robertson-Walker Metric. Modern Physics Letters A, 37, 2250016 (7pp). doi:https://doi.org/10.1142/S021773232250016X
• Melia, F. (2022). The Seemingly Preferred Cosmic Frame. Physica Scripta, 97, 045001 (10pp). doi:10.1002/asna.20224010
• Melia, F., & Lopez-Corredoira, M. (2022). Model Selection using Baryon Acoustic Oscillations in the Final SDSS-IV Release. International Journal of Modern Physics D, 31, 7. doi:10.1142/S0218271822500651
• Melia, F., & Wei, J. (2022). Exploring the Hubble Tension and Spatial Curvature from the Ages of Old Astrophysical Objects. The Astrophysical Journal, 928, 165 (8 pp). doi:10.3847/1538-4357/ac562c
• Melia, F., Sanchis-Lozano, M., Lopez-Corredoira, M., & Sanchis-Gual, N. (2022). Missing Large-angle Correlations versus Even-Odd Point-parity Imbalance in the Cosmic Microwave Background. Astronomy & Astrophysics, 660, A121 (14 pp). doi:10.1051/0004-6361/202142296
• Melia, F., Yennapureddy, M., Sultana, J., & Kazanas, D. (2022). Constraining f(R) Models with Cosmic Chronometers and the HII Galaxy Hubble Diagram. Monthly Notices of the Royal Astronomical Society, 514, 5827. doi:10.1093/mnras/stac1713
• MELIA, F. (2021). Classicalization of Quantum Fluctuations at the Planck Scale in the Rh=ct Universe. Physics Letters B, 818, 136362 (14pp). doi:10.21203/rs.3.rs-529452/v1
• Melia, F. (2021). Classicalization of Quantum Fluctuations at the Planck Scale. Physics Letters B, 818, 136632, 16. doi:10.1016/j.physletb.2021.136362
• Melia, F. (2021). The Anomalous 21-cm Absorption at High Redshifts. The European Physical Journal C, 81, id. 230 (10pp). doi:10.1140/epjc/s10052-021-09029-4
• Melia, F. (2021). The Origin of Rest-mass Energy. The European Physical Journal C, 81, 707 (12pp). doi:10.1140/epjc/s10052-021-09506-w
• Melia, F. (2021). Thermodynamics of the R_h=ct Universe: A Simplification of Cosmic Entropy. The European Physical Journal C, 81, id. 234 (11pp). doi:10.1140/epjc/s10052-021-09028-5
• Melia, F., & Qin, J. (2021). Test of the cosmic distance duality relation for arbitrary spatial curvature. Monthly Notices of the Royal Astronomical Society, 502, 3500–3509. doi:doi.org/10.1093/mnras/stab124
• Melia, F., & Yennapureddy, M. K. (2021). Structure Formation and the Matter Power Spectrum in the R_h=ct Universe. Physics of the Dark Universe, 31, id. 100752 (33pp). doi:10.1016/j.dark.2020.100752
• Melia, F., Ma, Q., & Wei, J. (2021). Evidence of a Truncated Primordial Power Spectrum from the CMB Large-scale Anomalies. Astronomy & Astrophysics, 655, A70, 8. doi:10.1051/0004-6361/202141251
• Melia, F. (2020). A Resolution of the Trans-Planckian problem in the R_h=ct universe. Astronomische Nachrichten, 341, 812--819. doi:10.1002/asna.202013813
• Melia, F. (2020). Reassessing Dust's Role in Forming the CMB. European Physical Journal Plus, 135, 511--547. doi:10.1140/epjp/s13360-020-00533-2
• Melia, F., & Liu, J. (2020). Viability of slow-roll inflation in light of the non-zero k_min measured in the CMB power spectrum. Proceedings of the Royal Society A, 476, id. 20200364 (7pp). doi:10.1098/rspa.2020.0364
• Melia, F., & Qin, J. (2020). Assessing Cosmic Acceleration with the Alcock-Paczynksi Effect in the SDSS-IV Quasar Catalog. Monthly Notices of the Royal Astronomical Society Letters, 499, L36--L40. doi:10.1093/mnrasl/slaa153
• Melia, F., & Wei, J. (2020). Cosmology-independent Estimate of the Hubble Constant and Spatial Curvature Using Time-delay Lenses and Quasars. The Astrophysical Journal, 897, id. 127 (10pp). doi:10.3847/1538-4357/ab959b
• Melia, F., & Wei, J. (2020). Model-independent Distance Calibration and Curvature Measurement using Quasars and Cosmic Chronometers. The Astrophysical Journal, 889(99), 5. doi:10.3847/1538-4357/ab5e7d
• Melia, F., Qin, J., & Zhang, T. (2020). Assessing cosmic acceleration with the Alcock–Paczyński effect in the SDSS-IV quasar catalogue. Monthly Notices of the Royal Astronomical Society: Letters, 499(1), L36-L40. doi:10.1093/mnrasl/slaa153
• Melia, F. (2019). A Cosmological Basis for E=mc^2. International Journal of Modern Physics A, 34(id. 1950055), 5. doi:10.1142/S0217751X19500556
• Melia, F. (2019). Cosmological test using the Hubble diagram of high-z quasars. Monthly Notices of the Royal Astronomical Society, 489(517), 7. doi:10.1093/mnras/stz2120
• Melia, F. (2019). Quantum Fluctuations at the Planck Scale. The European Physical Journal C Letters, 79(455), 4. doi:10.1140%2Fepjc%2Fs10052-019-6963-5
• Melia, F. (2019). Tantalizing New Physics from the Cosmic Purview. Modern Physics Letters A, 34(1930004), 22. doi:10.1142/S0217732319300040
• Melia, F. (2019). The Lapse Function in Friedmann-Lemaitre-Robertson-Walker Cosmologies. Annals of Physics, 411(167997), 5. doi:10.1016/j.aop.2019.167997
• Melia, F., & Leaf, K. (2019). Cosmological test using the high-redshift detection rate of FSRQs with the Square Kilometer Array. Monthly Notices of the Royal Astronomical Society, 487(2030), 9. doi:10.1093/mnras/stz1396
• Melia, F., & Leaf, K. (2019). Cosmological test using the high-redshift detection rate of FSRQs with the Square Kilometre Array. Monthly Notices of the Royal Astronomical Society, 487(2), 2030-2037. doi:10.1093/mnras/stz1396
• Melia, F., & Yennapureddy, M. K. (2019). A comparison of the R_h=ct and LCDM cosmologies based on the observed halo mass function. The European Physical Journal C, 79(571), 7. doi:10.1140/epjc/s10052-019-7082-z
• Melia, F., Ruan, C., Chen, Y., & Zhang, T. (2019). Using spatial curvature with HII galaxies and cosmic chronometers to explore the tension in H_0. The Astrophysical Journal, 881(137), 12. doi:10.3847/1538-4357/ab2ed0
• Melia, F., Sultana, J., & Kazanas, D. (2019). Testing Viable f(R) Models with the Angular-diameter Distance to Compact Quasar Cores. Physical Review D, 99(103505), 9. doi:10.1103/PhysRevD.99.103505
• Melia, F., Wan, H., Cao, S., & Zhang, T. (2019). Testing the R_h=ct Universe Jointly with the Redshift-dependent Expansion-rate and Angular-diameter and Luminosity Distances. Physics of the Dark Universe, 26(100405), 13. doi:10.1016/j.dark.2019.100405
• Melia, F. (2018). A Solution to the electroweak horizon problem in the R_h=ct universe. European Physical Journal C Letters, 78, 739. doi:10.1140/epjc/s10052-018-6231-0
• Melia, F. (2018). A comparison of the R_h=ct and LCDM cosmologies using the Cosmic Distance Duality Relation. Monthly Notices of the Royal Astronomical Society, 481, 4855. doi:10.1093/mnras/sty2596
• Melia, F. (2018). J1342+0928 Confirms the Cosmological Timeline in R_h=ct. Astronomy & Astrophysics, 615, A113. doi:10.1051/0004-6361/201832752
• Melia, F. (2018). J1342+0928 Supports the Timeline in the R_h = ct Cosmology. Astronomy & Astrophysics, A113 (5pp). doi:10.31219/osf.io/hq346
• Melia, F. (2018). Model Selection Based on the Angular-diameter Distance to Compact Quasar Cores. Europhysics Letters (EPL), 123, 39001. doi:10.1209/0295-5075/123/39001
• Melia, F. (2018). The Apparent (Gravitational) Horizon in Cosmology. American Journal of Physics, 86, 585. doi:10.1119/1.5045333
• Melia, F., & Leaf, K. (2018). A Two-point Diagnostic for the HII Galaxy Hubble Diagram. Monthly Notices of the Royal Astronomical Society, 474(4507), 7. doi:10.1093/mnras/stx3109
• Melia, F., & Leaf, K. (2018). Model Selection with Strong-lensing Systems. Monthly Notices of the Royal Astronomical Society, 478, 5104. doi:10.1093/mnras/sty1365
• Melia, F., & Lopez-Corredoira, M. (2018). Evidence of a truncated spectrum in the angular correlation function of the cosmic microwave background. Astronomy & Astrophysics, 610, A87. doi:10.1051/0004-6361/201732181
• Melia, F., & Yennapureddy, M. (2018). A Cosmological Solution to the Impossibly Early Galaxy Problem. Physics of the Dark Universe, 20, 65. doi:10.1016/j.dark.2018.03.003
• Melia, F., & Yennapureddy, M. (2018). Cosmological Tests with Strong Gravitational Lenses using Gaussian Processes. European Physical Journal C, 78, 258. doi:10.1140/epjc/s10052-018-5746-8
• Melia, F., & Yennapureddy, M. (2018). Model Selection Using Cosmic Chronometers with Gaussian Processes. Journal of Cosmology and Astroparticle Physics, 2018(2), 034. doi:10.1088/1475-7516/2018/02/034
• Melia, F., & Yennapureddy, M. (2018). The Maximum Angular-diameter Distance in Cosmology. Monthly Notices of the Royal Astronomical Society, 480, 2144. doi:10.1093/mnras/sty1962
• Melia, F., Ruan, C., & Zhang, T. (2018). Model-independent Test of the Cosmic Distance Duality Relation. The Astrophysical Journal, 866, 31. doi:10.3847/1538-4357/aaddfd
• Melia, F., Wei, J., Maier, R. S., & Wu, X. (2018). Cosmological Tests with the Joint Lightcurve Analysis. Europhysics Letters (EPL), 123, 59002. doi:10.1209/0295-5075/123/59002
• Melia, F., Wei, J., Maier, R. S., & Wu, X. (2018). Cosmological tests with the Joint Lightcurve Analysis. EPL (Europhysics Letters), 123(5), id 59002. doi:10.1209/0295-5075/123/59002
• Melia, F. (2017). Cosmological Perturbations without Inflation. Classical and Quantum Gravity, 34 (015011)(1), 19. doi:10.1088/1361-6382/34/1/015011
• Melia, F. (2017). Cosmological Perturbations without Inflation. Classical and Quantum Gravity, 34, 015011 (21pp). doi:10.31219/osf.io/tycqz
• Melia, F. (2017). The Linear Growth of Structure in the R_h=ct Universe. Monthly Notices of the Royal Astronomical Society, 464(2), 10. doi:10.1093/mnras/stw2493
• Melia, F. (2017). The Zero Active Mass Condition in Friedmann-Robertson-Walker Cosmologies. Frontiers of Physics (Springer), 12 (129802)(1), 5. doi:10.1007/s11467-016-0611-4
• Melia, F., & Fatuzzo, M. (2017). Unseen Progenitors of Luminous High-z Quasars in the R_h=ct Universe. The Astrophysical Journal, 846(129), 16.
• Melia, F., & Leaf, K. (2017). A two-point diagnostic for the H ii galaxy Hubble diagram. Monthly Notices of the Royal Astronomical Society, 474(4), 4507-4513. doi:10.1093/mnras/stx3109
• Melia, F., & Leaf, K. (2017). Analyzing H(z) Data Using Two-point Diagnostics. Monthly Notices of the Royal Astronomical Society, 470(2320), 13.
• Melia, F., & Lopez Correidora, M. (2017). Alcock-Paczynski Test with Model-independent BAO Peak. International Journal of Modern Physics D, 26 (1750055), 11. doi:10.1142/S0218271817500559
• Melia, F., & Lopez-Corredoira, M. (2017). Evidence of a truncated spectrum in the angular correlation function of the cosmic microwave background. Astronomy & Astrophysics, In press, 12.
• Melia, F., & Yennapureddy, M. (2017). Reconstruction of the HII Galaxy Hubble Diagram using Gaussian Processes. Journal of Cosmology and Astroparticle Physics, JCAP11(2017)029(029), 8.
• Melia, F., Wei, J., & Wu, X. (2017). Impact of a locally measured H_0 on the interpretation of cosmic chronometer data. The Astrophysical Journal, 835(270), 8.
• Melia, F. (2016). Constancy of the Cluster Gas Mass Fraction in the R_h = ct Universe. Proceedings of the Royal Society A, 472, 20150765 (17pp). doi:10.31219/osf.io/5e2q3
• Melia, F. (2016). Constancy of the cluster gas mass fraction in the Rh=ct Universe. Proceedings of The Royal Society A: Mathematical, Physical and Engineering Sciences, 472(2186), 20150765. doi:10.1098/rspa.2015.0765
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  The ratio of baryonic to dark matter densities is assumed to have remained constant throughout the formation of structure. With this, simulations show that the fraction fgas(z) of baryonic mass to ...
• Melia, F. (2016). Definitive Test of the R_h = ct Universe Using Redshift Drift. Monthly Notices of the Royal Astronomical Society Letters, 463, L61–L63. doi:10.31219/osf.io/hakwv
• Melia, F. (2016). Definitive Test of the R_h=ct Universe Using Redshift Drift. Monthly Notices of the Royal Astronomical Society Letters, 463(1), 3. doi:10.1093/mnrasl/slw157
• Melia, F. (2016). Physical Basis for the Symmetries in the Friedmann-Robertson-Walker Metric. Frontiers of Physics (Springer), 11 (119801)((4)), 7. doi:10.1007/s11467-016-0557-6
• Melia, F. (2016). The Cluster Gas Mass Fraction in the R_h=ct Universe. Proc. R. Soc. A, 472 (20150765), 17. doi:10.1098/rspa.2015.0765
• Melia, F., & Fatuzzo, M. (2016). The epoch of reionization in the Rh = ct universe. Monthly Notices of the Royal Astronomical Society, 456(4), 3422-3431. doi:10.1093/mnras/stv2902
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  The measured properties of the epoch of reionization (EoR) show that reionization probably began around z ˜ 12-15 and ended by z = 6. In addition, a careful analysis of the fluctuations in the cosmic microwave background indicate a scattering optical depth τ ˜ 0.066 ± 0.012 through the EoR. In the context of Λ cold dark matter, galaxies at intermediate redshifts and dwarf galaxies at higher redshifts now appear to be the principal sources of UV ionizing radiation, but only for an inferred (ionizing) escape fraction fion ˜ 0.2, which is in tension with other observations that suggest a value as small as ˜0.05. In this paper, we examine how reionization might have progressed in the alternative Friedmann-Robertson Walker cosmology known as the Rh = ct universe, and determine the value of fion required with this different rate of expansion. We find that Rh = ct accounts quite well for the currently known properties of the EoR, as long as its fractional baryon density falls within the reasonable range 0.026 ≲ Ωb ≲ 0.037. This model can also fit the EoR data with fion ˜ 0.05, but only if the Lyman continuum photon production is highly efficient and Ωb ˜ 0.037. These results are still preliminary, however, given their reliance on a particular form of the star formation rate density, which is still uncertain at very high redshifts. It will also be helpful to reconsider the EoR in Rh = ct when complete structure formation models become available.
• Melia, F., Lopez-Corredoira, M., Lusso, E., & Risaliti, G. (2016). Cosmological Tests with the QSO Hubble Diagram. International Journal of Modern Physics D, 25 (1650060)(5), 12. doi:10.1142/S0218271816500607
• Melia, F., Wei, J., & Wu, X. (2016). The HII Galaxy Hubble Diagram Strongly Favors R_h=ct Over LCDM. Monthly Notices of the Royal Astronomical Society, 463(2), 8. doi:10.1093/mnras/stw2057
• Melia, F., Zeng, H., & Zhang, L. (2016). Cosmological tests with the FSRQ gamma-ray luminosity function. Monthly Notices of the Royal Astronomical Society, 462(3), 9. doi:10.1093/mnras/stw1817
• Melia, F. (2015). Cosmological Implications of the CMB Large-scale Structure. The Astronomical Journal, 149(1), Id. 6 (10 pages). doi:10.1088/0004-6256/149/1/6
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  The Wilkinson Microwave Anisotropy Probe (WMAP) and Planck may have uncovered several anomalies in the full cosmic microwave background (CMB) sky that could indicate possible new physics driving the growth of density fluctuations in the early universe. These include an unusually low power at the largest scales and an apparent alignment of the quadrupole and octopole moments. In a ΛCDM model where the CMB is described by a Gaussian Random Field, the quadrupole and octopole moments should be statistically independent. The emergence of these low probability features may simply be due to posterior selections from many such possible effects, whose occurrence would therefore not be as unlikely as one might naively infer. If this is not the case, however, and if these features are not due to effects such as foreground contamination, their combined statistical significance would be equal to the product of their individual significances. In the absence of such extraneous factors, and ignoring the biasing due to posterior selection, the missing large-angle correlations would have a probability as low as ˜0.1% and the low-l multipole alignment would be unlikely at the ˜4.9% level; under the least favorable conditions, their simultaneous observation in the context of the standard model could then be likely at only the ˜0.005% level. In this paper, we explore the possibility that these features are indeed anomalous, and show that the corresponding probability of CMB multipole alignment in the {{R}h}=ct universe would then be ˜7-10%, depending on the number of large-scale Sachs-Wolfe induced fluctuations. Since the low power at the largest spatial scales is reproduced in this cosmology without the need to invoke cosmic variance, the overall likelihood of observing both of these features in the CMB is ≥slant 7%, much more likely than in ΛCDM, if the anomalies are real. The key physical ingredient responsible for this difference is the existence in the former of a maximum fluctuation size at the time of recombination, which is absent in the latter because of inflation.
• Melia, F. (2015). On recent claims concerning the Rh = ct Universe. Monthly Notices of the Royal Astronomical Society, 446(2), 1191-1194. doi:10.1093/mnras/stu2181
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  The Rh = ct universe is a Friedmann-Robertson-Walker (FRW) cosmology which, like Λ cold dark matter (ΛCDM), assumes the presence of dark energy in addition to (baryonic and non-luminous) matter and radiation. Unlike ΛCDM, however, it is also constrained by the equation of state (EOS) p = -ρ/3, in terms of the total pressure p and energy density ρ. One-on-one comparative tests between Rh = ct and ΛCDM have been carried out using over 14 different cosmological measurements and observations. In every case, the data have favoured Rh = ct over the standard model, with model selection tools yielding a likelihood ˜90-95 per cent that the former is correct, versus only ˜5-10 per cent for the latter. In other words, the standard model without the EOS p = -ρ/3 does not appear to be the optimal description of nature. Yet in spite of these successes - or perhaps because of them - several concerns have been published recently regarding the fundamental basis of the theory itself. The latest paper on this subject even claims - quite remarkably - that Rh = ct is a vacuum solution, though quite evidently ρ ≠ 0. Here, we address these concerns and demonstrate that all criticisms levelled thus far against Rh = ct, including the supposed vacuum condition, are unwarranted. They all appear to be based on incorrect assumptions or basic theoretical errors. Nevertheless, continued scrutiny such as this will be critical to establishing Rh = ct as the correct description of nature.
• Melia, F. (2015). The AGN Hubble Diagram and its Implications for Cosmology. Astrophysics and Space Science, 359, Id. 34 (8 pages). doi:10.1007/s10509-015-2483-4
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  We use a recently proposed luminosity distance measure for relatively nearby active galactic nuclei (AGNs) to test the predicted expansion of the Universe in the and cosmologies. This comparative study is particularly relevant to the question of whether or not the Universe underwent a transition from decelerated to accelerated expansion, which is believed to have occurred—on the basis of Type Ia SN studies—within the redshift range () that will eventually be sampled by these objects. We find that the AGN Hubble Diagram constructed from currently available sources does not support the existence of such a transition. While the scatter in the AGN data is still too large for any firm conclusions to be drawn, the results reported here nonetheless somewhat strengthen similar results of comparative analyses using other types of source. We show that the Akaike, Kullback, and Bayes Information Criteria all consistently yield a likelihood of that is closer to the "true" cosmology than is, though neither model adequately accounts for the data, suggesting an unnaccounted-for source of scatter.
• Melia, F. (2015). The Cosmic Equation of State. Astrophysics and Space Science, 356(2), 393-398. doi:10.1007/s10509-014-2211-5
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  The cosmic spacetime is often described in terms of the Friedmann-Robertson-Walker (FRW) metric, though the adoption of this elegant and convenient solution to Einstein's equations does not tell us much about the equation of state, p= wρ, in terms of the total energy density ρ and pressure p of the cosmic fluid. ΛCDM and the R h= ct Universe are both FRW cosmologies that partition ρ into (at least) three components, matter ρ m, radiation ρ r, and a poorly understood dark energy ρ de, though the latter goes one step further by also invoking the constraint w=-1/3. This condition is apparently required by the simultaneous application of the Cosmological principle and Weyl's postulate. Model selection tools in one-on-one comparisons between these two cosmologies favor R h= ct, indicating that its likelihood of being correct is ˜90 % versus only ˜10 % for ΛCDM. Nonetheless, the predictions of ΛCDM often come quite close to those of R h= ct, suggesting that its parameters are optimized to mimic the w=-1/3 equation-of-state. In this paper, we explore this hypothesis quantitatively and demonstrate that the equation-of-state in R h= ct helps us to understand why the optimized fraction Ω m≡ ρ m / ρ in ΛCDM today must be ˜0.27, an otherwise seemingly random variable. We show that when one forces ΛCDM to satisfy the equation-of-state w=( ρ r/3- ρ de)/ ρ, the value of the Hubble radius today, c/ H 0, can equal its measured value ct 0 only with Ω m˜0.27 when the equation-of-state for dark energy is w de=-1. (We also show, however, that the inferred values of Ω m and w de change in a correlated fashion if dark energy is not a cosmological constant, so that .) This peculiar value of Ω m therefore appears to be a direct consequence of trying to fit the data with the equation-of-state w=( ρ r/3- ρ de)/ ρ in a Universe whose principal constraint is instead R h= ct or, equivalently, w=-1/3.
• Melia, F., & McClintock, T. (2015). Supermassive Black Holes in the Early Universe. Proceedings of the Royal Society A, 471(20150449), 10.
• Melia, F., & McClintock, T. M. (2015). A Test of Cosmological Models Using High-z Measurements of H(z). The Astronomical Journal, 150(4), Id. 119 (6 pages). doi:10.1088/0004-6256/150/4/119
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  The recently constructed Hubble diagram using a combined sample of SNLS and SDSS-II SNe Ia, and an application of the Alcock-Paczyński (AP) test using model-independent Baryon Acoustic Oscillation (BAO) data, have suggested that the principal constraint underlying the cosmic expansion is the total equation-of-state of the cosmic fluid, rather than that of its dark energy. These studies have focused on the critical redshift range (0 ≲ z ≲ 2) within which the transition from decelerated to accelerated expansion is thought to have occurred, and they suggest that the cosmic fluid has zero active mass, consistent with a constant expansion rate. The evident impact of this conclusion on cosmological theory calls for an independent confirmation. In this paper, we carry out this crucial one-on-one comparison between the Rh = ct universe (a Friedmann-Robertson-Walker cosmology with zero active mass) and wCDM/ΛCDM, using the latest high-z measurements of H(z). Whereas the SNe Ia yield the integrated luminosity distance, while the AP diagnostic tests the geometry of the universe, the Hubble parameter directly samples the expansion rate itself. We find that the model-independent cosmic chronometer data prefer Rh = ct over wCDM/ΛCDM with a Bayes Information Criterion likelihood of ˜95% versus only ˜5%, in strong support of the earlier SNe Ia and AP results. This contrasts with a recent analysis of H(z) data based solely on BAO measurements which, however, strongly depend on the assumed cosmology. We discuss why the latter approach is inappropriate for model comparisons, and emphasize again the need for truly model-independent observations to be used in cosmological tests.
• Melia, F., Wei, J., & Wu, X. (2015). A Comparison of Cosmological Models Using Strong Gravitational Lensing Galaxies. The Astronomical Journal, 149(1), Id. 2 (10 pages). doi:10.1088/0004-6256/149/1/2
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  Strongly gravitationally lensed quasar-galaxy systems allow us to compare competing cosmologies as long as one can be reasonably sure of the mass distribution within the intervening lens. In this paper, we assemble a catalog of 69 such systems from the Sloan Lens ACS and Lens Structure and Dynamics surveys suitable for this analysis, and carry out a one-on-one comparison between the standard model, ΛCDM, and the {{R}h}=ct universe, which has thus far been favored by the application of model selection tools to other kinds of data. We find that both models account for the lens observations quite well, though the precision of these measurements does not appear to be good enough to favor one model over the other. Part of the reason is the so-called bulge-halo conspiracy that, on average, results in a baryonic velocity dispersion within a fraction of the optical effective radius virtually identical to that expected for the whole luminous-dark matter distribution modeled as a singular isothermal ellipsoid, though with some scatter among individual sources. Future work can greatly improve the precision of these measurements by focusing on lensing systems with galaxies as close as possible to the background sources. Given the limitations of doing precision cosmological testing using the current sample, we also carry out Monte Carlo simulations based on the current lens measurements to estimate how large the source catalog would have to be in order to rule out either model at a ˜ 99.7% confidence level. We find that if the real cosmology is ΛCDM, a sample of ˜ 200 strong gravitational lenses would be sufficient to rule out {{R}h}=ct at this level of accuracy, while ˜ 300 strong gravitational lenses would be required to rule out ΛCDM if the real universe were instead {{R}h}=ct. The difference in required sample size reflects the greater number of free parameters available to fit the data with ΛCDM. We point out that, should the {{R}h}=ct universe eventually emerge as the correct cosmology, its lack of any free parameters for this kind of work will provide a remarkably powerful probe of the mass structure in lensing galaxies, and a means of better understanding the origin of the bulge-halo conspiracy.
• Wei, J., Wu, X., & Melia, F. (2015). Cosmological tests using the angular size of galaxy clusters. Monthly Notices of the Royal Astronomical Society, 447(1), 479-485. doi:10.1093/mnras/stu2470
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  We use measurements of the galaxy-cluster angular size versus redshift to test and compare the standard model (ΛCDM) and the Rh = ct Universe. We show that the latter fits the data with a reduced χ ^2_dof=0.786 for a Hubble constant H0= 72.6_{-3.4}^{+3.8} km s- 1 Mpc- 1, and H0 is the sole parameter in this model. By comparison, the optimal flat Λ cold dark matter (ΛCDM) model, with two free parameters (including Ωm = 0.50 and H0=73.9_{-9.5}^{+10.6} km s- 1Mpc- 1), fits the angular-size data with a reduced χ ^2_dof=0.806. On the basis of their χ ^2_dof values alone, both models appear to account for the data very well in spite of the fact that the Rh = ct Universe expands at a constant rate, while ΛCDM does not. However, because of the different number of free parameters in these models, selection tools, such as the Bayes Information Criterion, favour Rh = ct over ΛCDM with a likelihood of ˜86 per cent versus ˜14 per cent. These results impact the question of galaxy growth at large redshifts. Previous work suggested an inconsistency with the underlying cosmological model unless elliptical and disc galaxies grew in size by a surprisingly large factor ˜6 from z ˜ 3 to 0. The fact that both ΛCDM and Rh = ct fit the cluster-size measurements quite well casts some doubt on the suggestion that the unexpected result with individual galaxies may be due to the use of an incorrect expansion scenario, rather than astrophysical causes, such as mergers and/or selection effects.
• Wei, J., Wu, X., & Melia, F. (2015). Testing Cosmological Models with Type Ic Superluminous Supernovae. The Astronomical Journal, 149(5), Id. 165 (11 pages). doi:10.1088/0004-6256/149/5/165
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  The use of type Ic super luminous supernovae (SLSNe Ic) to examine the cosmological expansion introduces a new standard ruler with which to test theoretical models. The sample suitable for this kind of work now includes 11 SLSNe Ic, which have thus far been used solely in tests involving the Λ cold dark matter (ΛCDM) model. In this paper, we broaden the base of support for this new, important cosmic probe by using these observations to carry out a one-on-one comparison between the {{R}h}=ct and ΛCDM cosmologies. We individually optimize the parameters in each cosmological model by minimizing the {{χ }2} statistic. We also carry out Monte Carlo simulations based on these current SLSNe Ic measurements to estimate how large the sample would have to be in order to rule out either model at a ˜99.7% confidence level. The currently available sample indicates a likelihood of ˜70-80% that the {{R}h}=ct universe is the correct cosmology versus ˜20-30% for the standard model. These results are suggestive, though not yet compelling, given the current limited number of SLSNe Ic. We find that if the real cosmology is ΛCDM, a sample of ˜240 SLSNe Ic would be sufficient to rule out {{R}h}=ct at this level of confidence, while ˜480 SLSNe Ic would be required to rule out ΛCDM if the real universe is instead {{R}h}=ct. This difference in required sample size reflects the greater number of free parameters available to fit the data with ΛCDM. If such SLSNe Ic are commonly detected in the future, they could be a powerful tool for constraining the dark-energy equation of state in ΛCDM, and differentiating between this model and the {{R}h}=ct universe.
• Wei, J., Wu, X., Melia, F., & Maier, R. S. (2015). A Comparative Analysis of the Supernova Legacy Survey Sample with LCDM and the R_h=ct Universe. The Astronomical Journal, 149(3), Id. 102 (11 pages). doi:10.1088/0004-6256/149/3/102
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  The use of Type Ia supernovae (SNe Ia) has thus far produced the most reliable measurement of the expansion history of the universe, suggesting that ΛCDM offers the best explanation for the redshift-luminosity distribution observed in these events. However, analysis of other kinds of sources, such as cosmic chronometers, gamma-ray bursts, and high-z quasars, conflicts with this conclusion, indicating instead that the constant expansion rate implied by the Rh = ct universe is a better fit to the data. The central difficulty with the use of SNe Ia as standard candles is that one must optimize three or four nuisance parameters characterizing supernova (SN) luminosities simultaneously with the parameters of an expansion model. Hence, in comparing competing models, one must reduce the data independently for each. We carry out such a comparison of ΛCDM and the Rh = ct universe using the SN Legacy Survey sample of 252 SN events, and show that each model fits its individually reduced data very well. However, since Rh = ct has only one free parameter (the Hubble constant), it follows from a standard model selection technique that it is to be preferred over ΛCDM, the minimalist version of which has three (the Hubble constant, the scaled matter density, and either the spatial curvature constant or the dark energy equation-of-state parameter). We estimate using the Bayes Information Criterion that in a pairwise comparison, the likelihood of Rh = ct is ˜90%, compared with only ˜10% for a minimalist form of ΛCDM, in which dark energy is simply a cosmological constant. Compared to Rh = ct, versions of the standard model with more elaborate parametrizations of dark energy are judged to be even less likely.
• Wei, J., Wu, X., Melia, F., Wang, F., & Yu, H. (2015). Age-Redshift Relationship of Old Passive Galaxies. The Astronomical Journal, 150(1), Id. 35 (13 pages). doi:10.1088/0004-6256/150/1/35
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  We use 32 age measurements of passively evolving galaxies as a function of redshift to test and compare the standard model (ΛCDM) with the {R}{{h}}={ct} universe. We show that the latter fits the data with a reduced {χ }{dof}2=0.435 for a Hubble constant {H}0={67.2}-4.0+4.5 km {{{s}}}-1 {{Mpc}}-1. By comparison, the optimal flat ΛCDM model, with two free parameters (including {{{Ω }}}{{m}}={0.12}-0.11+0.54 and {H}0={94.3}-35.8+32.7 km s-1 {{Mpc}}-1), fits the age-z data with a reduced {χ }{dof}2=0.428. Based solely on their {χ }{dof}2 values, both models appear to account for the data very well, though the optimized ΛCDM parameters are only marginally consistent with those of the concordance model ({{{Ω }}}{{m}}=0.27 and H0 = 70 km {{{s}}}-1 {{Mpc}}-1). Fitting the age-z data with the latter results in a reduced {χ }{dof}2=0.523. However, because of the different number of free parameters in these models, selection tools, such as the Akaike, Kullback and Bayes Information Criteria, favor {R}{{h}}={ct} over ΛCDM with a likelihood of ˜66.5%-80.5% versus ˜19.5%-33.5%. These results are suggestive, though not yet compelling, given the current limited galaxy age-z sample. We carry out Monte Carlo simulations based on these current age measurements to estimate how large the sample would have to be in order to rule out either model at a ˜ 99.7% confidence level. We find that if the real cosmology is ΛCDM, a sample
• Fatuzzo, M., & Melia, F. (2014). A numerical assessment of cosmic-ray energy diffusion through turbulent media. Astrophysical Journal, 784(2). doi:10.1088/0004-637X/784/2/131
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  Abstract: How and where cosmic rays are produced, and how they diffuse through various turbulent media, represent fundamental problems in astrophysics with far-reaching implications, both in terms of our theoretical understanding of high-energy processes in the Milky Way and beyond, and the successful interpretation of space-based and ground based GeV and TeV observations. For example, recent and ongoing detections, e.g., by Fermi (in space) and HESS (in Namibia), of γ-rays produced in regions of dense molecular gas hold important clues for both processes. In this paper, we carry out a comprehensive numerical investigation of relativistic particle acceleration and transport through turbulent magnetized environments in order to derive broadly useful scaling laws for the energy diffusion coefficients. © 2014. The American Astronomical Society. All rights reserved.
• Melia, F. (2014). Angular Correlation of the CMB in the R_h=ct Universe. Astronomy and Astrophysics, 561(A80).
• Melia, F. (2014). Angular correlation of the cosmic microwave background in the R _h = ct Universe. Astronomy and Astrophysics, 561.
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  Abstract: Context. The emergence of several unexpected large-scale features in the cosmic microwave background (CMB) has pointed to possible new physics driving the origin of density fluctuations in the early Universe and their evolution into the large-scale structure we see today. Aims. In this paper, we focus our attention on the possible absence of angular correlation in the CMB anisotropies at angles larger than ~60, and consider whether this feature may be the signature of fluctuations expected in the Rh = ct Universe. Methods. We calculate the CMB angular correlation function for a fluctuation spectrum expected from growth in a Universe whose dynamics is constrained by the equation-of-state p =-ρ/3, where p and ρ are the total pressure and density, respectively. Results. We find that, though the disparity between the predictions of ΛCDM and the WMAP sky may be due to cosmic variance, it may also be due to an absence of inflation. The classic horizon problem does not exist in the Rh = ct Universe, so a period of exponential growth was not necessary in this cosmology in order to account for the general uniformity of the CMB (save for the aforementioned tiny fluctuations of 1 part in 100 000 in the WMAP relic signal). Conclusions. We show that the Rh = ct Universe without inflation can account for the apparent absence in CMB angular correlation at angles θ ≠60 without invoking cosmic variance, providing additional motivation for pursuing this cosmology as a viable description of nature. © 2014 ESO.
• Melia, F. (2014). The Premature Formation of High Redshift Galaxies. The Astronomical Journal, 147(120).
• Melia, F. (2014). The high-z quasar Hubble Diagram. Journal of Cosmology and Astroparticle Physics, 2014(1).
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  Abstract: Two recent discoveries have made it possible for us to begin using high-z quasars as standard candles to construct a Hubble Diagram (HD) at z > 6. These are (1) the recognition from reverberation mapping that a relationship exists between the optical/UV luminosity and the distance of line-emitting gas from the central ionizing source. Thus, together with a measurement of the velocity of the line-emitting gas, e.g., via the width of BLR lines, such as Mg II, a single observation can therefore in principle provide a determination of the black hole's mass; and (2) the identification of quasar ULAS J1120+0641 at z = 7.085, which has significantly extended the redshift range of these sources, providing essential leverage when fitting theoretical luminosity distances to the data. In this paper, we use the observed fluxes and Mg II line-widths of these sources to show that one may reasonably test the predicted high-z distance versus redshift relationship, and we assemble a sample of 20 currently available high-z quasars for this exercise. We find a good match between theory and observations, suggesting that a more complete, high-quality survey may indeed eventually produce an HD to complement the highly-detailed study already underway (e.g., with Type Ia SNe, GRBs, and cosmic chronometers) at lower redshifts. With the modest sample we have here, we show that the Rh = ct Universe and ΛCDM both fit the data quite well, though the smaller number of free parameters in the former produces a more favorable outcome when we calculate likelihoods using the Akaike, Kullback, and Bayes Information Criteria. These three statistical tools result in similar probabilities, indicating that the Rh = ct Universe is more likely than ΛCDM to be correct, by a ratio of about 85% to 15%. © 2014 IOP Publishing Ltd and Sissa Medialab srl.
• Wei, J., Wu, X., & Melia, F. (2014). A Comparison of Cosmological Models Using Time Delay Lenses. The Astrophysical Journal, 788(190).
• Wei, J., Wu, X., Melia, F., Wei, D., & Feng, L. (2014). Cosmological tests using gamma-ray bursts, the star formation rate and possible abundance evolution. Monthly Notices of the Royal Astronomical Society, 439(4), 3329-3341.
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  Abstract: The principal goal of this paper is to use attempts at reconciling the Swift long gamma-ray bursts (LGRBs) with the star formation history (SFH) to compare the predictions of δ cold dark matter (δCDM) with those in the Rh = ct Universe. In the context of the former, we confirm that the latest Swift sample of GRBs reveals an increasing evolution in the GRB rate relative to the star formation rate (SFR) at high redshifts. The observed discrepancy between the GRB rate and the SFR may be eliminated by assuming a modest evolution parametrized as (1 + z)0.8 - perhaps indicating a cosmic evolution in metallicity. However, we find a higher metallicity cut of Z = 0.52 Z⊙ than was seen in previous studies, which suggested that LGRBs occur preferentially in metal-poor environments, i.e. Z ̃ 0.1-0.3 Z⊙. We use a simple powerlaw approximation to the high-z (>3.8) SFH, i.e. RSF α [(1 + z)/4.8]α, to examine how the high-z SFR may be impacted by a possible abundance evolution in the Swift GRB sample. For an expansion history consistent with δCDM, we find that the Swift redshift and luminosity distributions can be reproduced with reasonable accuracy if α = -2.41+1.87-2.09. For the Rh = ct Universe, the GRB rate is slightly different from that in δCDM, but also requires an extra evolutionary effect, with a metallicity cut of Z = 0.44 Z⊙. Assuming that the SFR and GRB rate are related via an evolving metallicity, we find that the GRB data constrain the slope of the high-z SFR in Rh = ct to be α = -3.60+2.45-2.45. Both cosmologies fit the GRB/SFR data rather well. However, in a one-on-one comparison using the Akaike information criterion, the best-fitting Rh = ct model is statistically preferred over the best-fitting δCDM model with a relative probability of ̃70 per cent versus ̃30 per cent. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
• Wu, X., Wei, J., & Melia, F. (2014). A COMPARISON OF COSMOLOGICAL MODELS USING TIME DELAY LENSES. The Astrophysical Journal, 788(2), 190. doi:10.1088/0004-637x/788/2/190
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  The use of time-delay gravitational lenses to examine the cosmological expansion introduces a new standard ruler with which to test theoretical models. The sample suitable for this kind of work now includes 12 lens systems, which have thus far been used solely for optimizing the parameters of ACDM. In this paper, we broaden the base of support for this new, important cosmic probe by using these observations to carry out a one-on-one comparison between competing models. The currently available sample indicates a likelihood of similar to 70%-80% that the R-h = ct universe is the correct cosmology versus similar to 20%-30% for the standard model. This possibly interesting result reinforces the need to greatly expand the sample of time-delay lenses, e.g., with the successful implementation of the Dark Energy Survey, the VST ATLAS survey, and the Large Synoptic Survey Telescope. In anticipation of a greatly expanded catalog of time-delay lenses identified with these surveys, we have produced synthetic samples to estimate how large they would have to be in order to rule out either model at a similar to 99.7% confidence level. We find that if the real cosmology is ACDM, a sample of similar to 150 time-delay lenses would be sufficient to rule out R-h = ct at this level of accuracy, while similar to 1000 time-delay lenses would be required to rule out ACDM if the real universe is instead R-h = ct. This difference in required sample size reflects the greater number of free parameters available to fit the data with ACDM.
• Melia, F. (2013). High-z quasars in the R _h = ct universe. Astrophysical Journal, 764(1).
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  Abstract: One cannot understand the early appearance of 109 M ⊙ supermassive black holes without invoking anomalously high accretion rates or the creation of exotically massive seeds, neither of which is seen in the local universe. Recent observations have compounded this problem by demonstrating that most, if not all, of the high-z quasars appear to be accreting at the Eddington limit. In the context of ΛCDM, the only viable alternative now appears to be the assemblage of supermassive black holes via mergers, as long as the seeds started forming at redshifts >40, but ceased being created by z ∼ 20-30. In this paper, we show that, whereas the high-z quasars may be difficult to explain within the framework of the standard model, they can instead be interpreted much more sensibly in the context of the R h = ct universe. In this cosmology, 5-20 M⊙ seeds produced after the onset of re-ionization (at z ≲ 15) could have easily grown to M ≳ 109 M⊙ by z ≳ 6, merely by accreting at the standard Eddington rate. © 2013. The American Astronomical Society. All rights reserved.
• Melia, F. (2013). Proper size of the visible Universe in FRW metrics with a constant spacetime curvature. Classical and Quantum Gravity, 30(15).
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  Abstract: In this paper, we continue to examine the fundamental basis for the Friedmann-Robertson-Walker (FRW) metric and its application to cosmology, specifically addressing the question: What is the proper size of the visible universe? There are several ways of answering the question of size, though often with an incomplete understanding of how far light has actually traveled in reaching us today from the most remote sources. The difficulty usually arises from an inconsistent use of the coordinates or an over-interpretation of the physical meaning of quantities such as the so-called proper distance R(t) = a(t)r, written in terms of the (unchanging) co-moving radius r and the universal expansion factor a(t). In this paper, we prove for the five non-trivial FRW metrics with a constant spacetime curvature that, when the expansion began from an initial singularity, the visible universe today has a proper size equal to Rh(t0/2), i.e., the gravitational horizon at half its current age. The exceptions are de Sitter and Lanczos, whose contents had pre-existing positions away from the origin. In so doing, we confirm earlier results showing the same phenomenon in a broad range of cosmologies, including ΛCDM, based on the numerical integration of null geodesic equations through an FRW metric. © 2013 IOP Publishing Ltd.
• Melia, F. (2013). The R_h=ct Universe Without Inflation. Astronomy and Astrophysics, 553(76).
• Melia, F., & Maier, R. S. (2013). Cosmic Chronometers in the R_h=ct Universe. Monthly Notices of the Royal Astronomical Society, 432(4), 2669-2675. doi:10.1093/mnras/stt596
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  The use of luminous red galaxies as cosmic chronometers provides us with an indispensable method of measuring the universal expansion rate H(z) in a model-independent way. Unlike many probes of the cosmological history, this approach does not rely on integrated quantities, such as the luminosity distance, and therefore does not require the pre-assumption of any particular model, which may bias subsequent interpretations of the data. We employ three statistical tools - the Akaike, Kullback and Bayes Information Criteria (AIC, KIC and BIC) - to compare the Λ cold dark matter (ΛCDM) model and the Rh = ct Universe with the currently available measurements of H(z), and show that the Rh = ct Universe is favoured by these model selection criteria. The parameters in each model are individually optimized by maximum likelihood estimation. The Rh = ct Universe fits the data with a reduced χ2dof=0.745 for a Hubble constant H0 = 63.2 ± 1.6 km s-1 Mpc-1, and H0 is the sole parameter in this model. By comparison, the optimal ΛCDM model, which has three free parameters (including H0 = 68.9 ± 3.3 km s-1 Mpc-1, Ωm = 0.32, and a dark-energy equation of state pde = -ρde), fits the H(z) data with a reduced χ2dof = 0.777. With these χ2dof values, the AIC yields a likelihood of ≈82 per cent that the distance-redshift relation of the Rh = ct Universe is closer to the correct cosmology, than is the case for ΛCDM. If the alternative BIC criterion is used, the respective Bayesian posterior probabilities are 91.2 per cent (Rh = ct) versus 8.8 per cent (ΛCDM). Using the concordance ΛCDM parameter values, rather than those obtained by fitting ΛCDM to the cosmic chronometer data, would further disfavour ΛCDM.
• Melia, F., & Maier, R. S. (2013). Cosmic chronometers in the R_h = ct universe. Monthly Notices of the Royal Astronomical Society, 432(4), 2669-2675.
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  Abstract: The use of luminous red galaxies as cosmic chronometers provides us with an indispensable method of measuring the universal expansion rate H(z) in a model-independent way. Unlike many probes of the cosmological history, this approach does not rely on integrated quantities, such as the luminosity distance, and therefore does not require the pre-assumption of any particular model, which may bias subsequent interpretations of the data. We employ three statistical tools . the Akaike, Kullback and Bayes Information Criteria (AIC, KIC and BIC) . to compare the δ cold dark matter (δCDM) model and the Rh = ct Universe with the currently available measurements of H(z), and show that the Rh = ct Universe is favoured by these model selection criteria. The parameters in each model are individually optimized by maximum likelihood estimation. The Rh =ct Universe fits the data with a reduced χ2 dof = 0.745 for a Hubble constant H0 = 63.2 ± 1.6 km s-1 Mpc-1, and H0 is the sole parameter in this model. By comparison, the optimal δCDM model, which has three free parameters (including H0 = 68.9 ± 3.3 km s.1 Mpc.1, σm = 0.32, and a dark-energy equation of state pde =-ρde), fits the H(z) data with a reduced χ2 dof = 0.777. With these χ2 dof values, the AIC yields a likelihood of .82 per cent that the distance.redshift relation of the Rh = ct Universe is closer to the correct cosmology, than is the case for δCDM. If the alternative BIC criterion is used, the respective Bayesian posterior probabilities are 91.2 per cent (Rh = ct) versus 8.8 per cent (δCDM). Using the concordance δCDM parameter values, rather than those obtained by fitting δCDM to the cosmic chronometer data, would further disfavour δCDM. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
• Melia, F., Melia, F., Maier, R. S., & Maier, R. S. (2013). Cosmic chronometers in the R-h = ct universe. Monthly Notices of the Royal Astronomical Society, 432(4), 2669-2675. doi:10.1093/mnras/stt596
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  Abstract: The use of luminous red galaxies as cosmic chronometers provides us with an indispensable method of measuring the universal expansion rate H(z) in a model-independent way. Unlike many probes of the cosmological history, this approach does not rely on integrated quantities, such as the luminosity distance, and therefore does not require the pre-assumption of any particular model, which may bias subsequent interpretations of the data. We employ three statistical tools . the Akaike, Kullback and Bayes Information Criteria (AIC, KIC and BIC) . to compare the δ cold dark matter (δCDM) model and the Rh = ct Universe with the currently available measurements of H(z), and show that the Rh = ct Universe is favoured by these model selection criteria. The parameters in each model are individually optimized by maximum likelihood estimation. The Rh =ct Universe fits the data with a reduced χ2 dof = 0.745 for a Hubble constant H0 = 63.2 ± 1.6 km s-1 Mpc-1, and H0 is the sole parameter in this model. By comparison, the optimal δCDM model, which has three free parameters (including H0 = 68.9 ± 3.3 km s.1 Mpc.1, σm = 0.32, and a dark-energy equation of state pde =-ρde), fits the H(z) data with a reduced χ2 dof = 0.777. With these χ2 dof values, the AIC yields a likelihood of .82 per cent that the distance.redshift relation of the Rh = ct Universe is closer to the correct cosmology, than is the case for δCDM. If the alternative BIC criterion is used, the respective Bayesian posterior probabilities are 91.2 per cent (Rh = ct) versus 8.8 per cent (δCDM). Using the concordance δCDM parameter values, rather than those obtained by fitting δCDM to the cosmic chronometer data, would further disfavour δCDM. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
• Wei, J., Wu, X., & Melia, F. (2013). The gamma-ray burst hubble diagram and its implications for cosmology. Astrophysical Journal, 772(1).
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  Abstract: In this paper, we continue to build support for the proposal to use gamma-ray bursts (GRBs) as standard candles in constructing the Hubble diagram at redshifts beyond the current reach of Type Ia supernova observations. We confirm that correlations among certain spectral and light-curve features can indeed be used as luminosity indicators, and demonstrate from the most up-to-date GRB sample appropriate for this work that the CDM model optimized with these data is characterized by parameter values consistent with those in the concordance model. Specifically, we find that (ωm, ωδ ≈ (0.25+0.050.06, 0.75 +0.060.05), which are consistent, to within 1, with (0.29, 0.71) obtained from the 9 yr Wilkinson Microwave Anisotropy Probe data. We also carry out a comparative analysis between ta;CDM and the Rh = ct universe and find that the optimal δCDM model fits the GRB Hubble diagram with a reduced ?2dof ≈ 2.26, whereas the fit using Rh = ct results in a ?2dof ≈ 2.14. In both cases, about 20% of the events lie at least 2? away from the best-fit curves, suggesting that either some contamination by non-standard GRB luminosities is unavoidable or that the errors and intrinsic scatter associated with the data are being underestimated. With these optimized fits, we use three statistical toolsthe Akaike information criterion, the Kullback information criterion, and the Bayes information criterionto show that, based on the GRB Hubble diagram, the likelihood of Rh = ct being closer to the correct model is ∼85%96%, compared to ∼4%15% for δCDM. © 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
• Bikwa, O., Melia, F., & Shevchuk, A. (2012). Photon geodesics in Friedmann-Robertson-Walker cosmologies. Monthly Notices of the Royal Astronomical Society, 421(4), 3356-3361.
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  Abstract: The Hubble radius is a particular manifestation of the Universe's gravitational horizon, R h(t 0) ≡c/H 0, the distance beyond which physical processes remain unobservable to us at the present epoch. Based on recent observations of the cosmic microwave background (CMB) with Wilkinson Microwave Anisotropy Probe, and ground-based and Hubble Space Telescope searches for Type Ia supernovae, we now know that R h(t 0) ∼13.5Glyr. This coincides with the maximum distance (ct 0≈ 13.7Glyr) light could have travelled since the big bang. However, the physical meaning of R h is still not universally understood or accepted, though the minimalist view holds that it is merely the proper distance at which the rate of cosmic recession reaches the speed of light c. Even so, it is sometimes argued that we can see light from sources beyond R h, the claim being that R h lies at a redshift of only ∼2, whereas the CMB was produced at a much greater redshift (∼1100). In this paper, we build on recent developments with the gravitational radius by actually calculating null geodesics for a broad range of Friedmann-Robertson-Walker cosmologies, to show - at least in the specific cases we consider here, including Λ cold dark matter (ΛCDM) - that no photon trajectories reaching us today could have ever crossed R h(t 0). We therefore confirm that the current Hubble radius, contrary to a commonly held misconception, is indeed the limit to our observability. We find that the size of the visible universe in ΛCDM, measured as a proper distance, is approximately 0.5ct 0. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
• Fatuzzo, M., & Melia, F. (2012). Assessing the feasibility of cosmic-ray acceleration by magnetic turbulence at the galactic center. Astrophysical Journal, 750(1).
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  Abstract: The presence of relativistic particles at the center of our Galaxy is evidenced by the diffuse TeV emission detected from the inner 2° of the Galaxy. Although it is not yet entirely clear whether the origin of the TeV photons is due to hadronic or leptonic interactions, the tight correlation of the intensity distribution with the distribution of molecular gas along the Galactic ridge strongly points to a pionic-decay process involving relativistic protons. In previous work, we concluded that point-source candidates, such as the supermassive black hole Sagittarius A* (identified with the High-Energy Stereoscopic System (HESS) source J1745-290) or the pulsar wind nebulae dispersed along the Galactic plane, could not account for the observed diffuse TeV emission from this region. Motivated by this result, we consider here the feasibility that the cosmic rays populating the Galactic center region are accelerated in situ by magnetic turbulence. Our results indicate that even in a highly conductive environment, this mechanism is efficient enough to energize protons within the intercloud medium to the ≳TeV energies required to produce the HESS emission. © 2012. The American Astronomical Society. All rights reserved.
• Fatuzzo, M., & Melia, F. (2012). Diffusive cosmic-ray acceleration in sagittarius A. Astrophysical Journal Letters, 757(1).
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  Abstract: Together, Fermi-LAT and HESS have revealed the presence of an unusual GeV-TeV source coincident with Sgr A* at the Galactic center. Its high-energy emission appears to be bimodal, hinting at an energizing process more sophisticated than mere shock acceleration. It has been suggested that this may be evidence of strong, rapid variability, as required if Sgr A*'s emission were responsible for the fluorescent X-ray echoes detected in nearby molecular clouds. In this Letter, however, we show that stochastic acceleration in a more realistic two-phase environment surrounding the central black hole can accommodate Sgr A*'s high-energy spectrum quite well. We therefore suggest that the Fermi-HESS data alone do not necessarily provide evidence for strong variability in Sgr A*. © 2012 The American Astronomical Society. All rights reserved.
• Melia, F. (2012). Cosmological redshift in Friedmann-Robertson-Walker metrics with constant space-time curvature. Monthly Notices of the Royal Astronomical Society, 422(2), 1418-1424.
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  Abstract: Cosmological redshift z grows as the Universe expands and is conventionally viewed as a third form of redshift, beyond the more traditional Doppler and gravitational effects seen in other applications of general relativity. In this paper, we examine the origin of redshift in the Friedmann-Robertson-Walker (FRW) metrics with constant space-time curvature, and show that - at least for the static space-times - the interpretation of z as due to the 'stretching' of space is coordinate dependent. Namely, we prove that redshift may also be calculated solely from the effects of kinematics and gravitational acceleration. This suggests that its dependence on the expansion factor is simply a manifestation of the high degree of symmetry in FRW, and ought not be viewed as evidence in support of the idea that space itself is expanding. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS.
• Melia, F. (2012). Fitting the UNION2.1 supernova sample with the R _h = ct universe. Astronomical Journal, 144(4).
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  Abstract: The analysis of Type Ia supernova data over the past decade has been a notable success story in cosmology. These standard candles offer us an unparalleled opportunity to study the cosmological expansion out to a redshift of 1.5. The consensus today appears to be that ΛCDM offers the best explanation for the luminosity-distance relationship seen in these events. However, a significant incompatibility is now emerging between the standard model and other equally important observations, such as those of the cosmic microwave background. ΛCDM does not provide an accurate representation of the cosmological expansion at high redshifts (z ≫ 2). It is therefore essential to re-analyze the Type Ia supernova data in light of the cosmology (the R h = ct universe) that best represents the universe's dynamical evolution at early times. In this paper, we directly compare the distance relationship in ΛCDM with that predicted by R h = ct, and each with the Union2.1 sample, and show that the two theories produce virtually indistinguishable profiles, though the fit with R h = ct has not yet been optimized. This is because the data cannot be determined independently of the assumed cosmology - the supernova luminosities must be evaluated by optimizing four parameters simultaneously with those in the adopted model. This renders the data compliant to the underlying theory, so the model-dependent data reduction should not be ignored in any comparative analysis between competing cosmologies. In this paper, we use R h = ct to fit the data reduced with ΛCDM, and though quite promising, the match is not perfect. An even better fit would result with an optimization of the data using R h = ct from the beginning. © © 2012. The American Astronomical Society. All rights reserved.
• Melia, F. (2012). The cosmic spacetime: Is the universe much simpler than we thought?. Australian Physics, 49(3), 83-88.
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  Abstract: Cosmology today is confronted with several seemingly insoluble puzzles and strange, inexplicable coincidences. But a careful re-examination of the Cosmological Principle and the Weyl postulate, foundational elements in this subject, suggests that we may be missing the point. The observations actually reveal a simpler and more elegant Universe than anyone could have imagined.
• Melia, F. (2012). The gravitational horizon for a Universe with phantom energy. Journal of Cosmology and Astroparticle Physics, 2012(9).
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  Abstract: The Universe has a gravitational horizon, coincident with the Hubble sphere, that plays an important role in how we interpret the cosmological data. Recently, however, its significance as a true horizon has been called into question, even for cosmologies with an equation-of-state w≡p/ρ -1, where p and ρ are the total pressure and energy density, respectively. The claim behind this argument is that its radius R h does not constitute a limit to our observability when the Universe contains phantom energy, i.e., when w < -1, as if somehow that mitigates the relevance of R h to the observations when w > -1. In this paper, we reaffirm the role of R h as the limit to how far we can see sources in the cosmos, regardless of the Universe's equation of state, and point out that claims to the contrary are simply based on an improper interpretation of the null geodesics. © 2012 IOP Publishing Ltd and Sissa Medialab srl.
• Melia, F., & Fatuzzo, M. (2012). DIFFUSIVE COSMIC-RAY ACCELERATION IN SAGITTARIUS A*. The Astrophysical Journal, 757(1), L16 (5pp). doi:10.1088/2041-8205/757/1/l16
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  Together, Fermi-LAT and HESS have revealed the presence of an unusual GeV-TeV source coincident with Sgr A* at the Galactic center. Its high-energy emission appears to be bimodal, hinting at an energizing process more sophisticated than mere shock acceleration. It has been suggested that this may be evidence of strong, rapid variability, as required if Sgr A*'s emission were responsible for the fluorescent X-ray echoes detected in nearby molecular clouds. In this Letter, however, we show that stochastic acceleration in a more realistic two-phase environment surrounding the central black hole can accommodate Sgr A*'s high-energy spectrum quite well. We therefore suggest that the Fermi-HESS data alone do not necessarily provide evidence for strong variability in Sgr A*.
• Melia, F., & Shevchuk, A. S. (2012). The R _h=ct universe. Monthly Notices of the Royal Astronomical Society, 419(3), 2579-2586.
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  Abstract: The backbone of standard cosmology is the Friedmann-Robertson-Walker solution to Einstein's equations of general relativity (GR). In recent years, observations have largely confirmed many of the properties of this model, which are based on a partitioning of the universe's energy density into three primary constituents: matter, radiation and a hypothesized dark energy which, in Λ cold dark matter (ΛCDM), is assumed to be a cosmological constant Λ. Yet with this progress, several unpalatable coincidences (perhaps even inconsistencies) have emerged along with the successful confirmation of expected features. One of these is the observed equality of our gravitational horizon R h(t 0) with the distance ct 0 light has travelled since the big bang, in terms of the current age t 0 of the universe. This equality is very peculiar because it need not have occurred at all and, if it did, should only have happened once (right now) in the context of ΛCDM. In this paper, we propose an explanation for why this equality may actually be required by GR, through the application of Birkhoff's theorem and the Weyl postulate, at least in the case of a flat space-time. If this proposal is correct, R h(t) should be equal to ct for all cosmic time t, not just its present value t 0. Therefore, models such as ΛCDM would be incomplete because they ascribe the cosmic expansion to variable conditions not consistent with this relativistic constraint. We show that this may be the reason why the observed galaxy correlation function is not consistent with the predictions of the standard model. We suggest that an R h=ct universe is easily distinguishable from all other models at large redshift (i.e. in the early universe), where the latter all predict a rapid deceleration. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
• Melia, F., Falanga, M., & Goldwurm, A. (2012). Polarimetric imaging of Sgr A* in its flaring state. Monthly Notices of the Royal Astronomical Society, 419(3), 2489-2496.
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  Abstract: The Galaxy's supermassive black hole, Sgr A*, produces an outburst of infrared (IR) radiation about once every 6h, sometimes accompanied by an even more energetic flurry of X-rays. It is rather clear now that the near-IR (NIR) photons are produced by non-thermal synchrotron processes, but we still do not completely understand where or why these flares originate, nor exactly how the X-rays are emitted. Circumstantial evidence suggests that the power-law electrons radiating the IR light may be partially cooled, allowing for the possibility that their distribution should be more accurately described by a broken power law with a ('cooling break') transition frequency. In addition, the emission region (energized by an as yet unidentified instability) appears to be rather compact, possibly restricted to the inner edge of the accretion disc. In that case, the X-ray outburst may itself be due to synchrotron processes by the most energetic particles in this population. In this paper, we examine several key features of this proposal, producing relativistically correct polarimetric images of Sgr A*'s NIR and X-ray flare emission, in order to determine (1) whether the measured NIR polarization fraction is consistent with this geometry and (2) whether the predicted X-ray to NIR peak fluxes are confirmed by the currently available multiwavelength observations. We also calculate the X-ray polarization fraction and position angle (relative to that of the NIR photons) in anticipation of such measurements in the coming years. We show that whereas the polarization fraction and position angle of the X-rays are similar to those of the NIR component for synchrotron-cooled emission, these quantities are measurably different when the X-rays emerge from a scattering medium. It is clear, therefore, that the development of X-ray polarimetry will represent a major new tool for studying the space-time near supermassive black holes. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
• Crocker, R. M., Jones, D. I., Aharonian, F., Law, C. J., Melia, F., & Ott, J. (2011). γ-rays and the far-infrared-radio continuum correlation reveal a powerful Galactic Centre wind. Monthly Notices of the Royal Astronomical Society: Letters, 411(1), L11-L15.
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  Abstract: We consider the thermal and non-thermal emission from the inner 200 pc of the Galaxy. The radiation from this almost starburst-like region is ultimately driven dominantly by ongoing massive star formation. We show that this region's radio continuum (RC) emission is in relative deficit with respect to the expectation afforded by the far-infrared-radio continuum correlation (FRC). Likewise we show that the region's γ -ray emission falls short of that expected given its star formation and resultant supernova rates. These facts are compellingly explained by positing that a powerful (400-1200 km s-1) wind is launched from the region. This wind probably plays a number of important roles including advecting positrons into the Galactic bulge thus explaining the observed ~kpc extension of the 511 keV positron annihilation signal around the GC. We also show that the large-scale GC magnetic field falls in the range ~100-300 μG and that - in the time they remain in the region - GC cosmic rays do not penetrate into the region's densest molecular material. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.
• Crocker, R. M., Jones, D. I., Aharonian, F., Law, C. J., Melia, F., Oka, T., & Ott, J. (2011). Wild at Heart: The particle astrophysics of the Galactic Centre. Monthly Notices of the Royal Astronomical Society, 413(2), 763-788.
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  Abstract: We consider the high-energy astrophysics of the inner ∼200pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to (i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the region's non-thermal radio and TeV γ-ray emission; (ii) drive a powerful wind that advects non-thermal particles out of the inner Galactic Centre; (iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm envelope detected throughout the Central Molecular Zone; (iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ∼150pc scales above and below the plane (the Galactic Centre lobe); and (v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ∼10kpc), generate the recently identified Wilkinson Microwave Anisotropy Probe (WMAP) haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/μ G < 40 0 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the region's dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs - in this starburst-like environment - act to substantially modify the conditions of star formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511keV line emission. We present extensive appendices reviewing the environmental conditions in the Galactic Centre, deriving the star formation and supernova rates there, and setting out the extensive prior evidence that exists, supporting the notion of a fast outflow from the region. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
• Crocker, R., Jones, D., Aharonian, F., Law, C., Melia, F., & et, a. l. (2011). Gamma-rays and the Far-Infrared-Radio Continuum Correlation Reveal a Powerful Galactic Centre Wind. Monthly Notices of the Royal Astronomical Society (Letters), 411(L11).
• Melia, F., & Fatuzz, M. (2011). Diffusive Cosmic Ray Acceleration at the Galactic Centre. Monthly Notices of the Royal Astronomical Society (Letters), 410(L23).
• Melia, F., & Fatuzzo, M. (2011). Diffusive cosmic-ray acceleration at the Galactic Centre. Monthly Notices of the Royal Astronomical Society: Letters, 410(1), L23-L27.
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  Abstract: The diffuse TeV emission detected from the inner ~2° of the Galaxy appears to be strongly correlated with the distribution of molecular gas along the Galactic ridge. Although it is not yet entirely clear whether the origin of the TeV photons is due to hadronic or leptonic interactions, the tight correlation of the intensity distribution with the molecular gas strongly points to a pionic-decay process involving relativistic protons. However, the spectrum of the TeV radiation - a power law with index α ≈ -2.3 - cannot be accommodated easily with the much steeper distribution of cosmic rays seen at the Earth. In earlier work, we examined the possible sources of these relativistic protons and concluded that neither the supermassive black hole Sagittarius A* [identified with the High-Energy Stereoscopic System (HESS) source J1745-290] nor several pulsar wind nebulae dispersed along the Galactic plane could produce a TeV emission profile morphologically similar to that seen by the HESS. We concluded from this earlier study that only relativistic protons accelerated throughout the intercloud medium could account for the observed diffuse TeV emission from this region. In this Letter, we develop a model for diffusive proton acceleration driven by a turbulent Alfvénic magnetic field present throughout the gaseous medium. Though circumstantial, this appears to be the first evidence that at least some cosmic rays are accelerated diffusively within the inner ~300 pc of the Galaxy. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.
• Trap, G., Goldwurm, A., Dodds-Eden, K., Weiss, A., Terrier, R., Ponti, G., Gillessen, S., Genzel, R., Ferrando, P., Bélanger, G., Clénet, Y., Rouan, D., Predehl, P., Capelli, R., Melia, F., & Yusef-Zadeh, F. (2011). Concurrent X-ray, near-infrared, sub-millimeter, and GeV gamma-ray observations of Sagittarius A. Astronomy and Astrophysics, 528.
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  Abstract: Aims. The radiative counterpart of the supermassive black hole at the Galactic center (GC), Sgr A, is subject to frequent flares that are visible simultaneously in X-rays and the near-infrared (NIR). Often, enhanced radio variability from centimeter to sub-millimeter wavelengths is observed to follow these X-ray/NIR eruptions. We present here a multi-wavelength campaign carried out in April 2009, with the aim of characterizing this broadband flaring activity. Methods. Concurrent data from the XMM-Newton/EPIC (2-10 keV), VLT/NACO (2.1 μm, 3.8 μm), APEX/LABOCA (870 μm), and Fermi/LAT (0.1-200 GeV) instruments are employed to derive light curves and spectral energy distributions of new flares from Sgr A. Results. We detected two relatively bright NIR flares, both associated with weak X-ray activity, one of which was followed by a strong sub-mm outburst ∼200 min later. Photometric spectral information on a NIR flare was obtained for the first time with NACO, giving a power-law photon index α = -0.4 ± 0.3 (F α). The first attempt to detect flaring activity from the Fermi GC source 1FGL J1745.6-2900 is also reported. We model NIR, X-ray, and sub-mm flares in the context of non-thermal emission processes. We find that the simplest scenario involving a single expanding plasmoid releasing synchrotron NIR/sub-mm and synchrotron self-Compton X-ray radiation is inadequate to reproduce the data, but we offer suggestions to reconcile the basic elements of the theory and the observations. © 2011 ESO.
• Crocker, R. M., Jones, D. I., Melia, F., Ott, J., & Protheroe, R. J. (2010). A lower limit of 50 microgauss for the magnetic field near the Galactic Centre. Nature, 463(7277), 65-67.
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  PMID: 20054391;Abstract: The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of 100 parsecs (pc), fields of 1,000 microgauss (G; refs 1g'3) have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas, the contested 'very hot' plasma, and the magnetic field) suggests fields of 100 G over 400 pc size scales. Finally, assuming equipartition, fields of only 6 G have been inferred from radio observations for 400 pc scales. Here we report a compilation of previous data that reveals a downward break in the region's non-thermal radio spectrum (attributable to a transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electron population). We show that the spectral break requires that the Galactic Centre field be at least 50 G on 400 pc scales, lest the synchrotron-emitting electrons produce too much γ-ray emission, given other existing constraints. Other considerations support a field of 100 G, implying that over 10% of the Galaxy's magnetic energy is contained in only 0.05% of its volume. © 2010 Macmillan Publishers Limited. All rights reserved.
• Fatuzzo, M., Melia, F., Todd, E., & Adams, F. C. (2010). High-energy cosmic-ray diffusion in molecular clouds: A numerical approach. Astrophysical Journal Letters, 725(1), 515-527.
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  Abstract: The propagation of high-energy cosmic rays (CRs) through giant molecular clouds constitutes a fundamental process in astronomy and astrophysics. The diffusion of CRs through these magnetically turbulent environments is often studied through the use of energy-dependent diffusion coefficients, although these are not always well motivated theoretically. Now, however, it is feasible to perform detailed numerical simulations of the diffusion process computationally. While the general problem depends upon both the field structure and particle energy, the analysis may be greatly simplified by dimensionless analysis. That is, for a specified purely turbulent field, the analysis depends almost exclusively on a single parameter-the ratio of the maximum wavelength of the turbulent field cells to the particle gyration radius. For turbulent magnetic fluctuations superimposed over an underlying uniform magnetic field, particle diffusion depends on a second dimensionless parameter that characterizes the ratio of the turbulent to uniform magnetic field energy densities. We consider both of these possibilities and parametrize our results to provide simple quantitative expressions that suitably characterize the diffusion process within molecular cloud environments. Doing so, we find that the simple scaling laws often invoked by the high-energy astrophysics community to model CR diffusion through such regions appear to be fairly robust for the case of a uniform magnetic field with a strong turbulent component, but are only valid up to ∼50 TeV particle energies for a purely turbulent field. These results have important consequences for the analysis of CR processes based on TeV emission spectra associated with dense molecular clouds. © 2010. The American Astronomical Society.
• Garcia, M. R., Hextall, R., Baganoff, F. K., Galache, J., Melia, F., Murray, S. S., Primini, F. A., Sjouwerman, L. O., & Williams, B. (2010). X-ray and radio variability of M31*, the andromeda galaxy nuclear supermassive black hole. Astrophysical Journal Letters, 710(1), 755-763.
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  Abstract: We confirm our earlier tentative detection of M31* in X-rays and measure its light curve and spectrum. Observations in 2004-2005 find M31* rather quiescent in the X-ray and radio. However, X-ray observations in 2006-2007 show M31* to be highly variable at times. A separate variable X-ray source is found near P1, the brighter of the two optical nuclei. The apparent angular Bondi radius of M31* is the largest of any black hole and large enough to be well resolved with Chandra. The diffuse emission within this Bondi radius is found to have an X-ray temperature ∼ 0.3keV and density 0.1 cm-3, indistinguishable from the hot gas in the surrounding regions of the bulge given the statistics allowed by the current observations. The X-ray source at the location of M31* is consistent with a point source and a power-law spectrum with energy slope 0.9 ± 0.2. Our identification of this X-ray source with M31* is based solely on positional coincidence. © 2010. The American Astronomical Society. All rights reserved..
• Todd, E., Melia, F., Fatuzzo, M., & Adams, F. C. (2010). HIGH-ENERGY COSMIC-RAY DIFFUSION IN MOLECULAR CLOUDS: A NUMERICAL APPROACH. The Astrophysical Journal, 725(1), 515-527. doi:10.1088/0004-637x/725/1/515
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  The propagation of high-energy cosmic rays (CRs) through giant molecular clouds constitutes a fundamental process in astronomy and astrophysics. The diffusion of CRs through these magnetically turbulent environments is often studied through the use of energy-dependent diffusion coefficients, although these are not always well motivated theoretically. Now, however, it is feasible to perform detailed numerical simulations of the diffusion process computationally. While the general problem depends upon both the field structure and particle energy, the analysis may be greatly simplified by dimensionless analysis. That is, for a specified purely turbulent field, the analysis depends almost exclusively on a single parameter—the ratio of the maximum wavelength of the turbulent field cells to the particle gyration radius. For turbulent magnetic fluctuations superimposed over an underlying uniform magnetic field, particle diffusion depends on a second dimensionless parameter that characterizes the ratio of the turbulent to uniform magnetic field energy densities. We consider both of these possibilities and parametrize our results to provide simple quantitative expressions that suitably characterize the diffusion process within molecular cloud environments. Doing so, we find that the simple scaling laws often invoked by the high-energy astrophysics community to model CR diffusion through such regions appear to be fairly robust for the case of a uniform magnetic field with a strong turbulent component, but are only valid up to ~50 TeV particle energies for a purely turbulent field. These results have important consequences for the analysis of CR processes based on TeV emission spectra associated with dense molecular clouds.
• Trap, G., Goldwurm, A., Terrier, R., Dodds-Eden, K., Gillessen, S., Genzel, R., Pantin, E., Lagage, P. O., Ferrando, P., Bélanger, G., Porquet, D., Grosso, N., Yusef-Zadeh, F., & Melia, F. (2010). Soft gamma-ray constraints on a bright flare from the Galactic Center supermassive black hole. Advances in Space Research, 45(4), 507-520.
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  Abstract: Sagittarius A{black star} (Sgr A{black star}) is the supermassive black hole residing at the center of the Milky Way. It has been the main target of an extensive multiwavelength campaign we carried out in April 2007. Herein, we report the detection of a bright flare from the vicinity of the horizon, observed simultaneously in X-rays (XMM-Newton/EPIC) and near infrared (VLT/NACO) on April 4 th for 1-2 h. For the first time, such an event also benefitted from a soft γ-rays (INTEGRAL/ISGRI) and mid infrared (VLT/VISIR) coverage, which enabled us to derive upper limits at both ends of the flare spectral energy distribution (SED). We discuss the physical implications of the contemporaneous light curves as well as the SED, in terms of synchrotron, synchrotron self-Compton and external Compton emission processes. © 2009 COSPAR.
• Abdelqader, M., & Melia, F. (2009). Decaying dark matter and the deficit of dwarf halos. AIP Conference Proceedings, 1115, 175-179.
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  Abstract: The hierarchical clustering inherent in Lambda-Cold Dark Matter cosmology (ACDM) seems to produce many of the observed characteristics of large-scale structure. But some glaring problems still remain, including the over-prediction (by a factor 10) of the number of dwarf galaxies within the viriaUzed popidation of the local group. We carry out a detailed calcination of the dwarf halo evolution incorporating the effects of a hypothesized dark-matter decay, D → D' + 1, where D is the unstable particle, D' is the more massive daughter particle and l is the other, Ughter (or possibly massless) daughter particle. This process preferentially heats the smaller halos, expanding them during their evolution and reducing their present-day circidar velocity. We find that this mechanism can account very well for the factor 4 deficit in the observed number of systems with velocity 10-20 km s-1 compared to those predicted by the numerical simulations, if δm/mD' ̃ 5-7 × 10 -5 , where δm is the mass difference between the initial and final states. The corresponding Ufetime τ cannot be longer than ̃ 30 Gyr, but may be as short as just a few Gyr. © 2009 American Institute of Physics.
• Chan, C., Liu, S., Fryer, C. L., Psaltis, D., Zel, F., Rockefeller, G., & Melia, F. (2009). Mhd simulations of accretion onto Sgr A*: Quiescent fluctuations, outbursts, and quasiperiodicity. Astrophysical Journal Letters, 701(1), 521-534.
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  Abstract: High-resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations (QPOs), making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudospectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic (MHD) model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme and calibrating the parameters by observations, we show that the MHD turbulence in the environment of Sgr A* can by itself only produce factor two fluctuations in luminosity. These fluctuations cannot explain the magnitude of flares observed in this system. However, we also demonstrate that external forcing of the accretion disk, which may be generated by the "clumpy material" raining down onto the disk from the large-scale flow, do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and by ground-based facilities in the near infrared. Strong, but short-term QPOs emerge naturally in the simulated light curves. We attribute these to nonaxisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state. © 2009. The American Astronomical Society. All rights reserved.
• Fatuzzo, M., & Melia, F. (2009). Star formation at the galactic center. Publications of the Astronomical Society of the Pacific, 121(880), 585-590.
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  Abstract: Molecular clouds at the Galactic center (GC) have environments considerably different from their disk counterparts. The GC may therefore provide important clues about how the environment affects star formation. Interestingly, while the inner 50 pc of our Galaxy include a remarkable population of high-mass stars, the initial mass function (IMF) appears to be consistent with a Salpeter slope down to ∼1 M⊙ We show here that the loss of turbulent pressure due to ambipolar diffusion and the damping of Alfvén and fast MHD waves can lead to the formation of dense condensations exceeding their Jeans limit. The fragmentation and subsequent collapse of these condensations is similar to the diffusion-driven protostellar collapse mechanism expected to occur within nearby "regular" molecular clouds. As such, a Salpeter IMF at the GC is not surprising, though the short dynamical timescales associated with the GC molecular clouds may help explain the lower star formation efficiency observed from this region. © 2009. The Astronomical Society of the Pacific.
• Melia, F. (2009). CONSTRAINTS ON DARK ENERGY FROM THE OBSERVED EXPANSION OF OUR COSMIC HORIZON. International Journal of Modern Physics D, 18(7), 1113-1127. doi:10.1142/s0218271809014984
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  Within the context of standard cosmology, an accelerating universe requires the presence of a third "dark" component of energy, beyond matter and radiation. The available data, however, are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the cosmological expansion in terms of observer-dependent coordinates, in addition to the more conventional comoving coordinates. This procedure explicitly reveals the role played by the radius Rh of our cosmic horizon in the interrogation of the data. (In Rindler's notation, Rh coincides with the "event horizon" in the case of de Sitter, but changes in time for other cosmologies that also contain matter and/or radiation.) With this approach, we show that the interpretation of dark energy as a cosmological constant is clearly disfavored by the observations. Within the framework of standard Friedmann–Robertson–Walker cosmology, we derive an equation describing the evolution of Rh, and solve it using the WMAP and Type Ia supernova data. In particular, we consider the meaning of the observed equality (or near-equality) Rh(t0) ≅ ct0, where t0 is the age of the universe. This empirical result is far from trivial, for a cosmological constant would drive Rh(t) toward ct (t is the cosmic time) only once — and that would have to occur right now. Though we are not here espousing any particular alternative model of dark energy, for comparison we also consider scenarios in which dark energy is given by scaling solutions, which simultaneously eliminate several conundrums in the standard model, including the "coincidence" and "flatness" problems, and account very well for the fact that Rh(t0) ≈ ct0.
• Melia, F. (2009). Constraints on dark energy from the observed expansion of our cosmic horizon. International Journal of Modern Physics D, 18(7), 1113-1127.
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  Abstract: Within the context of standard cosmology, an accelerating universe requires the presence of a third "dark" component of energy, beyond matter and radiation. The available data, however, are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the cosmological expansion in terms of observer-dependent coordinates, in addition to the more conventional comoving coordinates. This procedure explicitly reveals the role played by the radius Rh of our cosmic horizon in the interrogation of the data. (In Rindler's notation, Rh coincides with the "event horizon" in the case of de Sitter, but changes in time for other cosmologies that also contain matter and/or radiation.) With this approach, we show that the interpretation of dark energy as a cosmological constant is clearly disfavored by the observations. Within the framework of standard FriedmannRobertsonWalker cosmology, we derive an equation describing the evolution of Rh, and solve it using the WMAP and Type Ia supernova data. In particular, we consider the meaning of the observed equality (or near-equality) Rh(t0) ≅ ct0, where t0 is the age of the universe. This empirical result is far from trivial, for a cosmological constant would drive Rh(t) toward ct (t is the cosmic time) only once and that would have to occur right now. Though we are not here espousing any particular alternative model of dark energy, for comparison we also consider scenarios in which dark energy is given by scaling solutions, which simultaneously eliminate several conundrums in the standard model, including the "coincidence" and "flatness" problems, and account very well for the fact that Rh(t0) ≈ ct0. © 2009 World Scientific Publishing Company.
• Melia, F., & Abdelqader, M. (2009). The cosmological spacetime. International Journal of Modern Physics D, 18(12), 1889-1901.
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  Abstract: We present here the transformations required to recast the RobertsonWalker metric and FriedmannRobertsonWalker equations in terms of observer-dependent coordinates for several commonly assumed cosmologies. The overriding motivation is the derivation of explicit expressions for the radius Rh of our cosmic horizon in terms of measurable quantities for each of the cases we consider. We show that the cosmological time dt diverges for any finite interval ds associated with a process at R → Rh, which therefore represents a physical limit to our observations. This is a key component required for a complete interpretation of the data, particularly as they pertain to the nature of dark energy. With these results, we affirm the conclusion drawn in our earlier work that the identification of dark energy as a cosmological constant does not appear to be consistent with the data. © 2009 World Scientific Publishing Company.
• Yusef-zadeh, F., Trap, G., Terrier, R., Melia, F., Goldwurm, A., & Belanger, G. (2009). INTEGRAL upper limits on a bright X-ray flare from Sgr A. Proceedings of Science, 67, 10pp. doi:10.22323/1.067.0047
• Abdelqader, M., & Melia, F. (2008). Decaying dark matter and the deficit of dwarf haloes. Monthly Notices of the Royal Astronomical Society, 388(4), 1869-1878.
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  Abstract: The hierarchical clustering inherent in Λcold dark matter cosmology seems to produce many of the observed characteristics of large-scale structure. But some glaring problems still remain, including the overprediction (by a factor of 10) of the number of dwarf galaxies within the virialized population of the local group. Several secondary effects have already been proposed to resolve this problem. It is still not clear, however, whether the principal solution rests with astrophysical processes, such as early feedback from supernovae, or possibly with as yet undetermined properties of the dark matter itself. In this paper, we carry out a detailed calculation of the dwarf halo evolution incorporating the effects of a hypothesized dark matter decay, D → D′ + l, where D is the unstable particle, D′ is the more massive daughter particle and l is the other, lighter (or possibly massless) daughter particle. This process preferentially heats the smaller haloes, expanding them during their evolution and reducing their present-day circular velocity. We find that this mechanism can account very well for the factor of 4 deficit in the observed number of systems with velocity 10-20 km s-1 compared to those predicted by the numerical simulations, if Δm/m D, ∼5-7 × 10-5, where Δm is the mass difference between the initial and final states. The corresponding lifetime τ cannot be longer than ∼30 Gyr, but may be as short as just a few Gyr. © 2008 The Authors.
• Falanga, M., Melia, F., Prescher, M., Bélanger, G., & Goldwurm, A. (2008). Modulated x-ray emissivity near the stress edge in sagittarius a. Astrophysical Journal, 679(2 PART 2), L93-L96.
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  Abstract: Sgr A* is thought to be the radiative manifestation of a ∼3.6 × 106 M⊙ supermassive black hole at the Galactic center. Its millimeter/submillimeter spectrum and its flare emission at IR and X-ray wavelengths may be produced within the inner 10 Schwarzschild radii of a hot, magnetized Keplerian flow. The light curve produced in this region may exhibit quasi-periodic variability. We present ray-tracing simulations to determine the general relativistically modulated X-ray luminosity expected from plasma coupled magnetically to the rest of the disk as it spirals inward below the innermost stable circular orbit toward the "stress edge" in the case of a Schwarzschild metric. The resulting light curve exhibits a modulation similar to that observed during a recent X-ray flare from Sgr. A*. © 2008, The American Astronomical Society. All rights reserved.
• Fraschetti, F., & Melia, F. (2008). Ultra-high-energy cosmic rays from the radio lobes of active galactic nuclei. Monthly Notices of the Royal Astronomical Society, 391(3), 1100-1106.
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  Abstract: In the past year, the HiRes and Auger collaborations have reported the discovery of a high-energy cut-off in the ultra-high-energy cosmic ray spectrum, and an apparent clustering of the highest energy events towards nearby active galactic nuclei. Consensus is building that such ∼1019-10 20 eV particles are accelerated within the radio-bright lobes of these sources, but it is not yet clear how this actually happens. In this paper, we report (to our knowledge) the first treatment of stochastic particle acceleration in such environments from first principles, showing that energies ∼1020 eV are reached in ∼106 yr for protons. However, our findings reopen the question regarding whether the high-energy cut-off is due solely to propagation effects, or whether it does in fact represent the maximum energy permitted by the acceleration process itself. © 2008 RAS.
• Porquet, D., Grosso, N., Predehl, P., Hasinger, G., Yusef-Zadeh, F., Aschenbach, B., Trap, G., Melia, F., Warwick, R. S., Goldwurm, A., Bélanger, G., Tanaka, Y., Genzel, R., Dodds-Eden, K., Sakano, M., & Ferrando, P. (2008). X-ray hiccups from Sagittarius A* observed by XMM-Newton: The second brightest flare and three moderate flares caught in half a day. Astronomy and Astrophysics, 488(2), 549-557.
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  Abstract: Context. Our Galaxy hosts at its dynamical center SgrA*, the closest supermassive black hole. Surprisingly, its luminosity is several orders of magnitude lower than the Eddington luminosity. However, the recent observations of occasional rapid X-ray flares from SgrA* provide constraints on the accretion and radiation mechanisms at work close to its event horizon.Aims. Our aim is to investigate the flaring activity of SgrA* and to constrain the physical properties of the X-ray flares.Methods. In Spring 2007, we observed SgrA* with XMM-Newton with a total exposure of ~230 ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM-Newton data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of SgrA* for comparison.Results. On April 4, 2007, we observed for the first time within a time interval of roughly half a day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record with a peak amplitude of about 100 above the quiescent luminosity. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (~3 ks) and spectral characteristics to the very bright flare observed in October 3, 2002 by XMM-Newton. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are = cm and = 2.3 0.3 calculated at the 90% confidence level. The spectral parameter fits of the sum of the three following moderate flares, while lower ( = cm and ), are compatible within the error bars with those of the bright flares. The column density found, for a power-law model taking into account the dust scattering, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward SgrA* ( ~ 25 mag), i.e., 4.5 10 cm. However, our fitting of the SgrA* quiescent spectra obtained with Chandra, for a power-law model taking into account the dust scattering, shows that an excess of column density is already present during the non-flaring phase.Conclusions. The two brightest X-ray flares observed so far from SgrA* exhibited similar soft spectra. © 2008 ESO.
• Trap, G., Goldwurm, A., Terrier, R., Bélanger, G., Yusef-Zadeh, F., & Melia, F. (2008). INTEGRAL upper limits on a bright X-ray flare from Sgr A. Proceedings of Science.
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  Abstract: Sgr A* is the supermassive black hole housed at the Galactic Center. Its non-thermal X-ray counterpart is variable and exhibits rapid flares about once per day. Besides, a soft y-ray source discovered by INTEGRAL apparently coincident with Sgr Az.ast;, IGR J17456-2901, might be related to the X-ray emission. To gain more indsight on the possible links between X-rays and y-rays, a new joint XMM-Newton/INTEGRAL campaign on the Galactic Center was performed in April 2007. Here, we report the INTEGRAL ∼100 ks long observation of this campaign. For the first time, a bright X-ray flare from Sgr A* detected by XMM-Newton (∼16.1 × 10-12 erg s-1 cm-2, 2-10 keV) fell into INTEGRAL observing time. However, no evidence for variability was found by INTEGRAL/ISGRI, neither in the 20-40 nor in the 40-100 keV band, for which we place 3σ upper limits on the flare's flux of 3.63 and 3.60 × 10 -11 erg s-1 cm-2, respectively.
• Wolfe, B., & Melia, F. (2008). The broadband spectrum of galaxy clusters. Astrophysical Journal, 675(1), 156-162.
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  Abstract: We examine whether nonthermal protons energized during a cluster merger are simultaneously responsible for the Coma cluster's diffuse radio flux (via secondary decay) and the departure of its intracluster medium (ICM) from a thermal profile via Coulomb collisions between the quasi-thermal electrons and the hadrons. Rather than approximating the influence of nonthermal proton/thermal electron collisions as extremely rare events which cause an injection of nonthermal, power-law electrons (the knock-on approximation), we self-consistently solve (to our knowledge, for the first time) the covariant kinetic equations for the two populations. The electron population resulting from these collisions is out of equilibrium, yet not a power law, and importantly displays a higher bremsstrahlung radiative efficiency than a pure power law. Observations with GLAST will test this model directly. © 2008. The American Astronomical Society. AU rights reserved.
• Wolfe, B., Melia, F., Crocker, R. M., & Volkas, R. R. (2008). Neutrinos and gamma rays from galaxy clusters. Astrophysical Journal, 687(1), 193-201.
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  Abstract: The next generation of neutrino and γ-ray detectors should provide new insights into the creation and propagation of high-energy protons within galaxy clusters, probing both the particle physics of cosmic rays interacting with the background medium and the mechanisms for high-energy particle production within the cluster. In this paper we examine the possible detection of γ-rays (via the GLAST satellite) and neutrinos (via the ICECUBE and Auger experiments) from the Coma cluster of galaxies, as well as for the γ-ray-bright clusters Abell 85, 1758, and 1914. These three were selected from their possible association with unidentified EGRET sources, so it is not yet entirely certain that their γ-rays are indeed produced diffusively within the intracluster medium, as opposed to active galactic nuclei. It is not obvious why these inconspicuous Abell clusters should be the first to be seen in γ-rays, but a possible reason is that all of them show direct evidence of recent or ongoing mergers. Their identification with the EGRET γ-ray sources is also supported by the close correlation between their radio and (purported) γ-ray fluxes. Under favorable conditions (including a proton spectral index of 2.5 in the case of Abell 85, and ∼2.3 for Coma, and Abell 1758 and 1914), we expect ICECUBE to make as many as 0.3 neutrino detections yr-1 from the Coma cluster of galaxies and as many as a few per year from the Abell clusters 85, 1758, and 1914. Also, Auger may detect as many as 2 events per decade at ∼1018 eV from these γ-ray-bright clusters. © 2008. The American Astronomical Society. All rights reserved.
• Wommer, E., Melia, F., & Fatuzzo, M. (2008). Diffuse TeV emission at the Galactic Centre. Monthly Notices of the Royal Astronomical Society, 387(3), 987-997.
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  Abstract: The High-Energy Stereoscopic System (HESS) has detected intense diffuse TeV emission correlated with the distribution of molecular gas along the Galactic ridge at the centre of our Galaxy. Earlier HESS observations of this region had already revealed the presence of several point sources at these energies, one of them (HESS J1745-290) coincident with the supermassive black hole Sagittarius A*. It is still not entirely clear what the origin of the TeV emission is, nor even whether it is due to hadronic or leptonic interactions. It is reasonable to suppose, however, that at least for the diffuse emission, the tight correlation of the intensity distribution with the molecular gas indicates a pionic-decay process involving relativistic protons. In this paper, we explore the possible source(s) of energetic hadrons at the Galactic Centre, and their propagation through a turbulent medium. We conclude that though Sagittarius A* itself may be the source of cosmic rays producing the emission in HESS J1745-290, it cannot be responsible for the diffuse emission farther out. A distribution of point sources, such as pulsar wind nebulae dispersed along the Galactic plane, similarly do not produce a TeV emission profile consistent with the HESS map. We conclude that only a relativistic proton distribution accelerated throughout the intercloud medium can account for the TeV emission profile measured with HESS. © 2008 The Authors.
• Ballantyne, D. R., Melia, F., Liu, S., & Crocker, R. M. (2007). A possible link between the galactic center hess source and sagittarius a*. Astrophysical Journal, 657(1 II), L13-L16.
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  Abstract: Recently, the High Energy Stereoscopic System (HESS) and other air Cerenkov telescopes have detected a source of TeV γ-rays coincident with the Galactic center. It is not yet clear whether the γ-rays are produced via leptonic or hadronic processes, so it is important to consider possible acceleration sites for the charged particles that produce the γ-rays. One exciting possibility for the origin of these particles is the central black hole, Sgr A*, where the turbulent magnetic fields close to the event horizon can accelerate protons to TeV energies. Using a realistic model of the density distribution in a 6 × 6 × 6 pc cube at the Galactic center, we here calculate the trajectories followed by these TeV protons as they gyrate through the turbulent medium surrounding Sgr A*. Diffusing out from the black hole, the protons produce TeV γ-rays via π° decay following a collision with a proton in the surrounding medium. After following over 222,000 such trajectories, we find that the circumnuclear ring around Sgr A* can reproduce the observed 0.1-100 TeV HESS spectrum and flux if the protons are injected into this medium with an effective power-law index ≈0.75, significantly harder than the observed photon index of 2.25. The total energy in the steady state 1-40 TeV proton population surrounding Sgr A* is inferred to be ≈5 × 1045 ergs. Only 31% of the emitted 1-100 TeV protons encounter the circumnuclear torus, leaving a large flux of protons that diffuse outward to contribute to the Galactic ridge emission observed by HESS on scales of ≳1° © 2007. The American Astronomical Society. All rights reserved.
• Crocker, R. M., Jones, D., Ballantyne, D. R., & Melia, F. (2007). Radio synchrotron emission from secondary leptons in the vicinity of sagittarius A. Astrophysical Journal, 664(2 II), L95-L98.
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  Abstract: A pointlike source of ∼1 TeV γ-rays has recently been seen toward the Galactic center by HESS and other air Cerenkov telescopes. In recent work, we demonstrated that these γ-rays can be attributed to high-energy protons that (1) are accelerated close to the event horizon of the central black hole, Sgr A*, (2) diffuse out to approximately parsec scales, and (3) finally interact to produce γ-rays. The same hadronic collision processes will necessarily lead to the creation of electrons and positrons. Here we calculate the synchrotron emissivity of these secondary leptons in the same magnetic field configuration through which the initiating protons have been propagated in our model. We compare this emission with the observed ∼1 GHz radio spectrum of the inner few parsec region that we have assembled from archival data and new measurements that we have made with the Australia Telescope Compact Array. We find that our model predicts secondary synchrotron emission with a steep slope, consistent with the observations but with an overall normalization that is too large by a factor of ∼2. If we further constrain our theoretical γ-ray curve to obey the implicit EGRET upper limit on emission from this region, we predict radio emission that is consistent with observations; i.e., the hadronic model of gamma-ray emission can, simultaneously and without fine-tuning, also explain essentially all the diffuse radio emission detected from the inner few parsecs of the Galaxy. © 2007. The American Astronomical Society. All rights reserved.
• Crocker, R. M., Jones, D., Protheroe, R. J., Ott, J., Ekers, R., Melia, F., Stanev, T., & Green, A. (2007). The cosmic ray distribution in sagittarius B. Astrophysical Journal, 666(2 I), 934-948.
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  Abstract: The H.E.S.S. instrument has observed a diffuse flux of ∼TeV γ-rays from a large solid angle around the Galactic center (GC). This emission is correlated with the distribution of gas in the region, suggesting that the γ-rays originate in collisions between cosmic-ray hadrons (CRHs) and ambient matter. Of particular interest, H.E.S.S. has detected γ-rays from the Sagittarius (Sgr) B molecular cloud complex. Prompted by the suggestion of a hadronic origin for the γ-rays, we have examined archival 330 and 74 MHz Very Large Array radio data and 843 MHz Sydney University Molonglo Sky Survey data covering Sgr B, looking for synchrotron emission from secondary electrons and positrons (expected to be created in the same interactions that supply the observed γ-rays). Intriguingly, we have uncovered nonthermal emission, but at a level exceeding expectation. Adding to the overall picture, recent observations by the Atacama Pathfinder Experiment telescope show that the cosmic-ray ionization rate is 10 times greater in the Sgr B2 region of Sgr B than the local value. Lastly, Sgr B2 is also a very bright X-ray source. We examine scenarios for the spectra of CRHs and/or primary electrons that would reconcile all these different data. We determine that (1) a hard (∼E -2.2), high-energy (≳TeV) population of CRHs is unavoidably required by the H.E.S.S. γ-ray data, and (2) the remaining broadband, nonthermal phenomenology is explained either by a rather steep (∼E -2.9) spectrum of primary electrons or a (∼E-2.7) population of CRHs. Perhaps unsurprisingly, no single power-law population of either leptons or hadrons can explain the totality of broadband, nonthermal Sgr B phenomenology. © 2007. The American Astronomical Society. All rights reserved.
• Falanga, M., Melia, F., Tagger, M., Goldwurm, A., & Bélanger, G. (2007). General relativistic flux modulations from disk instabilities in sagittarius A*. Astrophysical Journal, 662(1 II), L15-L18.
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  Abstract: Near-IR and X-ray flares have been detected from the supermassive black hole Sgr A* at the center of our Galaxy with a (quasi-)period of ∼ 17-20 minutes, suggesting an emission region only a few Schwarzschild radii above the event horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its detailed light curve, yielding not only the highest quality period thus far, but also important structure reflecting the geometry of the emitting region. Recent MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave instability that enhances the accretion rate for several hours, possibly accounting for the observed flares. In this Letter, we carry out ray-tracing calculations in a Schwarzschild metric to determine as accurately as possible the light curve produced by general relativistic effects during such a disruption. We find that the Rossby wave-induced spiral pattern in the disk is an excellent fit to the data, implying a disk inclination angle of ≈77°. Note, however, that if this association is correct, the observed period is not due to the underlying Keplerian motion but rather to the pattern speed. The favorable comparison between the observed and simulated light curves provides important additional evidence that the flares are produced in Sgr A*'s inner disk. © 2007. The American Astronomical Society. All rights reserved.
• Melia, F. (2007). The cosmic horizon. Monthly Notices of the Royal Astronomical Society, 382(4), 1917-1921.
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  Abstract: The cosmological principle, promoting the view that the Universe is homogeneous and isotropic, is embodied within the mathematical structure of the Robertson-Walker (RW) metric. The equations derived from an application of this metric to the Einstein Field Equations describe the expansion of the Universe in terms of comoving coordinates, from which physical distances may be derived using a time-dependent expansion factor. These coordinates, however, do not explicitly reveal the properties of the cosmic space-time manifested in Birkhoff's theorem and its corollary. In this paper, we compare two forms of the metric - written in (the traditional) comoving coordinates, and a set of observer-dependent coordinates - first for the well-known de Sitter universe containing only dark energy, and then for a newly derived form of the RW metric, for a universe with dark energy and matter. We show that Rindler's event horizon - evident in the comoving system - coincides with what one might call the 'curvature horizon' appearing in the observer-dependent frame. The advantage of this dual prescription of the cosmic space-time is that with the latest Wilkinson Microwave Anisotropy Probe results, we now have a much better determination of the Universe's mass-energy content, which permits us to calculate this curvature with unprecedented accuracy. We use it here to demonstrate that our observations have probed the limit beyond which the cosmic curvature prevents any signal from having ever reached us. In the case of de Sitter, where the mass-energy density is a constant, this limit is fixed for all time. For a universe with a changing density, this horizon expands until de Sitter is reached asymptotically, and then it too ceases to change. © 2007 The Author. Journal compilation © 2007 RAS.
• Bélanger, G., Goldwurm, A., Renaud, M., Terrier, R., Melia, F., Lund, N., Paul, J., Skinner, G., & Yusef-Zadeh, F. (2006). A persistent high-energy flux from the heart of the milky way: INTEGRAL'S view of the Galactic center. Astrophysical Journal Letters, 636(1 I), 275-289.
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  Abstract: Highly sensitive imaging observations of the Galactic center (GC) at high energies with an angular resolution of order 10′ is a very recent development in the field of high-energy astrophysics. The IBIS/ISGRI imager on the INTEGRAL observatory detected for the first time a hard X-ray source, IGR J17456-2901, located within 1′ of Sagittarius A* (Sgr A*) over the energy range 20-100 keV. Here we present the results of a detailed analysis of approximately 7 × 106 s of observations of the GC obtained since the launch of INTEGRAL in 2002 October. Two years and an effective exposure of 4.7 × 106 s have allowed us to obtain more stringent positional constraints on this high-energy source and to construct its spectrum in the range 20-400 keV. Furthermore, by combining the ISGRI spectrum with the total X-ray spectrum corresponding to the same physical region around Sgr A* from XMM-Newton data collected during part of the γ-ray observations, we constructed and present the first accurate wideband high-energy spectrum for the central arcminutes of the Galaxy. Our complete and updated analysis of the emission properties of the INTEGRAL source shows that it is faint but persistent with no variability above 3 σ, contrary to what was alluded to in our first paper. This result, in conjunction with the spectral characteristics of the soft and hard X-ray emission from this region, suggests that the source is most likely not pointlike but rather that it is a compact yet diffuse nonthermal emission region. The centroid of IGR J17456-2901 is estimated to be R.A. = 17 h5m42s.5, decl. = -28°59′28″ (J2000.0), offset by 1′ from the radio position of Sgr A* and with a positional uncertainty of 1′. Its 20-400 keV luminosity at 8 kpc is L = (5.37 ± 0.21) × 1035 ergs s-1. A 3 σ upper limit on the flux at the electron-positron annihilation energy of 511 keV from the direction of Sgr A* is set at 1.9 × 10-4 photons cm-2 s-1. Very recently, the HESS collaboration presented the detection of a source of ∼TeV γ-rays also located within an arcminute of Sgr A*. We present arguments in favor of an interpretation that the photons detected by INTEGRAL and HESS arise from the same compact region of diffuse emission near the central black hole and that the supernova remnant Sgr A East could play an important role as a contributor of very high energy γ-rays to the overall spectrum from this region. There is also evidence for hard emission from a region located between the central black hole and the radio arc near / ∼ 0°.1 along the Galactic plane and known to contain giant molecular clouds. © 2006, The American Astronomical Society. All rights reserved.
• Bélanger, G., Terrier, R., Jager, O. D., Goldwurm, A., & Melia, F. (2006). Periodic modulations in an X-ray flare from sagittarius A*. Journal of Physics: Conference Series, 54(1), 420-426.
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  Abstract: We present the highly significant detection of a quasi-periodic fbx modulation with a period of 22.2min seen in the X-ray data of the Sgr A* flare of 2004 August 31. This flaring event, which lasted a total of about three hours, was detected simultaneously by EPIC on XMM-Newton and the NICMOS near-infrared camera on the HST. Given the inherent difficulty in, and the lack of readily available methods for quantifying the probability of a periodic signal detected over only several cycles in a data set where red noise can be important, we developed a general method for quantifying the likelihood that such a modulation is indeed intrinsic to the source and does not arise from background fluctuations. We here describe this Monte Carlo based method, and discuss the results obtained by its application to other XMM-Newton data sets. Under the simplest hypothesis that we witnessed a transient event that evolved, peaked and decayed near the marginally stable orbit of the supermassive black hole, this result implies that for a mass of 3.5 × 106 M, the central object must have an angular momentum corresponding to a spin parameter of a 0.22. © 2006 IOP Publishing Ltd.
• Fatuzzo, M., Adams, F. C., & Melia, F. (2006). Enhanced cosmic-ray flux and ionization for star formation in molecular clouds interacting with supernova remnants. Astrophysical Journal, 653(1 II), L49-L52.
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  Abstract: Molecular clouds interacting with supernova remnants may be subject to greatly enhanced irradiation by cosmic rays produced at the shocked interface between the ejecta and the molecular gas. Over the past decade, broadband observations have provided important clues about these relativistic particles, indicating that they may dominate over the locally observed cosmic-ray population by a significant amount. In this Letter, we estimate the enhancement and find that the cosmic-ray energy density can be up to ∼1000 times larger in the molecular cloud than in the field. This enhancement can last for a few million years and leads to a corresponding increase in the ionization fraction, which has important consequences for star formation. Ionization fractions in molecular cloud cores determine, in part, the rate of ambipolar diffusion, an important process in core formation and precollapse evolution. Ionization fractions in newly formed circumstellar disks affect the magnetorotational instability mechanism, which in turn affects the rate of disk accretion. As estimated here, the increased ionization acts to increase the ambipolar diffusion time by a factor of ∼30 and thereby suppresses star formation. In contrast, the increased ionization fraction reduces the sizes of dead zones in accretion disks (by up to an order of magnitude) and thus increases disk accretion rates (by a comparable factor). © 2006. The American Astronomical Society. All rights reserved.
• Fryer, C. L., Rockefeller, G., Hungerford, A., & Melia, F. (2006). The Sgr B2 X-ray echo of the galactic center supernova explosion that produced Sgr A east. Astrophysical Journal Letters, 638(2 I), 786-796.
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  Abstract: Using a combination of one-dimensional and three-dimensional hydrodynamic simulations, we have carried out the first in-depth analysis of the remnant's evolution and its various interactions: with the stellar winds flowing out from the inner ∼2 pc; with the supermassive black hole, Sgr A*; and with the 50 km s-1 molecular cloud behind and to the east of the nucleus. We have found that, unlike previous estimates, a rather "standard" supernova explosion with energy ∼1.5 × 1051 ergs would have been sufficient to create the remnant we see today and that the latter is probably only ∼1700 yr old. We have found that the passage of the remnant across the black hole would have enhanced the accretion rate onto the central object by less than a factor of 2. Such a small increase cannot explain the current Fe fluorescence observed from the molecular cloud Sgr B2; this fluorescence would have required an increase in the luminosity of Sgr A* by 6 orders of magnitude several hundred years ago. Instead, we have uncovered what appears to be a more plausible scenario for this transient irradiation: the interaction between the expanding remnant and the 50 km s-1 molecular cloud. The first impact would have occurred about 1200 yr after the explosion, producing a 2-200 keV luminosity of ∼1039 ergs s -1. During the intervening 300-400 yr, the dissipation of kinetic energy subsided considerably, leading to the much lower luminosity (∼10 36 ergs s-1 at 2-10 keV) we see today. © 2006. The American Astronomical Society. All rights reserved.
• Liu, S., Melia, F., & Petrosian, V. (2006). Stochastic electron acceleration during the near-infrared and X-ray flares in Sagittarius A. Astrophysical Journal Letters, 636(2 I), 798-803.
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  Abstract: Recent near-IR (NIR) and X-ray observations of Sagittarius A*'s spectrum have yielded several strong constraints on the transient energizing mechanism, justifying a reexamination of the stochastic acceleration model proposed previously for these events. We here demonstrate that the new results are fully consistent with the acceleration of electrons via the transit-time damping process. But more importantly, these new NIR and X-ray flares now can constrain the source size, the gas density, the magnetic field, and the wave energy density in the turbulent plasma. Future simultaneous multiwavelength observations with good spectral information will, in addition, allow us to study their temporal evolution, which will eventually lead to an accurate determination of the behavior of the plasma just minutes prior to its absorption by the black hole. © 2006. The American Astronomical Society. All rights reserved.
• Liu, S., Melia, F., Petrosian, V., & Fatuzzo, M. (2006). Stochastic acceleration in the galactic center hess source. Astrophysical Journal Letters, 647(2 I), 1099-1105.
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  Abstract: Stochastic acceleration of electrons interacting resonantly with a turbulent magnetic field in a small accretion torus appears to be the likely mechanism responsible for much of Sagittarius A*'s millimeter and shorter wavelength spectrum. The longer wavelength radiation is produced at larger radii by electrons either diffusing from smaller scales or accelerated in situ. An important prediction of this model is the ejection of a significant flux of relativistic protons from a magnetic-field-dominated acceleration site into the wind-shocked medium surrounding the black hole. Recently, several air Cerenkov telescopes, notably HESS, have detected TeV emission from the Galactic center, with characteristics hinting at p-p-induced pion decay process for the γ-ray emission. Given (1) the size of this acceleration region measured in the radio band and (2) the wind-injected ISM mapped with Chandra using the diffuse X-rays, it is feasible to test the idea that protons accelerated within ∼20 Schwarzschild radii of the black hole produce the TeV emission farther out. We show a fraction of TeV protons scattering about once within ∼3 pc of Sagittarius A* and the proton power (∼1037 ergs s -1) produced in concert with the 7 mm radio emission matches the TeV luminosity well. This model explains why the TeV source does not vary on a timescale of a year or less. The particle cascade generated by the p-p scatterings also produces bremsstrahlung, inverse Compton, and synchrotron emission at longer wavelengths from secondary particles. We compare these with current measurements and demonstrate that GLAST will detect this source during its one-year all-sky survey. © 2006. The American Astronomical Society. All rights reserved.
• Liu, S., Petrosian, V., Melia, F., & Fryer, C. L. (2006). A testable stochastic acceleration model for flares in sagittarius A*. Astrophysical Journal Letters, 648(2 I), 1020-1025.
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  Abstract: The near-IR and X-ray flares in Sagittarius A* are believed to be produced by relativistic electrons via synchrotron and synchrotron self-Comptonization, respectively. These electrons are likely energized by turbulent plasma waves through second-order Fermi acceleration that, in combination with the radiative cooling processes, produces a relativistic Maxwellian distribution in the steady state. This model has four principal parameters, namely the magnetic field B, the electron density n and temperature γcmec2, and the size of the flare region R. In the context of stochastic acceleration, the quantities Rn1/2B and γcRn should remain nearly constant in time. Therefore, simultaneous spectroscopic observations in the NIR and X-ray bands can readily test the model, which, if proven to be valid, may be used to determine the evolution of the plasma properties during an eruptive event with spectroscopic observations in either band or simultaneous flux density measurements in both bands. The formulae can be applied to other isolated or confined systems, where electrons are accelerated to relativistic energies by plasma wave turbulence and produce most of the emission via synchrotron processes.
• Melia, F., Fatuzzo, M., & Crocker, R. M. (2006). High-Energy Activity in the Unusually Soft TeV Source HESS J1804-216 toward the Galactic Center. arXiv: Astrophysics, 20pp. doi:10.48550/arXiv.astro-ph/0602330
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  In recent years, apparent anisotropies in the ~EeV cosmic ray (CR) flux arriving at Earth from the general direction of the galactic center have been reported from the analysis of AGASA and SUGAR data. The more recently commissioned Auger Observatory has not confirmed these results. HESS has now detected an unusually soft TeV source roughly coincident with the location of the previously claimed CR anisotropy. In this paper, we develop a model for the TeV emission from this object, consistent with observations at other wavelengths, and examine the circumstances under which it might have contributed to the $\sim$ EeV cosmic ray spectrum. We find that the supernova remnant G8.7-0.1 can plausibly account for all the known radiative characteristics of HESS J1804-216, but that it can accelerate cosmic rays only up to an energy $\sim 10^5$ GeV. On the other hand, the pulsar (PSR J1803-2137) embedded within this remnant can in principle inject EeV protons into the surrounding medium, but it cannot account for the broadband spectrum of HESS J1804-216. We therefore conclude that although G8.7-0.1 is probably the source of TeV photons originating from this direction, there is no compelling theoretical motivation for expecting a cosmic ray anisotropy at this location. However, if G8.7-0.1 is indeed correctly identified with HESS J1804-216, it should also produce a $\sim$ GeV flux detectable in a one-year all sky survey by GLAST.
• Tagger, M., & Melia, F. (2006). A possible rossby wave instability origin for the flares in sagittarius A. Astrophysical Journal Letters, 636(1 II), L33-L36.
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  Abstract: In recent years, near-IR and X-ray flares have been detected from the Galaxy's central radio point source, Sagittarius A* (Sgr A*), believed to be an ∼3 × 106 M⊙ supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-)periodic oscillation (QPO) of ∼ 17-20 minutes. The implied ∼3rs size of the emitter (where rs ≡ 2GM/c 2 is the Schwarzschild radius) points to an instability - possibly induced by accretion - near the marginally stable orbit (MSO) of a slowly spinning object. But Sgr A* is not accreting via a large, "standard" disk; instead, the low-density environment surrounding it apparently feeds the black hole with low angular momentum clumps of plasma that circularize within ∼(10-300)rs and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and we show that a Rossby wave instability, particularly in its magnetohydrodynamic (MHD) form, grows rapidly and produces a period of enhanced accretion lasting several hours. Both the amplitude of this response and its duration match the observed flare characteristics rather well. © 2006. The American Astronomical Society. All rights reserved.
• Wolfe, B., & Melia, F. (2006). Covariant kinetic theory with an application to the Coma cluster. Astrophysical Journal Letters, 638(1 I), 125-137.
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  Abstract: In this paper, we introduce a novel solution to the covariant Landau equation for a pure electron plasma. The method conserves energy and particle number and reduces smoothly to the Rosenbluth potentials of nonrelativistic theory. We find that the covariant relaxation timescale agrees with past results; however, as a full solution to the covariant Boltzmann equation, our technique allows for anisotropic or degenerate distributions, which makes it unique. To demonstrate the power of our solution in dealing with hot, astrophysical plasmas, we use this technique to show that one of the currently considered models, continuous stochastic acceleration of thermal electrons, for the hard X-ray emission in the Coma Cluster actually cannot work because the energy gained by the particles is distributed to the whole plasma on a timescale much shorter than that of the acceleration process itself. © 2006. The American Astronomical Society. All rights reserved.
• Wolfe, B., & Melia, F. (2006). Transrelativistic synchrotron emissivity, cross section, and polarization. Astrophysical Journal Letters, 637(1 I), 313-321.
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  Abstract: The spectrum and polarization produced by particles spiraling in a magnetic field undergo dramatic changes as the emitters transition from nonrelativistic to relativistic energies. However, none of the currently available methods for calculating the characteristics of this radiation field are adequate for the purpose of sustaining accuracy and speed of computation in the intensity, and none even attempt to provide a means of determining the polarization fraction other than in the cyclotron or synchrotron limits. But the transrelativistic regime, which we here find to lie between 5 × 107 and 5 × 109 K for a thermal plasma, is becoming increasingly important in high-energy astrophysical environments, such as in the intracluster medium, and in the accretion flows of supermassive black holes. In this paper, we present simple, yet highly accurate fitting formulae for the magnetobremsstrahlung (also known as cyclosynchrotron) emissivity, its polarization fraction (and content), and the absorption cross section. We demonstrate that both the harmonic and high-energy limiting behavior are well represented, incurring at most an error of ∼5% throughout the transition region. © 2006. The American Astronomical Society. All rights reserved.
• Bélanger, G., Goldwurm, A., Melia, F., Ferrando, P., Grosso, N., Porquet, D., Warwick, R., & Yusef-Zadeh, F. (2005). Repeated X-ray flaring activity in Sagittarius A. Astrophysical Journal Letters, 635(2 I), 1095-1102.
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  Abstract: Investigating the spectral and temporal characteristics of the X-rays coming from Sagittarius A* (Sgr A*) is essential to our development of a more complete understanding of the emission mechanisms in this supermassive black hole located at the center of our Galaxy. Several X-ray flares with varying durations and spectral features have already been observed from this object. Here we present the results of two long XMM-Newton observations of the Galactic nucleus carried out in 2004, for a total exposure time of nearly 500 ks. During these observations we detected two flares from Sgr A* with peak 2-10 keV luminosities about 40 times (LX ∼ 9 × 1034 ergs s-1) above the quiescent luminosity: one on 2004 March 31 and another on 2004 August 31. The total duration of the first flare was about 4 ks. The second, simultaneously detected in the near-IR, lasted about 10 ks. The combined fit to the EPIC spectra yields photon indices of about 1.5 and 1.9 for the first and second flares, respectively. This hard photon index strongly suggests the presence of an important population of nonthermal electrons during the event and supports the view that the majority of flaring events tend to be hard and not very luminous. © 2005. The American Astronomical Society. All rights reserved.
• Crocker, R. M., Fatuzzo, M., Jokipii, J. R., Melia, F., & Volkas, R. R. (2005). The AGASA and sugar anisotropies and TeV gamma rays from the Galactic center: A possible signature of extremely high energy neutrons. Astrophysical Journal Letters, 622(2 I), 892-909.
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  Abstract: Recent analysis of data sets from two extensive air shower cosmic-ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays toward the Galactic center region that "rums on" around 10 18 eV. We demonstrate that the anisotropy could be due to neutrons created in the Galactic center region through charge exchange in proton-proton collisions, where the incident, high-energy protons obey a ∼E-2 power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 (ascribed to p-p-induced neutral pion decay at GeV energies), together with a very reasonable spectral index of 2.2, predicts a neutron flux at ∼1018 eV fully consistent with the extremely high energy cosmic-ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air Cerenkov telescope at ∼TeV energies is almost equally compatible with the ∼1018 eV cosmic-ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We find a plausible resolution of this discrepancy in an effective two-source model. Finally, we argue that the shock acceleration is probably occurring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic center. In support of this contention we note that (1) the extended shell of this object could provide both of the sources suggested by the gamma-ray data and (2) the unusually strong magnetic field at this remnant, together with a perpendicular shock geometry, allow for acceleration of protons up to the extreme energies required to explain the cosmic-ray anisotropy. If the connection between the anisotropy and Sagittarius A East could be firmly established, it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near 1019 eV. © 2005. The American Astronomical Society. All rights reserved.
• Crocker, R. M., Melia, F., & Volkas, R. R. (2005). Neutrinos from the galactic center in the light of ITS gamma-ray detection at TeV energy. Astrophysical Journal Letters, 622(1 II), L37-L40.
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  Abstract: We reevaluate the event rate expected in km3-scale detectors for neutrinos from the direction of the Galactic center (GC) in light of recent spectral measurements obtained by the HESS instrument for ∼TeV γ-radiation from this direction. In the most plausible scenario the reevaluated event rate is smaller than that previously calculated - and here recalculated - on the basis of EGRET data. However, the GC TeV γ-ray detections by the Whipple, CANGAROO, and HESS instruments, together with the strong indications for an overabundance of cosmic rays coming from the GC at EeV energies, strengthen the expectation for a detectable, TeV-PeV GC neutrino signal from proton-proton interactions in that region. If the TeV γ-ray-EeV cosmic-ray anisotropy connection is correct, this signal will be detectable within a year and half for km3-scale neutrino detectors in the Northern Hemisphere at super-TeV energies and, significantly, should also be detectable in 1.6 yr by the South Polar IceCube detector at energies ≥1014 eV. The GC neutrino signal should also produce a detectable signal from neutrino showering and resonant W- production by v̄e in the volume of a km3-scale detector. © 2005. The American Astronomical Society. All rights reserved.
• Fatuzzo, M., & Melia, F. (2005). Primary versus secondary leptons in the EGRET supernova remnants. Astrophysical Journal Letters, 630(1 I), 321-331.
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  Abstract: The EGRET supernova remnants (SNRs) are all expanding into nearby dense molecular clouds, powering a shock at the interface where protons and electrons accelerate to relativistic energies. A viable mechanism for the emission of γ-rays in these sources is the decay of neutral pions created in collisions between the relativistic hadrons and protons in the ambient medium. But neutral pion decay alone cannot reproduce the whole high-energy spectrum, particularly below 100 MeV. A pion decay scenario thus requires a lepton population to fill in the lower part of the spectrum via bremsstrahlung emission. This population, however, is constrained by the SNR radio spectrum. Taking our cue from the behavior of Sgr A East, an energetic EGRET SNR at the Galactic center, we here examine the role played in these sources by secondary leptons - electrons and positrons produced in proton-proton scattering events and the ensuing particle cascades. We show that, while secondary leptons cannot account for the γ-rays below 100 MeV, they can account for the hard radio spectra observed from the EGRET SNRs. Thus, it appears that both primary and secondary leptons may be important contributors to the overall broadband emission from these sources, but if so, they must radiate most of their energy in different parts of the SNR-cloud environment. We show that shock acceleration in dense cores being overtaken by the expanding SNR shell can naturally lead to such a scenario. © 2005. The American Astronomical Society. All rights reserved.
• Melia, F. (2005). Astronomy: Odd company. Nature, 437(7062), 1105-.
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  PMID: 16237432;Abstract: Black holes cannot yet be seen directly, but their influence on surrounding stars is allowing them to be identified with increasing certainty. That those stars are there to be influenced, though, raises other questions. ©2005 Nature Publishing Group.
• Prescher, M., & Melia, F. (2005). Long-term radio modulation in sagittarius A* from spin-induced disk precession. Astrophysical Journal Letters, 632(2 I), 1048-1053.
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  Abstract: There is some evidence, although yet unconfirmed, that Sagittarius A*- the supermassive black hole at the Galactic center - emits its radio waves modulated with a ∼100 day period. What is intriguing about this apparent quasi periodicity is that although the amplitude of the modulation increases with decreasing wavelength (from 3.6 to 1.3 cm), the quasi period itself does not seem to depend on the frequency of the radiation. It is difficult to imagine how a binary companion, were that the cause of this modulation, could have escaped detection until now. Instead, it has been suggested that the spin-induced precession of a disk surrounding a slowly rotating black hole could have the right period to account for this behavior. In this paper, we examine how Sgr A*'s light curve could be modulated by this mechanism. We demonstrate that the partial occultation of a nonthermal halo by a compact, radio-opaque disk does indeed produce the observed frequency-dependent amplitude. This appears to be in line with other observational arguments suggesting that Sgr A*'s millimeter/submillimeter spectrum is produced by a ∼ 10 Schwarzschild radius disk, whereas its centimeter waves originate from a nonthermal particle distribution in a halo extending out to over 20 Schwarzschild radii. Interestingly, this model suggests that the observed period corresponds to half the precession period and that a nonaxisymmetric disk could produce a second period roughly twice as long as the first. © 2005. The American Astronomical Society. All rights reserved.
• Rockefeller, G., Fryer, C. L., & Melia, F. (2005). Spin-induced disk precession in sagittarius A. Astrophysical Journal Letters, 635(1 I), 336-340.
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  Abstract: In Sgr A* at the Galactic center, by far the closest and easiest supermassive black hole we can study, the observational evidence is increasingly pointing to the presence of a compact, hot, magnetized disk feeding the accretor. In such low Mach number plasmas, forces arising, e.g., from pressure gradients in the plasma, can altogether negate the warping of disks around Kerr black holes caused by the Bardeen-Petterson effect and can lead to coherent precession of the entire disk. In this paper, we present for the first time highly detailed three-dimensional smoothed particle hydrodynamics (SPH) simulations of the accretion disk evolution in Sgr A*, guided by observational constraints on its physical characteristics, and conclude that indeed the Bardeen-Petterson effect is probably absent in this source. Given what we now understand regarding the emission geometry in this object, we suggest that a ∼50-500 days modulation in Sgr A*'s spectrum, arising from the disk precession, could be an important observational signature; perhaps the ∼ 106 days period seen earlier in its radio flux, if confirmed, could be due to this process. On the other hand, if future observations do not confirm this long modulation in Sgr A*'s spectrum, this would be an indication either that the disk size or orientation is very different from current estimates, that the black hole is not spinning at all (unlikely), or that our current understanding of how it produces its radiative output is incorrect. © 2005. The American Astronomical Society. All rights reserved.
• Rockefeller, G., Fryer, C. L., Baganoff, F. K., & Melia, F. (2005). The X-ray ridge surrounding sagittarius a* at the galactic center. Astrophysical Journal Letters, 635(2 II), L141-L144.
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  Abstract: We present the first detailed simulation of the interaction between the supernova explosion that produced Sgr A East and the wind-swept inner ∼2 pc region at the Galactic center. The passage of the supernova ejecta through this medium produces an X-ray ridge ∼9″-15′ to the northeast of the supermassive black hole Sagittarius A* (Sgr A*). We show that the morphology and X-ray intensity of this feature match very well with recently obtained Chandra images, and we infer a supernova remnant age of less than 2000 yr. This young age - a factor of 3-4 lower than previous estimates - arises from our inclusion of stellar wind effects in the initial (preexplosion) conditions in the medium. The supernova does not clear out the central ∼0.2 pc region around Sgr A* and does not significantly alter the gas profile around the central black hole upon passage through the Galactic center. © 2005. The American Astronomical Society. All rights reserved.
• Rockefeller, G., Fryer, C. L., Melia, F., & Wang, Q. D. (2005). Diffuse X-rays from the Arches and Quintuplet clusters. Astrophysical Journal Letters, 623(1 I), 171-180.
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  Abstract: The origin and initial mass function of young stellar clusters near the Galactic center are still poorly understood. Two of the more prominent ones, the Arches and Quintuplet clusters, may have formed from a shock-induced burst of star formation, given their similar age and proximity to each other. Their unusual mass distribution, however, may be evidence of a contributing role played by other factors, such as stellar capture from regions outside the clusters themselves. Diffuse X-ray emission from these sources provides us with a valuable, albeit indirect, measure of the stellar mass-loss rate from their constituents. Using recent data acquired with Chandra, we can study the nature and properties of the outflow to not only probe the pertinent physical conditions, such as high metallicity, the magnetic field, and so forth, but also to better constrain the stellar distribution within the clusters, in order to identify their formative history. In this paper, we present a set of three-dimensional smoothed particle hydrodynamics simulations of the wind-wind interactions in both the Arches and Quintuplet clusters. We are guided primarily by the currently known properties of the constituent stars, though we vary the mass-loss rates in order to ascertain the dependence of the measured X-ray flux on the assumed stellar characteristics. Our results are compared with the latest observations of the Arches cluster. Our analysis of the Quintuplet cluster may be used as a basis for comparison with future X-ray observations of this source. © 2005. The American Astronomical Society. All rights reserved.
• Bélanger, G., Goldwurm, A., Goldoni, P., Paul, J., Terrier, R., Falanga, M., Ubertini, P., Bazzano, A., Santo, M. D., Winkler, C., Parmar, A. N., Kuulkers, E., Ebisawa, K., Roques, J. P., Lund, N., & Melia, F. (2004). Detection of hard X-ray emission from the Galactic nuclear region with INTEGRAL. Astrophysical Journal Letters, 601(2 II), L163-L166.
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  Abstract: This Letter presents the first results of an observational campaign to study the Galactic center with INTEGRAL, the International Gamma-Ray Astrophysics Laboratory. Mosaicked images were constructed using data obtained with ISGRI, the soft gamma-ray instrument of the coded aperture IBIS imager, in the energy ranges 20-40 and 40-100 keV. These give a yet unseen view of the high-energy sources of this region in hard X-rays and gamma rays with an angular resolution of 12′ (FWHM). We report on the discovery of a source, IGR J1745.6-2901, coincident with the Galactic nucleus Sgr A* to within 0′.9. Located at R.A. = 17h45m38s.5 (J2000.0), decl. = -29°01′15″ (J2000.0), the source is visible up to about 100 keV with a 20-100 keV luminosity at 8 kpc of (2.89 ± 0.41) × 1035 ergs s-1. Although the new INTEGRAL source cannot unequivocally be associated to the Galactic nucleus, this is the first report of significant hard X-ray emission from within the inner 10′ of the Galaxy and a contribution from the Galactic supermassive black hole itself cannot be excluded.
• Goldwurm, A., Bélanger, G., Goldoni, P., Paul, J., Terrier, R., Falanga, M., Ubertini, P., Bazzano, A., Santo, M. D., Winkler, C., Parmar, A. N., Kuulkers, E., Ebisawa, K., Roques, J. P., Skinner, G., Lund, N., Melia, F., & Yusef-Zadeh, F. (2004). The integral view of the galactic nucleus. European Space Agency, (Special Publication) ESA SP, 237-242.
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  Abstract: We present the preliminary results of the observational campaign performed in 2003 to study the Galactic Nucleus with INTEGRAL. The mosaicked images obtained with the IBIS/ISGRI coded aperture instrument in the energy range above 20 keV, give a yet unseen view of the high-energy sources of this region in hard X- and gamma-rays, with an angular resolution of 12′. We report on the discovery of a source, IGR J17456-2901, compatible with the instrument's point spread function and coincident with the Galactic Nucleus Sgr A*to within 0.9′. The source is visible up to 60-80 keV with a 20-100 keV luminosity at 8 kpc of 3 × 1035 erg s-1. Although we cannot unequivocally associate the new INTEGRAL source to the Galactic Nucleus, this is the first report of significant hard X-ray emission from within the inner 10′ of the Galaxy and a contribution from the galactic center supermassive black hole itself cannot be excluded. Here we discuss the results obtained and the perspectives for future observations of the Galactic Nucleus with INTEGRAL and other observatories.
• Liu, S., Petrosian, V., & Melia, F. (2004). Electron acceleration around the supermassive black hole at the galactic center. Astrophysical Journal Letters, 611(2 II), L101-L104.
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  Abstract: The recent detection of variable infrared emission from Sagittarius A* combined with its previously observed flare activity in X-rays provide compelling evidence that at least a portion of this object's emission is produced by nonthermal electrons. The polarization and variability of Sgr A*'s emission depend strongly on the observed wavelength, indicating distinct physical processes. We show here that acceleration of electrons by plasma wave turbulence in hot gases near the black hole's event horizon can, although with several theoretical uncertainties, reasonably account both for Sgr A*'s millimeter and shorter wavelength emission in the quiescent state and for the infrared and X-ray flares, induced either via an enhancement of the mass accretion rate onto the black hole or by reorganization of magnetic fields coupled to the accretion gas. High-energy electrons diffusing away from the acceleration site toward larger radii might account for Sgr A*'s emission at longer wavelengths. The acceleration model produces prominent IR flares accompanying X-ray bursts. Future coordinated multiwavelength observations will be able to test this model and constrain its parameters.
• Melia, F. (2004). Closing in on black holes. Physics World, 17(1), 20-.
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  Abstract: Observations using the Very Large Telescope (VLT) in Chile reveals new discoveries associated with the bright radio source known as Sagittarius A* located in the constellation Sagittarius. Findings show that the infrared light produced by Sagittarius A* during an explosion is actually modulated and fluctuates roughly every 17 minutes. Results through computer simulation suggest that the light was emitted by hot, compressed gas orbiting just above the event horizon of a supermassive black hole. These findings corroborate earlier data that Sagittarius A* is a supermassive black hole in space.
• O'brien, R., & Melia, F. (2004). Book review: The edge of infinity - supermassive black holes in the Universe (Melia). Journal of the British Astronomical Association, 114, 37-38.
• Rockefeller, G., Fryer, C. L., Melia, F., & Warren, M. S. (2004). Diffuse X-rays from the inner 3 parsecs of the galaxy. Astrophysical Journal Letters, 604(2 I), 662-670.
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  Abstract: Recent observations with the Chandra X-Ray Observatory have provided us with the capability to discriminate point sources, such as the supermassive black hole Sgr A*, from the diffuse emission within the inner 10″ of the Galaxy. The hot plasma producing the diffuse X-radiation, estimated at ≈7.6 × 1031 ergs s-1 arcsec-2 in the 2-10 keV band, has an rms electron density ≈26 cm-3 and a temperature kT ≈ 1.3 keV, with a total inferred mass of ≈0.1 M⊙. At least some of this gas must be injected into the interstellar medium via stellar winds. In the most recent census, about 25 bright, young stars have been identified as the dominant sources of the overall mass efflux from the Galactic center. In this paper we use detailed three-dimensional smoothed particle hydrodynamics (SPH) simulations to study the wind-wind interactions occurring in the inner 3 pc of the Galaxy, with a goal of understanding what fraction, if any, of the diffuse X-ray flux measured by Chandra results from the ensuing shock heating of the ambient medium. We conclude that this process alone can account for the entire X-ray flux observed by Chandra in the inner 10″ of the Galaxy. Understanding the X-ray morphology of the environment surrounding Sgr A* will ultimately provide us with a greater precision in modeling the accretion of gas onto this object, which appears to be relatively underluminous compared to its brethren in the nuclei of other galaxies.
• Fatuzzo, M., & Melia, F. (2003). A kinship between the EGRET supernova remnants and sagittarius a East. Astrophysical Journal Letters, 596(2 I), 1035-1043.
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  Abstract: Sgr A East appears to be a single, mixed-morphology 10,000 yr old supernova remnant (SNR) at the Galactic center. It also appears to belong to a class of remnants that have been observed and detected at 1720 MHz, the transition frequency of OH maser emission. However, if the EGRET source 3EG J1746-2852, coincident with the Galactic center, is itself associated with this object, it would endow it with a γ-ray luminosity almost 2 orders of magnitude greater than that of the other EGRET-detected SNRs. We here reconsider the viability of a pion-production mechanism as the source of the broadband emission observed from Sgr A East, and show that what connects these objects - and ultimately also accounts for their different γ-ray emissivity - is the very important interaction between the expanding SNR shell and the surrounding molecular cloud environment. The singularly high γ-ray luminosity of Sgr A East, as well as its unusually steep radio spectral index, can thereby be attributed to the high-density (nH = 103 cm-3), strongly magnetized (B ∼ 0.18 mG) environment in which it is located.
• Liu, S., Fromerth, M. J., & Melia, F. (2003). Shadowing of the nascent jet in NGC 4261 by a line-emitting supersonic accretion disk. Astrophysical Journal Letters, 596(2 I), 879-888.
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  Abstract: NGC 4261 (3C 270) is a low-luminosity radio galaxy with two symmetric kiloparsec-scale jets. Earlier Hubble Space Telescope observations indicated the presence of a 100 pc scale disk of cool dust and gas surrounding a central, supermassive (∼4.9 × 108 M⊙) black hole. The recent detection of free-free radio absorption by a small, geometrically thin disk, combined with earlier studies of the disk's large-scale properties, provides the strictest constraints to date on the nature of the accretion process in this system. We show here that a supersonic disk, illuminated by the active galactic nucleus (AGN), not only can account for the observed radio shadowing but can also produce the optical broad lines emitted from this region. At large radii, the gas is optically thin because the ram pressure due to turbulence is much larger than the thermal pressure of the gas. At smaller radii, but beyond a critical radius rc, line cooling dominates over gravitational dissipation and the gas is effectively cooled down to temperatures below 104 K. Within rc, however, heating due to the release of gravitational energy over-whelms line cooling and the gas falls onto the unstable portion of the cooling curve. Because cooling is quite inefficient under these conditions, the plasma is heated very quickly to a temperature close to its virial value as it falls toward the central engine. Thermal pressure of the gas dominates the turbulent ram pressure at a radius ∼2/3rc, below which the flow probably becomes advection dominated. The disk is optically thin to UV and X-ray radiation within r c, so the ionizing radiation from the AGN is preferentially absorbed near rc, affecting the disk structure significantly. To include the ensuing photoionization effect, we have used the algorithm CLOUDY with additional heating introduced by gravitational dissipation to calculate the temperature profile and line emission from the disk in a self-consistent manner. The results of our model calculation are consistent with current multiwavelength observations of the disk in this source.
• Liu, S., Liu, S., Melia, F., & Melia, F. (2003). A Relativistic Disk in Sagittarius A*. Astronomische Nachrichten, 324(SUPPL.1), 475-481.
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  Abstract: 10.1002/asna.200385045.abs The detection of a mm/Sub-mm "bump" in Sgr AA*'s radio spectrum suggests that at least a portion of its overall emission is produced within a compact accretion disk. This inference is strengthened by observations of strong linear polarization (at the 10 percent level) within this bump. No linear polarization has been detected yet at other wavelengths. Given that radiation from this source is produced on progressively smaller spatial scales with increasing frequency, the mm/Sub-mm bump apparently arises within a mere handful of Schwarzschild radii of the black hole. We have found that a small (10-Schwarzschild-radii) magnetized accretion disk can not only account for the spectral bump via thermal synchrotron processes, but that it can also reproduce the corresponding polarimetric results. In addition, the quiescent X-ray emission appears to be associated with synchrotron self-Comptonization, while X-ray flares detected from Sgr A* may be induced by a sudden enhancement of accretion through this disk. The hardening of the flare-state X-ray spectrum appears to favor thermal bremsstrahlung as the dominant X-ray emission mechanism during the transient event. This picture predicts correlations among the mm, IR, and X-ray flux densities, that appear to be consistent with recent multi-wavelength observations. Further evidence for such a disk in Sgr A* is provided by its radio variability. Recent monitoring of Sgr A* at cm and mm wavelengths suggests that a spectral break is manifested at 3 mm during cm/Sub-mm flares. The flat cm spectrum, combined with a weak X-ray flux in the quiescent state, rules out models in which the radio emission is produced by thermal synchrotron process in a bounded plasma. One possibility is that nonthermal particles may be produced when the large scale quasi-spherical inflow circularizes and settles down into the small accretion disk. Dissipation of kinetic energy associated with radial motion may lead to particle acceleration in shocks or via magnetic reconnection. On the other hand, the identification of a 106-day cycle in Sgr A*'s radio variability may signal a precession of the disk around a spinning black hole. The disk's characteristics imply rigid-body rotation, so the long precession period is indicative of a small black-hole spin with a spin parameter a/M around 0.1. It is interesting to note that such a small value of a/M would be favored if the nonthermal portion of Sgr A*'s spectrum is powered by a Blandford- Znajek type of process; in this situation, the observed luminosity would correspond to an outer disk radius of about 30 Schwarzschild radii. This disk structure is consistent with earlier hydrodynamical and recent MHD simulations and is implied by Sgr A*'s mm/Sub-mm spectral and polarimetric characteristics. For the disk to precess with such a long (106-day) period, the angular momentum flux flowing through it must be sufficiently small that any modulation of the total angular momentum is mostly due to its coupling with the black-hole spin. This requires that the torque exerted on the inner boundary of the disk via magnetic stresses is close to the angular momentum accretion rate associated with the infalling gas. Significant heating at the inner edge of the disk then leaves the gas marginally bounded near the black hole. A strong wind from the central region may ensue and produce a scaled down version of relativistic (possibly magnetized) jets in AGNs. Copyright © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Melia, F. (2003). Peering at the edge of time. Natural History, 112(5), 52-57.
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  Abstract: In search of the black hole at the center of out galaxy.
• Pessah, M. E., & Melia, F. (2003). On the Chandra Detection of Diffuse X-Ray Emission from Sgr A*. Astronomische Nachrichten, 324(SUPPL.1), 467-473.
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  Abstract: Kinematic studies of the stellar motions near Sgr A* have revealed the presence of several million solar masses of dark matter enclosed within 0.015 parsecs of the Galactic Center. However, it is not yet clear what fraction of this material is contained within a single point-like object, as opposed to an extended distribution of orbiting matter (e.g., in the form of neutron stars). Recent Chandra observations suggest that the X-ray emission from this source is partially diffuse. This result provides an important clue that can be used to set some constraints on the mass distribution surrounding the black hole. Here, we develop a simple model in which the diffuse emission is produced by a halo of neutron stars accreting from the gas falling toward the center. We discuss the various accretion mechanisms that are likely to contribute significantly to the X-ray flux, and show that a highly magnetized fraction of old neutron stars may account for the diffuse high-energy source. If this picture is correct, the upper bound to the mass of the central black hole is ≈2.2 × 106 M⊙. The core radius of the dark cluster must then be ≈0.06 pc. We also discuss the sensitivity of our results to the various assumptions made in our calculations. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Pessah, M., & Melia, F. (2003). Diffuse X-rays from a distributed component of dark matter surrounding sagittarius A. Astrophysical Journal Letters, 585(1 II), L29-L32.
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  Abstract: There is now compelling evidence for the presence of a few million solar masses of dark matter within 0.015 pc of the nonthermal radio source Sgr A&z.ast;. Recent Chandra observations suggest that the X-ray emission from this source is partially diffuse. This result provides an important clue that can be used to set some constraints on the mass distribution surrounding the black hole, and we here develop a simple model in which the diffuse emission is produced by a halo of neutron stars accreting from the gas falling toward the center. We discuss the various accretion mechanisms that are likely to contribute significantly to the X-ray flux and show that a highly magnetized fraction of old neutron stars may account for the diffuse high-energy source. If this picture is correct, the upper bound to the mass of the central black hole is ≈2.2 × 106 M⊙. The core radius of the dark cluster must then be ≈0.06 pc. We also discuss the sensitivity of our results to the various assumptions made in our calculations.
• Crocker, R. M., Melia, F., & Volkas, R. R. (2002). Searching for long-wavelength neutrino oscillations in the distorted neutrino spectrum of galactic supernova remnants. Astrophysical Journal, Supplement Series, 141(1), 147-155.
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  Abstract: We investigate the muon neutrino event rate in km3 neutrino telescopes due to a number of galactic supernova remnants (SNRs) expected on the basis of these objects' known γ-ray signals. We evaluate the potential of such instruments to detect breaks in the expected power-law behavior of these SNRs' neutrino signals. Such breaks are, in particular, induced by the long-wavelength neutrino oscillations predicted by various neutrino mixing schemes including pseudo-Dirac scenarios and the exact parity model. With 10 years' data, neutrino signals from Sgr A East, alone, should either exclude neutrino oscillations governed by a δm2 parameter close to 10-15 eV2 or, alternatively, discover such oscillations in the likely event that other explanations for any observed break can be excluded. If data from γ-ray observations are included in the analysis, then oscillations governed by a δm2 in the approximate range from 10-13 to 10-15 eV2 might be discovered or excluded. Terrestrial or solar system neutrino experiments do not have the capability to observe oscillations governed by such tiny δm2 values.
• Liu, S., & Melia, F. (2002). An accretion-induced X-ray flare in Sagittarius A*. Astrophysical Journal Letters, 566(2 II), L77-L80.
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  Abstract: The recent detection of a 3 hr X-ray flare from Sgr A* by Chandra provides very strong evidence for a compact emitting region near this supermassive black hole at the Galactic center. The millimeter/submillimeter spectrum and linear polarimetric properties of Sgr A* and its quiescent state X-ray flux density are consistent with a model in which low angular momentum gas captured at large radii circularizes to form a hot magnetized Keplerian flow within tens of Schwarzschild radii of the black hole's event horizon. In the quiescent state of Sgr A*, the X-ray emission appears to be produced by self-Comptonization of the millimeter/submillimeter synchrotron photons emitted in this region. In this Letter we show that the prominent X-ray flare seen in Sgr A* may be due to a sudden enhancement of accretion through the circularized flow. Depending on whether the associated response of the anomalous viscosity is to increase or decrease in tandem with this additional injection of mass, the X-ray photons during the outburst may be produced via either thermal bremsstrahlung (if the viscosity decreases) or synchrotron self-Comptonization (SSC; if the viscosity increases). However, the latter predicts a softer X-ray spectrum than was seen by Chandra, so it appears that a bremsstrahlung origin for the X-ray outburst is favored. A strong correlation is expected between the millimeter/submillimeter and X-ray fluxes when the flare X-rays are produced by SSC, while the correlated variability is strongest between the submillimeter/far-IR and X-rays when bremsstrahlung emission is dominant during the flare. In addition, we show that future coordinated multiwavelength observations planned for the 2002 and 2003 cycles may be able to distinguish between the accretion and jet scenarios.
• Liu, S., & Melia, F. (2002). Spin-induced disk precession in the supermassive black hole at the Galactic center. Astrophysical Journal Letters, 573(1 II), L23-L26.
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  Abstract: Sagittarius A* is a compact radio source at the Galactic center that is thought to be the radiative manifestation of a 2.6 × 106 M⊙ supermassive black hole. At least a portion of its spectrum-notably the millimeter/submillimeter "bump"-appears to be produced within the inner portion (r < 10rs) of a hot, magnetized Keplerian flow, whose characteristics are also consistent with the ∼10% linear polarization detected from this source at millimeter wavelengths. (The Schwarzschild radius, rs, for an object of this mass M is 2GM/c 2 ≈ 7.7 × 1011 cm, or roughly 1/20 AU.) The recent detection of a 106 day cycle in Sgr A*'s radio variability adds significant intrigue to this picture since it may signal a precession of the disk induced by the spin a of the black hole. The dynamical timescale near the marginally stable orbit around an object with this mass is ≈20 minutes. Thus, since the physical conditions associated with the disk around Sgr A* imply rigid-body rotation, a precession period of 106 days may be indicative of a small black hole spin if the circularized flow is confined to a region ∼30rs, for which a ≈ (M/10)(ro/30r s)5/2. The precession of a larger structure would require a bigger black hole spin. We note that a small value of a/M (
• Liu, S., Fromerth, M. J., & Melia, F. (2002). Line emission from cooling accretion flows in the nucleus of M31. Astrophysical Journal Letters, 565(2 I), 952-958.
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  Abstract: The recent Chandra X-ray observations of the nucleus of M31, combined with earlier Very Large Array radio and Hubble Space Telescope UV spectral measurements, provide the strictest constraint thus far available on the nature of accretion onto the supermassive black hole (called M31* hereafter) in this region. One of the two newly detected sources within roughly an arcsecond of M31* may be its X-ray counterpart. If not, the X-ray flux from the nucleus must be even lower than inferred previously. Some uncertainty remains regarding the origin of the UV excess from the compact component known as P2. In our earlier analysis, we developed a unified picture for the broadband spectrum of this source and concluded that M31* could be understood on the basis of the accretion model for Sgr A* at the Galactic center, although with several crucial differences. Contrary to the "standard" picture in which the infalling plasma attains temperatures in excess of 1010 K near the event horizon, the best-fit model for M31*, under the assumption that the UV radiation is in fact produced by this source, appears to correspond to a cool-branch solution arising from strong line cooling inside the capture radius. Starting its infall with a temperature of about 106 K in the postshock region, the plasma cools down efficiently to about 104 K toward smaller radii. An important prediction of this model is the appearance of a prominent UV spike from hydrogen line emission, which for simplicity was handled only crudely in the earlier work. It is our purpose here to model this line emission with significantly greater accuracy using the algorithm CLOUDY and to correctly take into account the attenuation along the line of sight. We show that this level of sophistication reproduces the currently available multiwavelength observations very well. Very importantly, we predict a spectrum with several additional prominent emission lines that can be used to verify the model with future high-resolution observations. A nondetection of the predicted line emission from M31* would then tilt the favored accretion picture in the direction of a hot Sgr A* type of model, although with only a single point remaining in the spectrum of M31*, additional observations at other wavelengths would be required to seriously constrain this system.
• Melia, F., Liu, S., & Patuzzo, M. (2002). Is thermal expansion driving the initial gas ejection in NGC 6251?. Astrophysical Journal Letters, 567(2 I), 811-816.
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  Abstract: The relativistic jets in active galactic nuclei (AGNs) are probably driven by the action of supermassive, spinning black holes. There is very little direct evidence for this, however, since the nuclei of active galaxies are difficult to study. This is now changing with new, high-resolution multiwavelength observations of nearby sources such as Sgr A* at the Galactic center and the nucleus of NGC 6251 (hereafter called NGC 6251*). In this paper, we explore the possibility that the radiative properties of the most compact region in NGC 6251* may be understood in the same sense as Sgr A*, though with some telling differences that may hint at the nature of jet formation. We show that observations of this object with ASCA, ROSAT, HST, and VLBI together may be suggesting a picture in which Bondi-Hoyle accretion from an ambient ionized medium feeds a standard disk accreting at ∼ 4.0 × 1022 g s-1. Somewhere near the event horizon, this plasma is heated to more than 1011 K, where it radiates via thermal synchrotron (producing a radio component) and self-Comptonization (accounting for a nonthermal X-ray flux). This temperature is much greater than its virial value and the hot cloud expands at roughly the sound speed (∼0.1c), after which it begins to accelerate on a parsec scale to relativistic velocities. In earlier work, the emission from the extended jet has been modeled successfully using nonthermal synchrotron self-Compton processes, with a self-absorbed inner core. In the picture we are developing here, the initial ejection of matter is associated with a self-absorbed thermal radio component that dominates the core emission on the smallest scales. The nonthermal particle distributions responsible for the emission in the extended jet are then presumably energized, e.g., via shock acceleration, within the expanding, hot gas. The power associated with this plasma represents an accretion efficiency of about 0.54, requiring dissipation in a prograde disk around a rapidly spinning black hole (with spin parameter a ∼ 1).
• Bromley, B. C., Melia, F., & Liu, S. (2001). Polarimetric imaging of the massive black hole at the galactic center. Astrophysical Journal Letters, 555(2 PART 2), L83-L86.
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  Abstract: The radio source Sagittarius A* in the Galactic center emits a polarized spectrum at millimeter and submillimeter wavelengths that is strongly suggestive of relativistic disk accretion onto a massive black hole. We use the well-constrained mass of Sgr A* and a magnetohydrodynamic model of the accretion flow to match both the total flux and polarization from this object. Our results demonstrate explicitly that the shift in the position angle of the polarization vector, seen at wavelengths near the peak of the millimeter to submillimeter emission from this source, is a signal of relativistic accretion flow in a strong gravitational field. We provide maps of the polarized emission to illustrate how the images of polarized intensity from the vicinity of the black hole would appear in upcoming observations with very long baseline radio interferometers (VLBIs). Our results suggest that near-term VLBI observations will be able to directly image the polarized Keplerian portion of the flow near the horizon of the black hole.
• Fromerth, M. J., & Melia, F. (2001). The formation of broad-line clouds in the accretion shocks of active galactic nuclei. Astrophysical Journal Letters, 549(1 PART 1), 205-214.
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  Abstract: Recent work on the gas dynamics in the Galactic center has improved our understanding of the accretion processes in galactic nuclei, particularly with regard to properties such as the specific angular momentum distribution, density, and temperature of the inflowing plasma. With the appropriate extrapolation of the physical conditions, this information can be valuable in trying to determine the origin of the broad-line region (BLR) in active galactic nuclei (AGNs). In this paper, we explore various scenarios for cloud formation based on the underlying principle that the source of plasma is ultimately that portion of the gas trapped by the central black hole from the interstellar medium. Based on what we know about the Galactic center, it is likely that in highly dynamic environments such as this, the supply of matter is due mostly to stellar winds from the central cluster. Winds accreting onto a central black hole are subjected to several disturbances capable of producing shocks, including a Bondi-Hoyle flow, stellar wind-wind collisions, and turbulence. Shocked gas is initially compressed and heated out of thermal equilibrium with the ambient radiation field; a cooling instability sets in as the gas is cooled via inverse-Compton and bremsstrahlung processes. If the cooling time is less than the dynamical flow time through the shock region, the gas may clump to form the clouds responsible for broad-line emission seen in many AGN spectra. Clouds produced by this process display the correct range of densities and velocity fields seen in broad emission lines. Very importantly, the cloud distribution agrees with the results of reverberation studies, in which it is seen that the central line peak (due to infalling gas at large radii) responds more slowly to continuum changes than the line wings, which originate in the faster moving, circularized clouds at smaller radii. Finally, we provide an example of fitting an observed line profile using the parameters specified by our model.
• Fromerth, M. J., Melia, F., & Leahy, D. A. (2001). A Monte Carlo study of the 6.4 keV emission at the galactic center. Astrophysical Journal Letters, 547(2 PART 2), L129-L132.
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  Abstract: Strong fluorescent Fe line emission at 6.4 keV has been observed from the Sagittarius B2 giant molecular cloud located in the Galactic center region. The large equivalent width of this line and the lack of an apparent illuminating nearby object indicate that a time-dependent source, currently in a low-activity state, is causing the fluorescent emission. It has been suggested that this illuminator is the massive black hole candidate Sagittarius A*, whose X-ray luminosity has declined by an unprecedented 6 orders of magnitude over the past 300 years. Here we report the results of our Monte Carlo simulations for producing this line under a variety of source configurations and characteristics. These indicate that the source may in fact be embedded within Sgr B2, although external sources give a slightly better fit to the data. The weakened distinction between the internal and external illuminators is due in part to the instrument-response function, which accounts for an enhanced equivalent width of the line by folding some of the continuum radiation in with the intrinsic line intensity. We also point out that although the spectrum may be largely produced by Kα emission in cold gas, there is some evidence in the data to suggest the presence of warm (∼105 K) emitting material near the cold cloud.
• Liu, S., & Melia, F. (2001). Accretion processes in the nucleus of M31. Astrophysical Journal Letters, 550(2 PART 2), L151-L154.
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  Abstract: The hypothesized supermassive black hole in the nucleus of M31 (M31*) has many features in common with Sgr A* at the Galactic center, yet they differ in several significant and important ways. Although M31* is probably 10 times heavier, its radio luminosity at 3.6 cm is only one-third that of Sgr A*. At the same time, M31* is apparently thousands of times more luminous in X-rays than its Galactic center counterpart. Thus, a comparative study of these objects can be valuable in helping us to understand the underlying physical basis for their activity. We show here that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the radius rc where the ambient gas is captured by the black hole. For typical conditions in the interstellar medium, the initial temperature [T(rc) ∼ 106-107 K] sits on the unstable branch of the cooling function. Depending on the actual value of T(rc) and the accretion rate, the plasma settles onto either a hot branch (attaining a temperature as high as 1010 K or so at small radii) or a cold branch, in which T drops to ∼104 K. Sgr A* is presumably a "hot" black hole. We show here that Very Large Array, Hubble Space Telescope, and Chandra observations of M31* reveal it to be a member of the "cold" black hole family. We discuss several predicted features in the spectrum of M31* that may be testable by future multiwavelength observations, including the presence of a prominent UV spike (from hydrogen line emission) that would be absent on the hot branch.
• Liu, S., & Melia, F. (2001). New constraints on the nature of radio emission in sagittarius A. Astrophysical Journal Letters, 561(1 PART 2), L77-L80.
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  Abstract: The millimeter to submillimeter spectrum of Sagittarius A* at the Galactic center, as well as its polarization characteristics, are consistent with the inner ∼10 Schwarzschild radii of a tight Keplerian emitting region of hot, magnetized, orbiting gas. This plasma may also be the source (through self-Comptonization) of the X-rays detected by Chandra. It has long been suspected that the circularization region between the quasi-spherical infall at large radii, and this inner zone, is responsible for producing the rest of Sgr A*'s spectrum. In this Letter, we report the results of a detailed study of this region, with several important conclusions that will be highly relevant to upcoming coordinated multiwavelength observations. First, the combination of existing centimeter and X-ray data preclude the possibility of producing the observed strong 1.36 GHz radio flux via thermal synchrotron within a bounded flow. If Sgr A*'s radio spectrum is produced by accreting gas, it appears that a nonthermal particle distribution is a necessity. This may not be surprising, given that the energy associated with the radial motion is probably dissipated by shocks before the gas circularizes, which can produce the required power-law distribution. Second, if this is the correct picture for how Sgr A*'s spectrum is produced, it appears that the Chandra-detected X-rays may originate either from self-Comptonization in the inner Keplerian region or from optically thin nonthermal synchrotron emission in the much larger circularization zone, extending up to 500 Schwarzschild radii or more. This is a question that should be answered by upcoming broadband observations, since the millimeter bump and X-rays are strongly correlated in the former case, whereas the X-rays are strongly correlated to the centimeter radio flux in the latter. In addition, X-rays produced in the circularized gas could show periodic or quasi-periodic variations but not those produced via nonthermal synchrotron emission much farther out.
• Melia, F. (2001). X-rays from the edge of infinity. Nature, 413(6851), 25-26.
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  PMID: 11544506;Abstract: The supermassive black hole at the centre of our Galaxy has a strong influence on its surroundings. Astronomers cannot yet see this beast directly but they now have a much better idea of its size.
• Melia, F., & Falcke, H. (2001). The supermassive black hole at the galactic center. Annual Review of Astronomy and Astrophysics, 39(1), 309-352.
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  Abstract: The inner few parsecs at the Galactic Center have come under intense scrutiny in recent years, in part due to the exciting broad-band observations of this region, but also because of the growing interest from theorists motivated to study the physics of black hole accretion, magnetized gas dynamics, and unusual star formation. The Galactic Center is now known to contain arguably the most compelling supermassive black hole candidate, weighing in at a little over 2.6 million suns. Its interaction with the nearby environment, comprised of clusters of evolved and young stars, a molecular dusty ring, ionized gas streamers, diffuse hot gas, and a hypernova remnant, is providing a wealth of accretion phenomenology and high-energy processes for detailed modeling. In this review, we summarize the latest observational results and focus on the physical interpretation of the most intriguing object in this region - the compact radio source Sgr A *, thought to be the radiative manifestation of the supermassive black hole.
• Melia, F., & Kowalenko, V. (2001). Magnetic field dissipation in converging flows. Monthly Notices of the Royal Astronomical Society, 327(4), 1279-1287.
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  Abstract: Converging flows (e.g., gas accreting on to compact objects) are often ionized and magnetized. As the gas in these systems compresses towards smaller radii, flux conservation acts to intensify the magnetic field B, which can attain superequipartition values. (Throughout this paper, equipartition is meant to imply a comparison between the energy density in the field and that of the particles only, not including turbulence.) Since such a field probably cannot remain anchored in the gas, it is often assumed that the field intensity in excess of equipartition (i.e., Beq) is dissipated as heat, and that B therefore saturates at its Beq value -the so-called 'equipartition assumption'. In this paper we make an attempt at developing a model for magnetic field dissipation based on resistive magnetic tearing, in order to provide a more realistic means of determining the evolution of B in cases where the contribution to the spectrum from magnetic bremsstrahlung is important. We find that the violation of equipartition can vary in degree from large to small radii, and in either direction. Thus the spectrum predicted on the basis of the equipartition assumption is not always an adequate representation of the actual state of the system. However, several major shortcomings remain in our formulation. For example, our approach in this paper is to consider the turbulence as being initiated primarily by hydrodynamic processes. Arguing that the magnetic field is frozen into the highly ionized plasma, we therefore adopt a magnetic field spatial distribution that mirrors that of the gas. This may be valid only when the field is subequipartition, for otherwise the turbulent cascade may be influenced primarily by magnetic dissipation, rather than the hydrodynamics. In the application of this work to systems such as Sgr A* at the Galactic Centre, our approach may therefore break down at small radii (i.e., several Schwarzschild radii) where the magnetic field can in fact become superequipartition, for which a complete treatment of magnetic turbulence would need to be considered. However, the dominant emission mechanism in sources such as this appears to be magnetic bremsstrahlung over the full extent of the accreting region, so that even with these limitations, the bulk of the spectrum is influenced significantly by the qualitative results presented here.
• Melia, F., Bromley, B. C., Liu, S., & Walker, C. K. (2001). Measuring the black hole spin in Sagittarius A. Astrophysical Journal Letters, 554(1 PART 2), L37-L40.
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  Abstract: The polarized millimeter/submillimeter radiation from Sagittarius A* is apparently produced by a Keplerian structure whose peak emission occurs within several Schwarzschild radii (rs ≡ 2GMIc2) of the black hole. The Chandra X-ray counterpart, if confirmed, is presumably the self-Comptonized component from this region. In this Letter, we suggest that submillimeter timing observations could yield a signal corresponding to the period P0 of the marginally stable orbit and therefore point directly to the black hole's spin a. Sgr A*'s mass is now known to be (2.6 ± 0.2) × 106 M⊙ (an unusually accurate value for supermassive black hole candidates), for which 2.7 minutes < P0 < 36 minutes, depending on the value of a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild black hole (a = 0) should have -P0 ≈ 20 minutes. The identification of the orbital frequency with the innermost stable circular orbit is made feasible by the transition from optically thick to thin emission at submillimeter wavelengths. With stratification in the emitter, the peak of the submillimeter bump in Sgr A*'s spectrum is thus produced at the smallest radius. We caution, however, that theoretical uncertainties in the structure of the emission region may still produce some ambiguity in the timing signal. Given that Sgr A*'s flux at v ∼ 1 mm is several janskys, these periods should lie within the temporal resolving capability of submillimeter telescopes using bolometric detectors. A determination of P0 should provide not only a value of a, but it should also define the angular momentum vector of the orbiting gas in relation to the black hole's spin axis. By analogy with low-mass X-ray binaries and Galactic black hole candidates, Sgr A* may also display quasi-periodic oscillations, which can reveal additional features in the geometry of the accreting gas. In addition, since the X-ray flux detected by Chandra appears to be the self-Comptonized millimeter-to-submillimeter component, these temporal fluctuations may also be evident in the X-ray signal.
• Melia, F., Liu, S., & Coker, R. (2001). A magnetic dynamo origin for the submillimeter excess in sagittarius A. Astrophysical Journal Letters, 553(1 PART 1), 146-157.
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  Abstract: The submillimeter bump observed in the spectrum of Sgr A * appears to indicate the existence of a compact emitting component within several Schwarzschild radii, rs, of the nucleus at the Galactic center. This is interesting in view of the predicted circularized flow within ∼5-10rs, based on detailed multidimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this unusual object. In this paper, we examine the physics of magnetic field generation by a Keplerian dynamo subject to the conditions pertaining to Sgr A *, and show that the submillimeter bump can be produced by thermal synchrotron emission in this inner region. This spectral feature may therefore be taken as indirect evidence for the existence of this circularization. In addition, the self-Comptonization of the submillimeter bump appears to produce an X-ray flux exceeding that due to bremsstrahlung from this region, which may account for the X-ray counterpart to Sgr A * discovered recently by Chandra. However, the required accretion rate in the Keplerian flow is orders of magnitude smaller than that predicted by the Bondi-Hoyle simulations. We speculate that rapid evaporation, in the form of a wind, may ensue from the heating associated with turbulent mixing of gas elements with large eccentricity as they settle down into a more or less circular (i.e., low-eccentricity) trajectory. The spectrum of Sgr A * longward of ∼1-2 mm may be generated outside of the Keplerian flow, where the gas is making a transition from a quasi-spherical infall to a circularized pattern.
• Rafelski, J., Melia, F., & Fatuzzo, M. (2001). Electron-Positron Annihilation Radiation from Sagittarius A East at the Galactic Center. The Astrophysical Journal, 549(1), 293-302. doi:10.1086/319069
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  Maps of the Galactic electron-positron annihilation radiation show evidence for three distinct and significant features: (1) a central bulge source, (2) emission in the Galactic plane, and (3) an enhancement of emission at positive latitudes above the Galactic center. In this paper, we explore the possibility that Sgr A East, a very prominent radio structure surrounding the Galactic nucleus, may be a significant contributor to the central bulge feature. The motivation for doing so stems from a recently proposed link between this radio object and the EGRET γ-ray source 2EG J1746-2852. If this association is correct, then Sgr A East is also expected to be a source of copious positron production. The results presented here show that indeed Sgr A East must have produced a numerically significant population of positrons, but also that most of them have not yet had sufficient time to thermalize and annihilate. As such, Sgr A East by itself does not appear to be the dominant current source of annihilation radiation, but it will be when the positrons have cooled sufficiently and they have become thermalized. This raises the interesting possibility that the bulge component may be a result of the relics of earlier explosive events like the one that produced Sgr A East.
• Coker, R. F., & Melia, F. (2000). The role of magnetic field dissipation in the black hole candidate Sagittarius A*. Astrophysical Journal Letters, 534(2 PART 1), 723-733.
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  Abstract: The compact, nonthermal radio source Sgr A* at the Galactic center appears to be coincident with a ∼2.6 × 106 M⊙ pointlike object. Its energy source may be the release of gravitational energy as gas from the interstellar medium descends into its deep potential well. However, simple attempts at calculating the radiative spectrum and flux based on this picture have come tantalizingly close to the observations, yet have had difficulty in accounting for the unusually low efficiency in this source. Regardless of whether the radiating particles in the accretion flow are thermal or nonthermal, there now appear to be two principal reasons for this low conversion rate of dissipated energy into radiation: (1) the plasma separates into two temperatures, with the protons attaining a significantly higher temperature than that of the radiating electrons; and (2) the magnetic field B is subequipartition, which reduces the magnetic bremsstrahlung emissivity, and therefore the overall power of Sgr A*. In this paper, we investigate the latter with a considerable improvement over what has been attempted before. In particular, rather than calculating B based on some presumed model (e.g., equipartition with the thermal energy of the gas), we instead infer its distribution with radius empirically with the requirement that the resulting spectrum matches the observations. Our assumed Ansatz for B(r) is motivated in part by earlier calculations of the expected magnetic dissipation rate due to reconnection in a compressed flow. We find reasonable agreement with the observed spectrum of Sgr A* as long as its distribution consists of three primary components: an outer equipartition field, a roughly constant field at intermediate radii (∼103 Schwarzschild radii), and an inner dynamo (more or less within the last stable orbit for a nonrotating black hole), which increases B to about 100 G. The latter component accounts very well for the observed submillimiter hump in this source.
• Crocker, R. M., Melia, F., & Volkas, R. R. (2000). Oscillating neutrinos from the galactic center. Astrophysical Journal, Supplement Series, 130(2), 339-350.
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  Abstract: It has recently been demonstrated that the γ-ray emission spectrum of the EGRET-identified central Galactic source 2EG J1746-2852 can be well fitted by positing that these photons are generated by the decay of π0's produced in p-p scattering at or near an energizing shock. Such scattering also produces charged pions which decay leptonically. The ratio of γ-rays to neutrinos generated by the central Galactic source can be accurately determined, and a well-defined and potentially measurable high-energy neutrino flux at Earth is unavoidable. An opportunity, therefore, to detect neutrino oscillations over an unprecedented scale is offered by this source. In this paper we assess the prospects for such an observation with the generation of neutrino Čerenkov telescopes now in the planning stage. We determine that the next generation of detectors may well find an oscillation signature in the Galactic center (GC) signal.
• Falcke, H., Melia, F., & Agol, E. (2000). Viewing the shadow of the black hole at the Galactic center. Astrophysical Journal Letters, 528(1 PART 2), L13-L16.
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  Abstract: In recent years, evidence for the existence of an ultracompact concentration of dark mass associated with the radio source Sagittarius A* in the Galactic center has become very strong. However, unambiguous proof that this object is indeed a black hole is still lacking. A defining characteristic of a black hole is the event horizon. To a distant observer, the event horizon casts a relatively large "shadow" with an apparent diameter of ∼10 gravitational radii that is due to the bending of light by the black hole, and this shadow is nearly independent of the black hole spin or orientation. The predicted size (∼30 μas) of this shadow for Sgr A* approaches the resolution of current radio interferometers. If the black hole is maximally spinning and viewed edge-on, then the shadow will be offset by ∼8 μas from the center of mass and will be slightly flattened on one side. Taking into account the scatter broadening of the image in the interstellar medium and the finite achievable telescope resolution, we show that the shadow of Sgr A* may be observable with very long baseline interferometry at submillimeter wavelengths, assuming that the accretion flow is optically thin in this region of the spectrum. Hence, there exists a realistic expectation of imaging the event horizon of a black hole within the next few years.
• Fromerth, M. J., & Melia, F. (2000). Determination of the central mass in active galactic nuclei using cross-correlation lags and velocity dispersions. Astrophysical Journal Letters, 533(1 PART 1), 172-175.
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  Abstract: We here estimate the mass M of the central object in five active galactic nuclei (AGNs), using the most recent reverberation data obtained by the AGN Watch Consortium. The cross-correlation function (CCF) centroids of the broad Lyα γ1216 and C IV γ1549 lines are used to estimate the size of the broadline region (BLR) in these sources. We calculate the velocity dispersions of these lines in the root mean square (rms) spectra and then use our results to estimate M. We argue that our technique of calculating the velocity dispersion should work in the general case of an arbitrary line profile, unlike methods that depend on the measurement of the full-width at half-maximum (FWHM) of the broad line. We also show that our results agree with the FWHM method in the limit of a normal (Gaussian) line profile. The masses calculated here are considerably smaller than those calculated with the previous generation of reverberation data.
• Melia, F. (2000). The heart of the milky way. American Scientist, 88(4), 346-353.
• Melia, F., Liu, S., & Coker, R. (2000). Polarized millimeter and submillimeter emission from Sagittarius A* at the Galactic center. Astrophysical Journal Letters, 545(2 PART 2), L117-L120.
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  Abstract: The recent detection of significant linear polarization at millimeter and submillimeter wavelengths in the spectrum of Sagittarius A * (if confirmed) will be a useful probe of the conditions within several Schwarzschild radii (rS) of the event horizon at the Galactic center. Hydrodynamic simulations of gas flowing in the vicinity of this object suggest that the infalling gas circularizes when it approaches within 5-25rS of the black hole. We suggest that the submillimeter "excess" of emission seen in the spectrum of Sgr A * may be associated with radiation produced within the inner Keplerian region and that the observed polarization characteristics provide direct evidence for this phenomenon. The overall spectrum from this region, including the high-energy component due to bremsstrahlung and inverse Compton scattering processes, is at or below the recent Chandra measurement and may account for the X-ray source if it turns out to be the actual counterpart to Sgr A *.
• Nayakshin, S., Rappaport, S., & Melia, F. (2000). Time-dependent disk models for the microquasar GRS 1915 + 105. Astrophysical Journal Letters, 535(2 PART 1), 798-814.
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  Abstract: During the past three years, the Galactic black hole microquasar GRS 1915 + 105 has exhibited a bewildering diversity of large-amplitude, chaotic variability in X-rays. Although it is generally accepted that the variability in this source results from an accretion disk instability, the exact nature of the instability remains unknown. Here we investigate different accretion disk models and viscosity prescriptions in order to provide a basic explanation for some of the exotic temporal behavior in GRS 1915 + 105. We discuss a range of possible accretion flow geometries. Any geometrically thick disk (e.g., an advection-dominated accretion flow [ADAF] or a "slim" accretion disk) has trouble explaining the very long cycle times unless the α-parameter is exceedingly small (∼10-4). In addition, the rise/fall timescales in GRS 1915 + 105 can be a factor of 100 shorter than the cycle times, whereas thick disks predict that these two timescales should be comparable. We thus concentrate on geometrically thin (though not necessarily standard) Shakura-Sunyaev type disks. We argue that X-ray observations clearly require a quasi-stable accretion disk solution at a high accretion rate at which radiation pressure begins to dominate, which excludes the standard α-viscosity prescription. We have therefore devised a simplified model of a disk with a corona and a modified viscosity law that has a quasi-stable upper branch, and we have developed a code to solve the time-dependent equations to study the evolution of this configuration. Via numerical simulations, we show that the model does account for several gross observational features of GRS 1915 + 105, including its overall cyclic behavior on timescales of ∼100-1000 s. On the other hand, the rise/fall timescales are not as short as those observed, no rapid oscillations on timescales ≲10 s emerge naturally from the model, and the computed cycle-time dependence on the average luminosity is stronger than is found in GRS 1915 + 105. We then consider, and numerically test, a more elaborate model that includes the "cold" disk, a corona, and plasma ejections from the inner disk region that occur when the luminosity of the source is near the Eddington luminosity. The inclusion of a jet allows us to reproduce several additional observed features of GRS 1915 + 105. We conclude that the most likely structure of the accretion flow in this source is that of a cold disk with a modified viscosity law, plus a corona that accounts for much of the X-ray emission and unsteady plasma ejections that occur when the luminosity of the source is high. The disk is geometrically thin (as required by the data) because most of the accretion power is drained by the corona and the jet.
• Yusef-Zadeh, F., Melia, F., & Wardle, M. (2000). The galactic center: An interacting system of unusual sources. Science, 287(5450), 85-91.
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  Abstract: The region bounded by the inner tens of light-years at the center of the Milky Way Galaxy contains five principal components that coexist within the central deep well of gravitational potential. These constituents are a black hole candidate (Sgr A*) with a mass equivalent to 2.6 ± 0.2 × 106 solar masses, a surrounding cluster of evolved stars, a complex of young stars, molecular and ionized gas clouds, and a powerful supernova-like remnant. The interaction of these components is responsible for many of the phenomena occurring in this complex and unique portion of the Galaxy. Developing a consistent picture of the primary interactions between the components at the Galactic center will improve our understanding of the nature of galactic nuclei in general, and will provide us with a better-defined set of characteristics of black holes. For example, the accretion of stellar winds by Sgr A* appears to produce far less radiation than indicated by estimates based on models of galactic nuclei.
• Coker, R., Melia, F., & Falcke, H. (1999). Accretion disk evolution with wind infall. II. Results of three-dimensional hydrodynamical simulations with an illustrative application to sagittarius. Astrophysical Journal Letters, 523(2 PART 1), 642-653.
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  Abstract: In the first paper of this series, using analytic tools, we examined how the evolution and structure of a massive accretion disk may be influenced significantly by the deposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Such a mass influx impacts the long-term behavior of the disk by providing additional sources of viscosity and heating. In this paper, we make a significant improvement over this earlier work by incorporating the results of three-dimensional hydrodynamical simulations of the large-scale accretion from an ambient medium into the disk evolution equations developed previously. We discuss in detail two models, one with the axis of the disk parallel to and the second with the axis oriented perpendicular to the large scale Bondi-Hoyle flow. We find that the mass inflow rate onto the disk within logarithmic annuli is roughly constant with radius and that the impacting wind carries much less specific angular momentum than Keplerian. We also find, in general, that the infrared spectrum of a wind-fed disk system is steeper than that of a Shakura-Sunyaev configuration, due mainly to the dissipation of the wind's kinetic energy at the disk's surface. In applying our results to the Galactic center black hole candidate Sgr A*, accreting from nearby stellar winds, we demonstrate that a high wind inflow rate of the order of 10-4 M⊙ yr-1 cannot be incorporated into a fossil disk without a significant dissipation of kinetic energy at all radii. Such a high dissipation would violate current infrared and near-infrared limits on the observed spectrum of Sgr A*.
• Kowalenko, V., & Melia, F. (1999). Towards incorporating a turbulent magnetic field in an accreting black hole model. Monthly Notices of the Royal Astronomical Society, 310(4), 1053-1061.
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  Abstract: A model proposed by Melia & Ruffert to evaluate the spectrum and radiation flux for accretion on to a black hole makes use of the 'equipartition assumption' in which the magnetic, turbulent and gravitational energy densities are assumed to be in approximate equilibrium for distances below the accretion radius, where Bondi-Hoyle infall begins. As a consequence, the mechanism for the dissipation of the magnetic field and the resulting effect on the flow of the accreting gas have not been treated quantitatively. Here we examine alternative approaches for modelling the dissipation of magnetic fields and turbulent flow to see how these may be incorporated into the model. The results of our study should be immediately applicable to the ever-improving measurements of the spectrum and size of the massive black hole at our Galactic Centre, in particular producing a more accurate estimate of its mass. Combined with greatly refined kinematic studies of this region, our work may constrain the dark matter concentration in the nucleus of our Galaxy.
• Melia, F., & Coker, R. (1999). Stellar gas flows into a dark cluster potential at the Galactic center. Astrophysical Journal Letters, 511(2 PART 1), 750-760.
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  Abstract: The evidence for the presence of a concentration of dark matter at the Galactic center (GC) is now very compelling. There is no question that the stellar and gas kinematics within ≈0.01 pc is dominated by underluminous matter in the form of either a massive black hole, a highly condensed distribution of stellar remnants, or a more exotic source of gravity. The unique, compact radio source Sgr A* appears to be coincident with the center of this region, but its size (less than about 6 × 1013 cm at λ7 mm) is still significantly smaller than the current limiting volume enclosing this mass. Sgr A* may be the black hole, if the dark matter distribution is pointlike. If not, we are left with a puzzle regarding its nature and a question of why this source should be so unique and lie only at the GC. In this paper, we examine an alternative to the black hole paradigm, that the gravitating matter is a condensed cluster of stellar remnants, and study the properties of the GC wind flowing through this region. Some of this gas is trapped in the cluster potential, and we study in detail whether this hot, magnetized gas is in the proper physical state to produce Sgr A*'s spectrum. We find that at least for the GC environment, the temperature of the trapped gas never attains the value required for significant GHz emission. In addition, continuum (mostly bremsstrahlung) emission at higher frequencies is below the current measurements for this source. We conclude that the cluster potential is too shallow for the trapped GC wind to account for Sgr A*'s spectrum, which instead appears to be produced only within an environment that has a steep-gradient potential like that generated by a black hole.
• Sarcevic, I., Melia, F., & Markoff, S. (1999). High-Energy Emission from Relativistic Particles in Sagittarius A*. The Astrophysical Journal, 522(2), 870-878. doi:10.1086/307656
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  EGRET onboard the Compton Gamma Ray Observatory has recently detected a high-energy source at the Galactic center. However, it is not yet clear whether the γ-rays are produced by the hypothesized massive black hole, Sgr A*, or whether the emitter is diffuse. The lack of variability in the γ-ray flux, up to an amplitude of about 20%, seems to suggest the latter. But hydrodynamical simulations of the accretion process point to a fluctuation amplitude in the inflowing mass rate below this level on a timescale comparable to the current observation time line. Thus, if the γ-ray flux is directly tied to the dissipation of gravitational energy, e.g., if it is produced by relativistic particles energized by a shock within the infalling plasma, we cannot rule Sgr A* out as the source of the γ-rays on this basis alone. In earlier work, we demonstrated that the γ-ray spectrum from the Galactic center may be consistent with the radiative decay of pions produced via proton-proton interactions in this shock acceleration scenario. However, this study was incomplete for several reasons. In this paper, we extend this work significantly by making several improvements to our treatment of the particle physics. We also make use of the new EGRET data published since our last paper. Our analysis now indicates that the hypothesized massive black hole is probably not the source of the γ-rays. Understanding the nature of the Galactic center γ-ray source may ultimately depend on whether or not future γ-ray detectors will have the capability of resolving it. In this paper we consider a point source, but if the γ-rays are in fact associated with diffuse emission, GLAST may be able to image the source with a spatial resolution of ~30'' to 5'.
• Schmidt, G. D., Hoard, D. W., Szkody, P., Melia, F., Honeycutt, R. K., & Wagner, R. M. (1999). Accretion in the high-field magnetic cataclysmic variable AR Ursae Majoris. Astrophysical Journal Letters, 525(1 PART 1), 407-419.
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  Abstract: AR UMa is the highest field example known of the magnetic cataclysmic variables, with a polar field Bp = 230 MG. We report here the results of a long program of photometry, spectroscopy, and spectropolarimetry of the object that was undertaken with the aim of gaining a better understanding of the role that the white dwarf field plays in shaping the properties of the magnetic systems. Only three accretion episodes, reaching mv ≤ 15 and covering a total of 9 months, were detected during the 4 yr of available photometry. Fortunately, the onset of one episode coincided with a week-long observing run, resulting in spectrophotometry and linear and circular spectropolarimetry that document the event. The long intervals of quiescence have enabled a determination of the orbital ephemeris that is now sufficiently precise to phase observations 20 yr old. The high-state observations lead to a number of conclusions regarding the system geometry. The white dwarf magnetic axis is inclined rather little to the spin axis, but the dipole is twisted in azimuth such that it lies nearly perpendicular to the stellar line of centers. Tomographic analysis of a wide variety of atomic species indicates that the white dwarf likely accretes in twin funnels that split off a ballistic gas stream reaching 30%-50% of the distance between the stars. A narrow high-velocity (KNHV ≈ 700 km s-1) emission-line component offset in phase from the main features suggests the existence of an additional gas stream, but the interpretation of this is not yet resolved. The system is viewed from a moderate inclination, which places the disfavored (retrograde) magnetic pole on the observed hemisphere at all times. We have investigated the possible effect that the white dwarf magnetosphere might have in restricting mass flow through L1, and therefore in giving rise to the unique, protracted low-accretion states of this object. However, the mechanisms considered are too weak and/or would give rise to consequences that are at odds with low-state observations. At the same time, the high magnetic field in AR UMa has yielded a new, powerful observational tool: phase-resolved Zeeman spectroscopy of the emission lines produced in the accretion stream(s). Future high-quality observations and sophisticated modeling of these features hold promise for three-dimensional reconstructions of the gas flow in high-field magnetic variables.
• Fatuzzo, M., & Melia, F. (1998). The hard X-ray to gamma-ray spectrum in the egret active galactic nuclei. Astrophysical Journal Letters, 508(2 PART II), 601-607.
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  Abstract: EGRET (20 MeV-30 GeV), on board the Compton Gamma Ray Observatory, has observed high-energy emission from about ∼ 40-50 active galactic nuclei. Theoretical models of this emission based on the upscattering of thermal disk photons by cooling, relativistic electrons can successfully account for the EGRET observations, but they predict a considerably greater X-ray flux than that actually observed in a majority of these sources. This inconsistency may be an indication that the particles are energized during the Compton scattering process, since the X-ray emission is produced by the lowest energy electrons, whose density may be relatively small because of the acceleration. Such a situation may arise as a result of resistive field generation in electromagnetic acceleration schemes, which we here explore. A key feature of this model is the assumed existence of a current associated with the azimuthal component Bψ of the underlying magnetic field by a slight imbalance in the energy distributions of outwardly moving, relativistic electrons and protons produced at the disk surface via shock acceleration. The generation of an electric field (via magnetic field line reconnection) is thus required to maintain the current in the presence of a resistivity induced by the radiative drag on the relativistic electrons. We show that the resulting spectrum can exhibit a significant deficit of X-rays compared with gamma rays. In addition, because of the unidirectional flow of the current associated with Bψ, this model would predict that the electrons are energized relative to the protons in the outflow only on one side of the disk. They should be decelerated on the reverse side. As such, we would anticipate that any given blazar should have a ∼50% probability of being gamma-ray-bright, which appears to be consistent with the observed ratio. © 1998. The American Astronomical Society. All rights reserved.
• Hinz, P., Hege, K., Lloyd-hart, M., Melia, F., & Mccarthy, D. W. (1998). 9.5. The “double nucleus” of M31 in J, H, and K. Symposium - International Astronomical Union, 184, 391-392. doi:10.1017/s0074180900085326
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  Hubble Space Telescope images of the nucleus of M31 show a double-peaked structure with the primary peak being offset from the center by approximately 0.5″. We observed the central 13″ of M31 in the J, H, and Ks passbands to determine the nuclear structure in the near-infrared. Observations were taken at the MMT Observatory, using a low-order adaptive optics system, FASTTRAC II (Gray et. al. 1995). The diffraction limit for the system is 0.25″ in K band. PSF images showed correction to 0.5″ FWHM. Uncorrected images showed the seeing to be about 1″. The images were deconvolved using several methods to check for consistency. We used Iterative-Blind Deconvolution, Richardson-Lucy, and Wiener filter algorithms, getting similar results for each. Measurements suggest the PSF in the deconvolved images is approximately 0.35″ FWHM.
• Melia, F., & Coker, R. F. (1998). 7.14. Gaseous accretion flows in the inner parsec of the Galaxy. Symposium - International Astronomical Union, 184, 319-320. doi:10.1017/s0074180900085107
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  Many characteristics of galactic nuclei may be associated with the accretion of ambient gas by a central concentration of mass. Using a 3D hydrodynamical code, we have been simulating this accretion process for Sgr A∗, the compact nonthermal source at the center of the Milky Way, in order to realistically model the gaseous flows in the inner parsec of our Galaxy. In the most recent simulations, we have taken into account the multi-point-like distribution of wind sources and we find that the structure of the flow can be significantly different from that due to a uniform medium. We here present our results concerning the mass and angular momentum accretion rates and discuss how these may be used to set constraints on our Galaxy's central engine.
• Melia, F., Fatuzzo, M., Yusef-Zadeh, F., & Markoff, S. (1998). A self-consistent model for the broadband spectrum of Sagittarius A East at the Galactic center. Astrophysical Journal Letters, 508(1 PART II), L65-L69.
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  Abstract: Sgr A East is a very prominent elongated shell structure surrounding (although off-centered from) the Galactic nucleus. Its energetics (∼4 × 1052 ergs), based on the power required to carve out the radio synchrotron remnant within the surrounding dense molecular cloud, appear to be extreme compared to the total energy (∼1051 ergs) released in a typical supernova (SN) explosion. Yet it shares several characteristics in common with SN remnants (SNRs), the most significant of which is the ∼0.1-10 GeV γ-ray spectrum measured by EGRET, if we associate the Galactic center source 2EG J1746-2852 with this nonthermal shell. We here show that the highest energy component in Sgr A East's spectrum, like that of SNRs, can be fitted with the γ-rays produced in π0 decays. Further, we demonstrate in a self-consistent manner that the leptons released in the associated π± decays produce an e± distribution that can mimic a power law with index ∼3, like that inferred from the VLA data for this source. These relativistic electrons and positrons also radiate by bremsstrahlung and inverse Compton scattering with the intense IR and UV fields from the nucleus. We show that the overall emissivity calculated in this way may account for Sgr A East's broadband spectrum ranging from GHz frequencies all the way to TeV energies, where Whipple has thus far set an upper limit to the flux corresponding to a 2.5 σ signal above the noise. © 1998. The American Astronomical Society. All rights reserved.
• Melia, F., Yusef-Zadeh, F., & Fatuzzo, M. (1998). A model of the egret source at the galactic center: Inverse compton scattering within sagittarius a east and its halo. Astrophysical Journal Letters, 508(2 PART II), 676-679.
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  Abstract: Continuum low-frequency radio observations of the Galactic center reveal the presence of two prominent radio sources, Sgr A East and its surrounding halo, containing nonthermal particle distributions with power-law indexes ∼ 2.5-3.3 and ∼2.4, respectively. The central 1-2 pc region is also a source of intense (stellar) UV and (dust-reprocessed) far-IR radiation that bathes these extended synchrotronemitting structures. A recent detection of γ-rays (2EG J1746-2852) from within ∼1° of the Galactic center by EGRET onboard the Compton Gamma Ray Observatory (CGRO) shows that the emission from this environment extends to very high energies. We suggest that inverse Compton scattering between the power-law electrons inferred from the radio properties of Sgr A East and its halo and the UV and IR photons from the nucleus may account for the possibly diffuse γ-ray source as well. We show that both particle distributions may be contributing to the γ-ray emission, although their relevant strength depends on the actual physical properties (such as the magnetic field intensity) in each source. If this picture is correct, the high-energy source at the Galactic center is extended over several arcminutes, which can be tested with the next generation of γ-ray and hard X-ray missions. © 1998. The American Astronomical Society. All rights reserved.
• Misra, R., Chitnis, V. R., & Melia, F. (1998). A fit to the simultaneous broadband spectrum of cygnus x-1 using the transition disk model. Astrophysical Journal Letters, 495(1 PART I), 407-412.
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  Abstract: We have used the transition disk model to fit the simultaneous broadband (2-500 keV) spectrum of Cygnus X-1 from OSSE and Ginga observations. In this model, the spectrum is produced by saturated Comptonization within the inner region of the accretion disk, where the temperature varies rapidly with radius. In an earlier attempt, we demonstrated the viability of this model by fitting the data from EXOSAT, XMPC balloon, and OSSE observations, although these were not made simultaneously. Since the source is known to be variable, however, the results of this fit were not conclusive. In addition, since only one set of observations was used, the good agreement with the data could have been a chance occurrence. Here we improve considerably upon our earlier analysis by considering four sets of simultaneous observations of Cygnus X-1, using an empirical model to obtain the disk temperature profile. The vertical structure is then obtained using this profile, and we show that the analysis is self-consistent. We demonstrate conclusively that the transition disk spectrum is a better fit to the observations than that predicted by the soft-photon Comptonization model. In particular, although the transition disk model has only one additional parameter, the χ2 value is reduced and there are no systematic residuals. Since the temperature profile is obtained by fitting the data, the unknown viscosity mechanism need not be specified. The disk structure can then be used to infer the viscosity parameter α, which appears to vary with radius and luminosity. This behavior can be understood if a depends intrinsically on the local parameters such as density, height, and temperature. However, because of uncertainties in the radiative transfer, quantitative statements regarding the variation of a cannot yet be made. © 1998. The American Astronomical Society. All rights reserved.
• Nayakshin, S., & Melia, F. (1998). Self-consistent Fokker-Planck treatment of particle distributions in astrophysical plasmas. Astrophysical Journal, Supplement Series, 114(2), 269-288.
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  Abstract: High-energy, multicomponent plasmas in which pair creation and annihilation, lepton-lepton scattering, lepton-proton scattering, and Comptonization all contribute to establishing the particle and photon distributions are present in a broad range of compact astrophysical objects. The different constituents are often not in equilibrium with each other, and this mixture of interacting particles and radiation can produce substantial deviations from a Maxwellian profile for the lepton distributions. Earlier work has included much of the microphysics needed to account for electron-photon and electron-proton interactions, but little has been done to handle the redistribution of the particles as a result of their Coulomb interaction with themselves. The most detailed analysis thus far for finding the exact electron distribution appears to have been done within the framework of nonthermal models, where the electron distribution is approximated as a thermal one at low energy with a nonthermal tail at higher energy. Recent attention, however, has been focused on thermal models. Our goal here is to use a Fokker-Planck approach in order to develop a fully self-consistent theory for the interaction of arbitrarily distributed particles and radiation to arrive at an accurate representation of the high-energy plasma in these sources. We derive Fokker-Planck coefficients for an arbitrary electron distribution and correct an earlier expression for the diffusion coefficient used by previous authors. We conduct several tests representative of two dominant segments of parameter space. For high source compactness of the total radiation field, l ∼ 102, we find that although the electron distribution deviates substantially from a Maxwellian, the resulting photon spectra are insensitive to the shape of the exact electron distribution, in accordance with some earlier results. For low source compactness, l ∼ few, and an optical depth ≲0.2, however, we find that both the electron distribution and the photon spectra differ strongly from what they would be in the case of a Maxwellian distribution. In addition, for all values of compactness, we find that different electron distributions lead to different positron number densities and proton equilibrium temperatures. This means that the ratio of radiation pressure to proton pressure is strongly dependent on the lepton distribution, which might lead to different configurations of hydrostatic equilibrium. This, in turn, may change the compactness, optical depth, and heating and cooling rates and therefore lead to an additional change in the spectrum. An important result of our analysis is the derivation of useful, approximate analytical forms for the electron distribution in the case of strongly non-Maxwellian plasmas.
• Sarcevic, I., Melia, F., & Markoff, S. (1998). 7.8. Particle cascades in Sgr A∗: the possibility of observing their γ-ray signature. Symposium - International Astronomical Union, 184, 307-308. doi:10.1017/s0074180900085041
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  The recent detection of a γ-ray flux from the direction of the Galactic center by EGRET on the Compton GRO raises the question of whether this is a point source (possibly coincident with the massive black hole candidate Sgr A∗) or a diffuse emitter. Using the latest experimental particle physics data and theoretical models, we have examined in detail the γ-ray spectrum produced by synchrotron, inverse Compton scattering and mesonic decay resulting from the interaction of relativistic protons with hydrogen accreting onto a point-like object. Such a distribution of high-energy baryons may be expected to form within an accretion shock as the inflowing gas becomes supersonic. This scenario is motivated by hydrodynamic studies of Bondi-Hoyle accretion onto Sgr A∗, which indicate that many of its radiative characteristics may ultimately be associated with energy liberated as this plasma descends down into the deep potential well. Earlier attempts at analyzing this process concluded that the EGRET data are inconsistent with a massive point-like object (Mastichiadis & Ozernoy, 1994). Our results demonstrate that a more careful treatment of the physics of p-p scattering suggests that a ~ 10 6 M ⊙ black hole may be contributing to this high-energy emission.
• Coker, R. F., & Melia, F. (1997). Hydrodynamical accretion onto sagittarius A* from distributed point sources. Astrophysical Journal Letters, 488(2 PART II), L149-L152.
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  Abstract: Spectral and kinematic studies suggest that the nonthermal radio source Sgr A*, located at the center of the Milky Way, is a supermassive compact object with a mass ∼(2-3) × 106 M⊙. Winds from nearby stars, located ≈0.06 pc to the east of Sgr A*, should, in the absence of any outflow from the putative black hole itself, be accreting onto this object. We report the results of the first three-dimensional Bondi-Hoyle hydrodynamical numerical simulations of this process under the assumption that the Galactic center wind is generated by several different point sources (here assumed to be 10 pseudorandomly placed stars). Our results show that the accretion rate onto the central object can be higher than in the case of a uniform flow since wind-wind shocks dissipate some of the bulk kinetic energy and lead to a higher capture rate for the gas. However, even for this highly nonuniform medium, most of the accreting gas carries with it a relatively low level of specific angular momentum, although large transient fluctuations can occur. Additionally, the post-bow shock focusing of the gas can be substantially different than that for a uniform flow, but it depends strongly on the stellar spatial distribution. We discuss how this affects the morphology of the gas in the inner 0.15 pc of the Galaxy and the consequences for accretion disk models of Sgr A*. © 1997. The American Astronomical Society. All rights reserved.
• Falcke, H., & Melia, F. (1997). Accretion disk evolution with wind infall. I. General solution and application to Sagittarius A. Astrophysical Journal Letters, 479(2 PART I), 740-751.
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  Abstract: The evolution of an accretion disk can be influenced significantly by the deposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Such a mass influx impacts the long-term behavior of the disk by providing additional sources of viscosity and heating. In this paper, we derive and solve the disk equations when these effects are taken into account. We present a survey of models with various wind configurations and demonstrate that the disk spectrum may then differ substantially from that of a standard α-disk. In particular, it is likely that a wind-fed disk has a significant infrared bump due to the deposition of energy in its outer region. We apply some of the results of our calculations to the Galactic center black hole candidate Sagittarius A* and show that if a fossil disk is present in this source, it must have a very low viscosity parameter (α ≲ 10-4) and the Bondi-Hoyle wind must be accreting with a very high specific angular momentum to prevent it from circularizing in the inner disk region where its impact would be most noticeable. © 1997. The American Astronomical Society. All rights reserved.
• Hollywood, J. M., & Melia, F. (1997). General relativistic effects on the infrared spectrum of thin accretion disks in active galactic nuclei: Application to sagittarius A. Astrophysical Journal, Supplement Series, 112(2), 423-455.
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  Abstract: The possibility that some portion of the infrared (IR) radiation emanating from active galactic nuclei (AGNs) may arise from disklike structures of ionized plasma accreting onto massive black holes motivates the investigation of the effects of strong gravitational fields in the vicinity of emitting particles on the observed radiation. Numerous previous studies have been incomplete in several respects: (1) they have neglected to take into account the observed specific power flux contribution of radiation emitted from the underside of the disk and gravitationally lensed into the upper half-hemisphere, (2) they have considered only a limited range of observing positions and hole spins, and (3) many have been restricted to examination of the steady state flux arising from homogeneous disks. The present study develops, within the context of the optically thick, geometrically thin accretion disk model, a set of new calculational techniques based on an analysis of the algebraic properties of the effective potential functions governing photon propagation in the Kerr metric; ancillary techniques, such as that of "extended images," are introduced and employed to illustrate aspects of general relativistic image formation that affect the observed time-dependent flux arising from a thermally inhomogeneous accretion disk. The contribution of the first-orbit disk images, including the effects of disk self-blocking, to the observed flux are fully taken into account for the entire range of observing positions and hole spins for both the steady state and time-dependent cases. The procedure is illustrated by application of the results to the paradigm case of the Galactic center black hole candidate Sagittarius A*. Current observations are somewhat contaminated because of poor angular resolution, making this exercise still only an illustrative examination of the method. However, the future deployment of the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on HST should provide data with sufficient sensitivity for direct comparison with our calculated K-band fluxes and light curves. Of particular interest in this comparison is the expected reversal of the disk's angular momentum in Sgr A* on a timescale of 10 yr or so. We discuss the distinct spectral signature of a retrograde disk versus that of a prograde configuration and demonstrate the feasibility of observing this transition.
• Marietta, E., & Melia, F. (1997). Particle acceleration and high-energy emission in the EGRET AGNs. Advances in Space Research, 19(1), 109-112.
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  Abstract: Prior to the EGRET observations, jet formation models generally treated the acceleration and radiation mechanisms separately, since very little was known of the physical environment where the particles are initially energized. Because the high-energy emission from these sources presumably originates close to the central engine, the EGRET spectral measurements offer us the first opportunity to seriously model the early jet formation phase within ∼ 10 - 100 Schwarzschild radii of the nucleus. A viable mechanism for producing the high-energy gamma-rays is the Compton upscattering of ambient low-energy photons (e.g., from a disk) by relativistically moving particles. However, it is well known that the resulting Compton drag on the particles can significantly retard their progress, which results in a particle-photon-induced resistivity. Thus, if the energizing force on the particles is associated with an AGN magnetospheric phenomenon, as many have hypothesized, the electromagnetic acceleration is fully dependent on the magnitude of the photon drag and hence is connected directly to the radiative emission itself. These two processes, i.e., the particle acceleration and the associated Compton upscattering, eventually need to be treated self-consistently. Our primary goal in this preliminary set of calculations is to establish the range of blackhole masses, the spin rate, and the permissible magnetic field configurations that produce spectra and fluxes consistent with the EGRET observations. This analysis also predicts the degree of beaming and inclination effects, both of which should bear directly on theories that attempt to unify the radio-loud AGNs into a single class. As an illustration, we apply our model to QSO CTA 102 and obtain a reasonable agreement with the observed flux and spectral index. Published by Elsevier Science on behalf of COSPAR.
• Melia, F. (1997). High-energy spectrum of the galactic center black hole Sgr A. Advances in Space Research, 19(1), 71-74.
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  Abstract: The massive blackhole candidate Sgr A* may be accreting from an ambient Galactic center wind at a rate ≈ 1022 g s-1. Dissipational processes within the large-scale quasi-spherical infall, from 50 rg to 105 rg (where rg = 3 × 1011 cm is the Schwarzschild radius for M = 106 M⊙) can account well for the observed radio spectrum and flux (due to cyclotron/synchrotron emission) and its X-ray/Gamma-ray luminosity (apparently due to Bremsstrahlung emission). Small scale instabilities associated with the stagnation region of the flow produce fluctuations in the accretion rate with an amplitude of up to 30% and a time scale of several months to over a year. This may account for the long term variability of the high-energy (and radio) luminosity observed from Sgr A*. Although the average accreted angular momentum is approximately zero, these instabilities also induce fluctuations in the specific angular momentum of the accreted gas that lead to a circularized flow at distances less than about 20 rg. Optically thick emission from this "disk" (roughly the size of Mercury's orbit) is the origin of the IR flux recently detected from the location of Sgr A*, but is not itself a significant source of high-energy emission. Published by Elsevier Science on behalf of COSPAR.
• Melia, F., & Hollywood, J. M. (1997). GENERAL RELATIVISTIC EFFECTS ON THE INFRARED SPECTRUM OF THIN ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI: APPLICATION TO SAGITTARIUS A*. Astrophysical Journal Supplement Series, 112(2), 423-455. doi:10.1086/313036
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  The possibility that some portion of the infrared (IR) radiation emanating from active galactic nuclei (AGNs) may arise from disklike structures of ionized plasma accreting onto massive black holes motivates the investigation of the effects of strong gravitational fields in the vicinity of emitting particles on the observed radiation. Numerous previous studies have been incomplete in several respects: (1) they have neglected to take into account the observed specific power flux contribution of radiation emitted from the underside of the disk and gravitationally lensed into the upper half-hemisphere, (2) they have considered only a limited range of observing positions and hole spins, and (3) many have been restricted to examination of the steady state flux arising from homogeneous disks. The present study develops, within the context of the optically thick, geometrically thin accretion disk model, a set of new calculational techniques based on an analysis of the algebraic properties of the effective potential functions governing photon propagation in the Kerr metric; ancillary techniques, such as that of extended images, are introduced and employed to illustrate aspects of general relativistic image formation that affect the observed time-dependent flux arising from a thermally inhomogeneous accretion disk. The contribution of the first-orbit disk images, including the effects of disk self-blocking, to the observed flux are fully taken into account for the entire range of observing positions and hole spins for both the steady state and time-dependent cases. The procedure is illustrated by application of the results to the paradigm case of the Galactic center black hole candidate Sagittarius A*. Current observations are somewhat contaminated because of poor angular resolution, making this exercise still only an illustrative examination of the method. However, the future deployment of the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on HST should provide data with sufficient sensitivity for direct comparison with our calculated K-band fluxes and light curves. Of particular interest in this comparison is the expected reversal of the disk's angular momentum in Sgr A* on a timescale of 10 yr or so. We discuss the distinct spectral signature of a retrograde disk versus that of a prograde configuration and demonstrate the feasibility of observing this transition.
• Melia, F., & Kowalenko, V. (1997). Dissipation of Turbulent Magnetic Fields in Accreting Black Holes. International Astronomical Union Colloquium, 163, 749-750. doi:10.1017/s0252921100043773
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  Calculations of the spectrum resulting from accretion onto a massive black hole often make use of the “equipartition assumption” in order to estimate the magnetic field intensity. Thus, the mechanism for the dissipation of the magnetic field and the resulting dynamical influence on the gas have not been treated quantitatively, nor self-consistently. Here, we introduce an alternative approach for modelling the magnetic field dissipation from basic principles, using current ideas about turbulent fields and tearing mode instabilities.
• Misra, R., & Melia, F. (1997). The evolution of the optically thick accretion disk in nova muscae. Astrophysical Journal Letters, 484(2), 848-854.
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  Abstract: We here model the soft X-ray flux from the black hole X-ray nova, GS 1124-68 (Nova Muscae, GRS 1124-68) as emission from an optically thick accretion disk with Comptonization. Our results suggest that the disk's inner edge has a radius the dependence of which on the accretion rate is a power law, and that the slope of this function changes between the different spectral states. This disk configuration retains a hot inner region, which can account for the hard X-rays, although its structure appears to be different during the soft and ultra-soft states. © 1997. The American Astronomical Society. All rights reserved.
• Misra, R., Chitnis, V. R., Melia, F., & Rao, A. R. (1997). A transition disk model fit for the broadband X-ray spectrum of cygnus X-1. Astrophysical Journal Letters, 487(1 PART I), 388-395.
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  Abstract: The broadband (2-500 keV) data for Cygnus X-l from observations by EXOSAT, OSSE, and the XMPC balloon, have been fitted to the transition disk model. In this model the emission is from the inner region of an accretion disk where the temperature is a rapidly varying function of radius and the radiative mechanism is saturated Comptonization. We fit the data to an empirical model and obtain the temperature profile that would give rise to the observed spectrum. Then we solve for the disk structure using this profile and show that the analysis is self-consistent. An advantage of this method is that the viscosity mechanism need not be specified. We find that the transition model spectrum seems to be a better fit compared to a power law with exponential cutoff. In particular, a second component (with peak around 100 keV) that has been used in the past to explain the spectrum is not required here. We emphasize the need to conduct simultaneous broadband observations of this source in order to test ideas such as those presented here. © 1997. The American Astronomical Society. All rights reserved.
• Nayakshin, S., & Melia, F. (1997). Big blue bump and transient active regions in seyfert galaxies. Astrophysical Journal Letters, 484(2 PART II), L103-L106.
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  Abstract: An important feature of the EUV spectrum (known as the Big Blue Bump, hereafter BBB) in Seyfert galaxies is the narrow range in its cutoff energy Ec from source to source, even though the luminosity changes by 4 orders of magnitude. Here we show that if the BBB is due to accretion disk emission, then in order to account for this "universality" in the value of Ec, the emission mechanism is probably optically thin bremsstrahlung. In addition, we demonstrate that the two-phase model with active regions localized on the surface of the cold disk is consistent with this constraint if the active regions are very compact and are highly transient, i.e., they evolve faster than one dynamical timescale. © 1997. The American Astronomical Society. All rights reserved.
• Nayakshin, S., & Melia, F. (1997). Magnetic flares and the observed τ_t ∼ 1 in seyfert galaxies. Astrophysical Journal Letters, 490(1 PART II), L13-L16.
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  Abstract: We here consider the pressure equilibrium during an intense magnetic flare above the surface of a cold accretion disk. Under the assumption that the heating source for the plasma trapped within the flaring region is an influx of energy transported inward with a group velocity close to c, e.g., by magnetohydrodynamic waves, this pressure equilibrium can constrain the Thomson optical depth, ΤT, to be of order unity. We suggest that this may be the reason why τT ∼ 1 in Seyfert galaxies. We also consider whether current data can distinguish between the spectrum produced by a single X-ray-emitting region with τT ∼ 1 and that formed by many different flares spanning a range of τT. We find that the current observations do not yet have the required energy resolution to permit such a differentiation. Thus, it is possible that the entire X-ray/γ-ray spectrum of Seyfert galaxies is produced by many independent magnetic flares with an optical depth of 0.5 < τT < 2. © 1997. The American Astronomical Society. All rights reserved.
• Fatuzzo, M., & Melia, F. (1996). An in-depth study of magnetospheric emission in soft gamma-ray repeaters. Astrophysical Journal Letters, 464(1 PART I), 316-331.
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  Abstract: The association of soft gamma-ray repeaters (SGRs) with supernova remnants strongly suggests that these transient events are produced in young neutron star environments, thereby supporting the view that SGR bursts result when these stars undergo structural adjustments. Observations at X-ray and radio wavelengths of the plerions which surround two of the three known SGR sources suggest that bursts are accompanied by an outflow of energized particles. In our previous work, we showed that crustal disturbances associated with glitching activity occurring on a neutron star excite a spectrum of outwardly propagating sheared Alfvén waves. Since these waves load and energize the stellar magnetosphere, an ensuing mildly relativistic stellar wind is produced. This preliminary work reproduced some of the key SGR burst features. We extend here our study of the underlying physical processes and apply our results to the three known SGR sources, thereby determining the magnetospheric conditions during bursting events. The results of our calculations allow us to determine whether SGR bursts can have an observable impact on their surrounding plerion environments. In light of the similarity between G10.0-0.3 and the Crab Nebula, we consider the possibility that glitching activity on the Crab pulsar produces SGR-like bursts and show that the wisp activity correlated with a 1969 glitching event can be explained within the framework of our model. If this scenario is correct, then a neutron star glitch, which we associate with an SGR event, can indeed produce observable flux changes in the surrounding medium, but apparently only if the total released energy is ≳1041 ergs for a source distance of ∼1.5 kpc. Although the numerical estimates that lead to this conclusion are uncertain, they nonetheless point to a potentially interesting observational signature that could support the glitch and sheared Alfvén wave scenario developed here. © 1996. The American Astronomical Society. All rights reserved.
• Haller, J. W., & Melia, F. (1996). Inferring spherical mass distributions using the projected mass estimator. Astrophysical Journal Letters, 464(2 PART I), 774-787.
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  Abstract: Various workers have applied the projected mass estimator (PME) to infer the radial mass distribution M(r) in stellar systems ranging in scale from the Galactic center to dwarf spheroidals galaxies and globular cluster systems of external galaxies. The PME was originally used to infer the total mass of stellar systems according to the relation M = f〈νz,i2r⊥,i〉/G, where the factor f depends on the stellar orbit distribution (e.g., isotropic) and how coextensive the mass distribution is compared to the tracer population. We examine the general expression of the PME for a spherically symmetric mass distribution and an arbitrary sampling volume. For a cylinder centered on the distribution, which corresponds to the observational case of evaluating the PME within apertures of increasing radius R, boundary terms that arise from the finite sampling volume make appreciable contributions when R → 0. Numerical calculations and Monte Carlo simulations demonstrate that the PME can overestimate M(r) by factors of at least 3-4 inside the core of a self-gravitating distribution. More importantly, the functional form of the PME as R → 0 is attributable to the normalization f = f(R), not the function M(r). Analytic "η-models" show that the PME can infer the presence of a compact object at the center of a stellar distribution only when its mass greatly exceeds the mass of the cluster. Attempts to overcome volume incompleteness by computing the PME within a series of concentric annuli are also examined. The precise term that gives rise to M(r) in the case of spherical sampling cancels out when the tracer is sampled in concentric annuli. What is measured is an average of M(r) along the line of sight, weighted by the local tracer density and velocity dispersion gradients. The PME often yields results that resemble M(r), but this is not precisely what is being measured. The generalized estimator can be compared with observations if one has some model of the mass distribution and the tracer population. © 1996. The American Astronomical Society. All rights reserved.
• Melia, F. (1996). A hydrodynamic model for the formation of the galactic center "minicavity". Astrophysical Journal Letters, 460(1 PART II), L33-L36.
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  Abstract: The minicavity in the streamer to the southwest of the dynamical center of the Galaxy and the unique, nonthermal radio source Sgr A appear to be physically connected by a chain of plasma "blobs" that may be in transit from the latter to the former. Recent observations at radio and infrared wavelengths have provided clear evidence that the cavity has an opening radius of ∼1″ ≈ 0.04 pc and radiates with a power Lcav ≈ 6 X 1037 ergs s-1. Here, we model the formation of the cavity as the result of an impact between the streamer gas and the post-bow shock gas collimated by a massive (∼106 M⊙) black hole coincident with Sgr A. Multiwavelength observations, including those of He I, Brα, and Brγ line emission in the inner ∼0.3 pc region of the Galaxy provide strong evidence for the presence of an ambient Galactic center wind with velocity vw ≈ 500-700 km s-1 and mass-loss rate ∼ 3-4 × 10-3 M⊙ yr-1. We show that the Bondi-Hoyle process responsible for the accretion of ∼ 1022 g s-1 by Sgr A also produces a downstream, focused flow with a radius very similar to that of the minicavity and a mechanical luminosity about 2.5 larger than Lcav. In addition, the size and density of the blobs appear to be consistent with the gas characteristics in this flow. © 1996. The American Astronomical Society. All rights reserved Printed in U.S.A.
• Misra, R., & Melia, F. (1996). A transition disk model for cygnus X-1. Astrophysical Journal Letters, 467(1 PART I), 405-418.
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  Abstract: The standard cold accretion disk theory for black hole systems such as Cygnus X-1 is not self-consistent when the effective optical depth is less than unity. In this paper, we develop a scheme for solving the disk structure that is valid at all optical depths. We show that the disk typically consists of three regions: (a) an outer cold disk, (b) a middle transition zone, and (c) an inner two-temperature disk. The emergent spectrum from the transition region is due primarily to a sum of Wien peaks instead of the soft-photon Comptonization within a geometrically thick, hot inner region as proposed in earlier models, and it matches well with the observed X-ray power-law continuum from these sources. In the picture developed here, we include the possibility of the presence of a magnetic field having an equipartition pressure with the plasma. We suggest that variations in this magnetic field and the corresponding viscosity parameter a can account for the two long-term X-ray states of Cygnus X-1. The transition disk region is subject to both thermal and secular instabilities, but it appears that these do not transform the disk into a two-temperature configuration (Taam & Lin 1984). Instead, we speculate that these instabilities give rise to structural fluctuations which we observe as quasi-periodic oscillations. © 1996. The American Astronomical Society. All rights reserved.
• Misra, R., & Melia, F. (1996). The crucial effects of advection on the structure of hot accretion disks. Astrophysical Journal Letters, 465(2 PART I), 869-873.
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  Abstract: Optically thick, radiation pressure-dominated accretion disks are known to undergo a secular instability and to evolve into optically thin configurations that can account for the gross X-ray and γ-ray characteristics of black hole systems such as Cyg X-1 and 1E 1740.7-2942. Recent analyses have shown that in the inner, bremsstrahlung self-Comptonized portions of these disks, an electron-positron pair runaway can occur above a critical accretion rate. However, these studies have mostly ignored the impact of advection on the structure of the hot disk. Ongoing observations of these sources by, e.g., the Compton Gamma Ray Observatory, and the upcoming timing studies of X-ray novae (such as Nova Muscae) with XTE, are motivating efforts to better understand the physics of these systems. In this paper, we include the crucial effects of proton thermal energy advection and show that the disk structure is modified substantially. Pair runaway seems to be completely suppressed. Instead we confirm the existence of a physically different critical accretion rate above which no self-consistent steady state solution exists. We suggest that the hot disk is probably dynamic above this rate, which may provide observationally significant timing signatures. © 1996. The American Astronomical Society. All rights reserved.
• Hollywood, J. M., & Melia, F. (1995). The effects of redshifts and focusing on the spectrum of an accretion disk in the Galactic center black hole candidate sagittarius A. Astrophysical Journal Letters, 443(1 PART 2), L17-L20.
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  Abstract: There are firm indications that Sgr A*, a compact, nonthermal radio source at the Galactic center, may be powered by the dissipation of gravitational energy as gas trapped from an ambient wind descends down the potential well, first through a quasi-spherical inflow (extending out to ∼3 × 1016 cm) and then through a small accretion disk at ≲5-10 Schwarzschild radii. Earlier three-dimensional Bondi-Hoyle numerical simulations have indicated that fluctuations in the accreted specific angular momentum can lead to a variability in the disk flux on a timescale of years. With greatly improved flux measurements at K and H, and the hint of a ∼10 minute modulation in the IR luminosity, it is crucial to model the disk emission much more precisely than has been attempted thus far. In this Letter we take into account the effects of Doppler and gravitational redshifts, the light-travel time factor, and the light bending near the black hole to determine the measurable spectrum of Sgr A* in the increasingly important 1013 Hz ≲ v ≲ 1016 Hz frequency range. We find that the relativistic disk spectrum is much softer than its Newtonian counterpart, with a predicted UV flux roughly an order of magnitude smaller than had previously been anticipated. In addition, we find that when the physical conditions in the disk are taken to be consistent with the properties of the quasi-spherical infall (specifically, in terms of the accretion rate and disk size), only a slowly spinning or Schwarzschild black hole appears to fit the observations. Our calculations also reveal that the disk flux is much more weakly dependent on the observer's inclination angle than had been suspected on the basis of earlier Newtonian estimates.
• Mccarthy, D. W., Melia, F., Mccarthy, D. W., & Close, L. M. (1995). Infrared photometry of the black hole candidate Sagittarius A. The Astrophysical Journal, 439(2), 682-686. doi:10.1086/175207
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  An infrared source has been imaged within 0.2 +/- 0.3 arcseconds of the unique Galactic center radio source Sgr A* High angular resolution (averaged value of the Full Width at Half Maximum (FWHM) approximately 0.55 arcseconds) was achieved by rapid (approximately 50 Hz) real-time images motion compensation. The source's near-infrared magnitudes (K = 12.1 +/- 0.3, H = 13.7 +/- 0.3, and J = 16.6 +/- 0.4) are consistent with a hot object reddened by the local extinction A(sub v) approximately 27). At the 3 sigma level of confidence, a time series of 80 images limits the source variability to less than 50% on timescales from 3 to 30 minutes. The photometry is consistent with the emission from a simple accretion disk model for a approximately 1 x 10(exp 6) solar mass black hole. However, the fluxes are also consistent with a hot luminous (L approximately 10(exp 3.5) to 10(exp 4-6) solar luminosity) central cluster star positionally coincident with Sgr A*.
• Mccarthy, D. W., Melia, F., Mccarthy, J. D., Mccarthy, D. W., Hollywood, J. M., Dekeyser, T. A., & Close, L. M. (1995). GENERAL RELATIVISTIC FLUX MODULATIONS IN THE GALACTIC CENTER BLACK HOLE CANDIDATE SAGITTARIUS A. The Astrophysical Journal Letters, 448(1), L21-L26. doi:10.1086/309584
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  The proximity of the unique radio source Sgr A* at the Galactic center is allowing us to make unprecedented observations of the spectral formation region near the event horizon of a massive black hole. Near-infrared flux measurements hint at a possible 10.4 minute modulation period in this source, which suggests an emission surface with a Keplerian velocity near the last stable orbit of a 106 M☉ Schwarzschild object. In this Letter, we set up the fully general relativistic framework for calculating such modulations, which we suggest might arise from orbiting disk inhomogeneities. The application of our method to the currently available data shows excellent agreement between the calculated light curve and the observed temporal profile, promising exciting future observations of Sgr A* with NICMOS on Hubble Space Telescope.
• Melia, F., & Fatuzzo, M. (1995). Physics of magnetospheric emission in soft gamma-ray repeaters. Astrophysical Journal Letters, 438(2), 904-907.
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  Abstract: Soft gamma-ray repeaters appear to be relatively young, strongly magnetized neutron stars embedded within plerions. The observationally inferred presence of teragauss fields and energetic outflows in these sources provides some support for a model in which the bursts results from crustal disturbances that load and energize the magnetosphere with the ensuing generation of sheared Alfvén waves. We here determine accurately the structure of the particle efflux and calculate the spectrum of the resultant radiation, incorporating the key effects due to mildly relativistic magnetic bremsstrahlung, a frequency-dependent photospheric radius and angle-dependent boosting of the intensity for emitting elements at different inclinations. We find that the average photospheric radius during the transient event is ∼4 stellar radii and that the particles attain a Lorentz factor ∼13.2 by the time the gas becomes optically thin. The calculated spectrum is a weighted integral over the boosted intensity determined from the physical conditions (i.e., magnetic field, particle density, and temperature) at the photospheric radius corresponding to the relevant frequency. When applied to SGR 1806-20, this procedure yields a distance D ≈ 10 kpc to this source, which appears to be consistent with the range (∼8-17 kpc) inferred from its apparent coincidence with the supernova remnant G10.0-0.3.
• Misra, R., & Melia, F. (1995). Hot accretion disks with electron-positron pair winds. Astrophysical Journal Letters, 449(2), 813-825.
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  Abstract: It is well known that optically thick, geometrically thin accretion disks in black hole systems are unstable in their inner region. Evolving into hot, optically thin configurations, these environments can easily account for the gross features of the X-ray and γ-ray emission from sources such as Cygnus X-1 and 1E 1740.7-2942. Recent analysis of the stability criteria for such disks, however, have pointed to a critical accretion rate ṁcr above which electron-positron pair creation exceeds annihilation, and a runaway ensues, rendering the two-temperature corona unstable. On the assumption that they are 10 solar mass black holes, both these sources appear to be close to ṁcr - indeed, 1E 1740.7-2942 seems to exceed it. Knowing where objects such as this lie with respect to the stability cutoff is essential not only in terms of understanding the physical conditions near the central black hole, but because these systems are also prodigious sources of outflowing, energetic positrons the contribution to the pair flux from particles escaping the corona can be substantial when ṁ is close to ṁcr. However, drawing definite conclusions from the currently available analysis is premature given the approximate nature of the earlier studies. In this paper, we refine these analyses by employing a more accurate treatment of the Comptonization process throughout the corona, and we also include the energy loss associated with the escaping pairs in the overall energy balance equation. The pair outflux depends critically on the velocity of the escaping particles, which we here consider in the two extreme values of zero and the thermal velocity. We find that the combined effect of these improvements results in an overall increase in ṁcr by a factor of ∼0.5-3 when the pairs escape with the thermal velocity, but that the improved treatment of the Comptonization actually decreases ṁcr by about 30% when this velocity is zero. An application to Cygnus X-1 (assuming a mass of 10 M⊙) shows that this source is supercritical if the velocity of the escaping pairs is zero, but it is subcritical when the escaping pair flux is maximal. In the latter case, as much as ∼50% of the power emitted by the corona is carried off by escaping electron-positron pairs. Under some conditions, this positron number flux can account for the annihilation line radiation and the extended radio jets observed in 1E 1740.7-2942.
• Haller, J. W., & Melia, F. (1994). Near-infrared observations of the 1E 1740.7-2942 field at the galactic center. Astrophysical Journal Letters, 423(2 PART 2), L109-L112.
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  Abstract: 1E 1740.7-2942 is thought to be a Cygnus X-1-like black hole near the Galactic center, although several important questions regarding its nature and possible connection to the narrow e+-e- annihilation line seen from this region remain unanswered. Here we report on HK photometry of this field which shows that the distribution of near-infrared stars is consistent with a population lying near the Galactic center similar to that of the late-type Baade's window population with an average foreground extinction AV = 30 ± 5 mag and a substantially greater value of AV = 55 ± 5 mag associated with stars centered at the position of the molecular cloud enshrouding 1E 1740.7-294.2. We argue that the known distribution of bulge stars and the observed "shadowing" by the cloud constrain its position to lie within ∼300 pc of the Galactic center. We show that the implied total hydrogen column density in this direction is (1.05 ± 0.53) × 1023 atoms cm-2, of which (0.48 ± 0.24) × 1023 cm-2 is attributable to the cloud itself. This is consistent with the value required to account for the attenuation in the 4-300 keV spectrum of 1E 1740.7-2942 in its standard state, and we therefore conclude that 1E 1740.7-2942 is at least as distant as the molecular cloud. The total mass of the observed obscuring material associated with the cloud is (4.6 ± 2.3) × 103 M⊙, an order of magnitude below that derived under the assumption that the molecular cloud is predominantly composed of gas traced by HCO+ emission with an average number density greater than 5 × 104 atoms cm-3. The corresponding average cloud density of ∼5 × 103 cm-3 appears to be inconsistent with the value required to account for the narrow electron-positron line flux variations if the leptons trace out straight-line trajectories.
• Melia, F. (1994). An accreting black hole model for sagittarius A*. II. a detailed study. Astrophysical Journal Letters, 426(2), 577-585.
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  Abstract: Sgr A* is a unique, compact radio source at the Galactic center whose characteristics suggest that it may be a massive (i.e., ∼ 106 M⊙) black hole accreting from an ambient wind in that region. Earlier (simplified) calculations suggested that its 108-1020 Hz spectrum could be derived from bremsstrahlung and magnetic bremsstrahlung emission from plasma descending toward the event horizon at a rate of roughly 1022 g s-1. Here, we introduce several significant improvements to the model, including (1) an exact treatment of the cyclotron/ synchrotron emissivity that is valid for all temperatures, (2) the actual determination of the temperature distribution in the inflow, and (3) the effect on the spectrum should the accreting plasma have a residual angular momentum, possibly forming a disk at small radii. We find that the most likely value of the mass in this improved model is ≈2 ± 1 × 106 M⊙, close to the range inferred earlier, but about a factor of 2 greater than the previous "best-fit" number. The main reason for this difference is that the more realistic (new) formulation of the magnetic bremsstrahlung emissivity has fluctuations with frequency that decrease the overall line-of-sight intensity, thereby pointing to a slightly larger mass in order to account for the observed spectrum. We also find that a slight excess of angular momentum in the accreting gas may be necessary in order to account for the IR luminosity from this source. Such an excess is consistent with the results of ongoing three-dimensional simulations that will be reported elsewhere.
• Ruffert, M., & Melia, F. (1994). Hydrodynamical 3D Bondi-Hoyle accretion onto the Galactic Center blackhole candidate SGR A*. Astronomy and Astrophysics Letters, 288, L29-L32.
• Yusef-Zadeh, F., Cotton, W., Wardle, M., Melia, F., & Roberts, D. A. (1994). Anisotropy in the angular broadening of Sagittarius A* at the Galactic center. Astrophysical Journal Letters, 434(2 PART 2), L63-L66.
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  Abstract: We present the results of a λ = 20 cm VLA2 observation of the compact Galactic center radio source Sgr A*. The scatter-broadened image is elongated in the east-west direction, with an axial ratio of 0.56 ± 0.22 and a position angle of 82° ± 1°.8. A similar shape and orientation has been found previously at shorter wave-lengths using VLBI and VLBA. Both the major and minor axes follow the λ2 law appropriate for scattering by turbulence in the intervening medium. We argue that the scattering occurs in thin ionized surface layers of molecular clouds lying in the central 100 pc of the Galaxy. The ionized regions are characterized by electron densities of 104 cm-3 and milligauss magnetic field strengths, and are responsible for the diffuse free-free emission and the Faraday rotation in the region.
• Fatuzzo, M., & Melia, F. (1993). A detailed study of sheared Alfvén waves and their possible application to gamma-ray bursts. Astrophysical Journal Letters, 407(2), 680-686.
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  Abstract: Strong particle acceleration in neutron-star environments may result in the upscattering of soft photons to gamma-ray energies. Sheared Alfvén waves, which may be generated by several plausible disturbances in the stellar crust, can energize the charges to Lorentz factors in excess of 105. However, a full understanding of the physics of these MHD waves is lacking due to the complications arising from the charge separation within the sheared region. Here, we present a detailed study of this mechanism, with particular attention to a comparison of the nonrelativistic and relativistic domains. We find that the charge separation is consistent with our previous classical MHD solutions and that it provides a natural description of the sheared current via a relativistic flow. These waves can apparently produce the particle distributions that are necessary to produce the γ-ray fluxes observed in gamma-ray bursts, regardless of whether the sources are Galactic or extragalactic.
• Fatuzzo, M., & Melia, F. (1993). Focusing of alfvénic wave power in the context of gamma-ray burst emissivity. Astrophysical Journal Letters, 414(2 PART 2), L89-L92.
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  Abstract: Highly dynamic magnetospheric perturbations in neutron star environments can naturally account for the features observed in gamma-ray burst spectra. The source distribution, however, appears to be extragalactic. Although noncatastrophic isotropic emission mechanisms may be ruled out on energetic and timing arguments, MHD processes can produce strongly anisotropic γ-rays with an observable flux out to distances of ∼1-2 Gpc. Here we show that sheared Alfvén waves propagating along open magnetospheric field lines at the poles of magnetized neutron stars transfer their energy dissipationally to the current sustaining the field misalignment and thereby focus their power into a spatial region ∼1000 times smaller than that of the crustal disturbance. This produces a strong (observable) flux enhancement along certain directions. We apply this model to a source population of "turned-off" pulsars that have nonetheless retained their strong magnetic fields and have achieved alignment at a period of ≳5 seconds.
• Melia, F., & Fatuzzo, M. (1993). Understanding soft gamma-ray repeaters in the context of the extragalactic radio pulsar origin of gamma-ray bursts. Astrophysical Journal Letters, 408(1 PART 2), L9-L12.
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  Abstract: Both the soft gamma-ray repeaters (SGRs) and the classical gamma-ray burst (GRB) sources may be neutron stars undergoing structural adjustments that produce the observed transient γ-ray events. We propose a unified scenario, in which a cosmological population of young radio pulsars is responsible for both phenomena, and we argue that whether the radiative emission associated with a pulsar "glitch" is seen as a GRB or as an SGR event, is determined primarily by the direction of our line of sight relative to the stellar spin axis. We show that whereas the classical GRB spectrum arises from Compton upscattering by charges accelerated along the viewing direction, the SGR burst spectrum is due to the thermalization of Alfvén wave energy away from this direction and is roughly that of a blackbody, whose characteristic temperature is correlated to the burst luminosity. Within the context of this model, the difference in time histories between the two classes results from selection effects, since their detection probabilities imply that we are far more likely to see nearby, weak (and presumably short) events as SGR bursts and the more distant, powerful, and longer events as GRBs. The crudely inferred source distance to SGR 0526-66 and SGR 1806-20 is ≈ 170 kpc and ≈ 18 kpc, respectively, consistent with their apparent positioning within the LMC (the former) and the Galactic plane (the latter). If the crustal adjustments occur during the first ∼ 50,000 yr of a pulsar's lifetime, this model predicts that we should be able to see approximately two SGR sources within the galaxy, in agreement with current observations.
• Melia, F., & Misra, R. (1993). A self-consistent model for the long-term gamma-ray spectral variability of Cygnus X-1. Astrophysical Journal Letters, 411(2), 797-801.
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  Abstract: The long-term transitions of the black hole candidate Cygnus X-1 (between the states γ1, γ2, and γ3) include the occasional appearance of a strong ∼ MeV bump (γ1), whose strength appears to be anticorrelated with the continuum flux (≲ 400 keV) due to the Compton upscattering of cold disk photons by the inner, hot corona. We develop a self-consistent disk picture that accounts naturally for these transitions and their corresponding spectral variations. We argue that the bump is due to the self-Comptonization of bremsstrahlung photons emitted predominantly near the plane of the corona itself. Our results suggest that a decrease by a factor of ≈ 2 in the viscosity parameter α is responsible for quenching this bump and driving the system to the γ2 state, whereas a transition from γ2 to γ3 appears to be induced by an increase of about 25% in the accretion rate Ṁ. In view of the fact that most of the transitions observed in this source seem to be of the γ2-γ3 variety, we conclude that much of the long-term gamma-ray spectral variability in Cygnus X-1 is due to these small fluctuations in Ṁ. The unusual appearance of the γ1 state apparently reflects a change in the dissipative processes within the disk.
• Misra, R., & Melia, F. (1993). Formation of a jet in the Galactic center black hole candidate 1E 1740.7-2942. Astrophysical Journal Letters, 419(1 PART 2), L25-L28.
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  Abstract: The Galactic center object IE 1740.7-2942 is apparently a source of e+e- annihilation radiation and has recently been associated with a double-sided radio jet at λ = 6 and λ = 20 cm. We here extend our earlier model for the y-ray continuum emission to encompass the formation of the jet, and discuss the ensuing jet structure. We find that due to γγ interactions in the intense radiation field produced within the inner ∼20 Schwarzschild radii (rg) of the accretion disk, more than 5 × 1043 pairs s-1 stream outward and are accelerated by Compton scatterings to velocities as high as ∼0.7c. Roughly 90% of these pairs annihilate at the base of the nascent jet (within ∼40 rg of the black hole) forming the broad (∼240 keV) annihilation line observed by SIGMA on GRANAT. The remainder (≲6 × 1042 pairs s-1) escape into the enshrouding cloud and beyond, and presumably contribute to the formation of both the steady and the time-variable components of the narrow annihilation line observed from the bulge. However, before the pairs annihilate, they radiate via synchrotron emission as they spiral in the cloud's ambient magnetic field. We show that the physical conditions required to account for the broad γ-ray line seen in the high state and the implied conditions in the normal state are consistent with the pair density and velocity required to form the large-scale jet, the jet's shape, and its radio spectrum and luminosity.
• Tamblyn, P., & Melia, F. (1993). Reconciliation of the disparate gamma-ray burst catalogs in the context of a cosmological source distribution. Astrophysical Journal Letters, 417(1 PART 2), L21-L24.
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  Abstract: It is well-known that gamma-ray burst spectra often display a break at energies ≲400 keV, with some exceptions extending to several MeV. Modeling of a cosmological source population is thus nontrivial when comparing the catalogs from instruments with different energy windows since this spectral structure is redshifted across the trigger channels at varying levels of sensitivity. We here include this important effect in an attempt to reconcile all the available data sets and show that a model in which bursts have a "standard" spectral break at 300 keV and occur in a population uniformly distributed in a q0 = 1/2 universe with no evolution can account very well for the combined set of observations. We show that the source population cannot be truncated at a minimum redshift zmin beyond ∼0.1, and suggest that a simple follow-on instrument to BATSE, with the same trigger window, no directionality and 18 times better sensitivity might be able to distinguish between a q0 = 0.1 and a q0 = 0.5 universe in 3 years of full sky coverage, provided the source population has no luminosity evolution.
• Melia, F. (1992). An accreting black hole model for sagittarius A. Astrophysical Journal Letters, 387(1 PART 2), L25-L28.
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  Abstract: Several observations, notably of broad He I, Brα, and Brγ emission lines from the vicinity of IRS 16, indicate the presence of a strong circumnuclear wind near the dynamical center of the Galaxy. Sgr A*, a hypothesized supermassive object situated ≈0.06 pc to the west of IRS 16, should be accreting from this wind if it is not itself a source of gaseous outflow, for which there is currently no observational evidence. Here, we calculate the spectrum and flux of radiation resulting from this process, and show that they are consistent with the data over at least 12 decades of frequency. Together with the kinematic studies of the stellar and gas distributions in this region, our model argues strongly in favor of Sgr A * being a ≳ 106 M⊙ black hole.
• Melia, F. (1992). The nucleus of M31. Astrophysical Journal Letters, 398(2 PART 2), L95-L98.
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  Abstract: High-resolution measurements of the rapid rotation and large velocity dispersion in the nucleus of the Andromeda nebula (M31) hint at the possible influence of a massive black hole in that region. The recent detection of an unresolved radio source coincident with the dynamical center of the galaxy has evoked a direct comparison of M31's nucleus with the compact, nonthermal radio source Sgr A* at the center of our own Galaxy. We calculate the broad-band spectrum expected of a 107 M⊙ black hole accreting from an ambient galactic wind in the nucleus of M31, and we discuss several testable predictions of this model: (1) when resolved, this source will probably be elongated, perhaps in the direction of the semistellar nucleus BD +40° 148; (2) its radio spectral index is expected to be ∼0.4 (compared with ∼0.25 for Sgr A*); (3) the radio component should steepen sharply between ∼1010 and 1011 Hz; (4) if an association between this radio source and a nearby Einstein X-ray source is confirmed, its radio luminosity should be variable on a time of ≲1 yr; and (5) the variability of its radio and X-ray fluxes should be anticorrelated.
• Melia, F., & Fatuzzo, M. (1992). Gamma-ray bursts from extragalactic radio pulsars. Astrophysical Journal Letters, 398(2 PART 2), L85-L88.
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  Abstract: The distribution of gamma-ray bursts (GRBs) detected by BATSE, in combination with the spectral data gathered over the past two decades, argue for a cosmological population of neutron star sources. We demonstrate that GRB spectra may be understood in the context of the Compton upscattering of typical radio pulsar spectra, which are steep (i.e., having a flux density Fv ∼ να with α ∼ -1 to -4) above a low-frequency cutoff νmin at ≲500 MHz. We suggest that the existence of νmin is the principal reason why GRB spectra often exhibit a break εbreak at ∼0.2-3 MeV. Due to magnetic suppression, the scattering cross section for the radio photons ∝ν2. As such, this model predicts a γ-ray power spectral index μ ∼ α + 2 + 1 above the break, though the prevalence of steeper radio spectra in the brightest pulsars implies a biasing of μ toward the bottom of this range. We find that the probability of detecting a burst in progress from any given source is ≈5 ×10-11, implying an individual stellar burst rate of about 1 every 50 yr if all active pulsars are involved, or about 1 every 2-3 yr for the very young members of this class. In addition, the energy released per burst ranges from ∼1041 ergs to ∼5 × 1043 ergs. This rate and energy release coincide with those pertaining to the macro- and microglitches seen in the periods of many such sources. We conclude that GRBs may simply be the crustal adjustments responsible for the now familiar timing noise observed in young pulsars. A simple (though important) requirement of this model - that εbreak should be correlated with the burst luminosity - seems to be in agreement with the recent analysis of SIGNE data.
• Melia, F., & Zylstra, G. J. (1992). An interpretation of the multipeaked structure in X-ray bursts. Astrophysical Journal Letters, 398(1 PART 2), L53-L56.
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  Abstract: Some X-ray bursts from sources such as 4U/MXB 1636-53, 4U 1608-52, and possibly MXB 1728-34, display a multipeaked structure in the bolometric flux that is difficult to reconcile with the standard thermonuclear flash model or with the effects due to photospheric expansion and contraction. Understanding this phenomenon is important primarily because it might be due to a variable Thomson optical depth along a line of sight threading the burst-induced accretion disk corona. As such, multipeaked bursts offer us some hope of learning more about the still poorly understood accretion disk geometry in these systems. Here, we explore the conditions under which these burst profiles might be observed, in light of the somewhat surprising recent results from the radiative-hydrodynamic simulation of the coronal structure. We find that double- (and triple-) peaked bursts are produced at inclination angles 40° ≲ i ≲ 65°. Interestingly, the orbital information for 4U/MXB 1636-53 and 4U 1608-52 suggests i < 70°, in these sources. The orbital inclination of MXB 1728 - 34 is not yet known.
• Melia, F., Jokipil, J. R., & Narayanan, A. (1992). A determination of the mass of Sagittarius A * from its radio spectral and source size measurements. Astrophysical Journal Letters, 395(2 PART 2), L87-L90.
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  Abstract: There is growing evidence that Sgr A* may be a million solar mass black hole accreting from the Galactic center wind. A consideration of the spectral and source size characteristics associated with this process can offer at least two distinct means of inferring the mass M, complementing the more traditional dynamical arguments. We show that M is unmistakably correlated with both the radio spectral index and the critical wave-length below which the intrinsic source size dominates over the angular broadening due to scattering in the interstellar medium. Current observations can already rule out a mass much in excess of 2 × 106 M⊙ and suggest a likely value close to 1 × 106 M⊙, in agreement with an earlier study matching the radio and highenergy spectral components. We anticipate that such a mass may be confirmed with the next generation of source-size observations using milliarcsecond angular resolution at 0.5-1 cm wavelengths.
• Melia, F., Zylstra, G. J., & Fryxell, B. (1992). Radiative-hydrodynamical simulations of X-ray burst-induced accretion disk coronae. Astrophysical Journal Letters, 396(1 PART 2), L27-L30.
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  Abstract: X-ray burst sources, which contain a neutron star that explosively burns freshly accreted fuel every few hours, emit a variable X-ray flux that increases by an order of magnitude or more during the transient event. These are ideal systems in which to explore the structure of the surrounding accretion disk and its response to the changing conditions at the central source. In particular, a burst-induced accretion disk corona can significantly alter the observable characteristics of the burst profile. Here we use a fully self-consistent radiative-hydrodynamical algorithm developed earlier to study the "steady" state coronae in order to simulate the response of the evaporated outflow to a time-dependent irradiation of the disk's surface. We find that the burst definition is altered at both low and high inclination angles i. At i ≳ 45°, the detectable flux is enhanced (as much as 50%) by X-rays scattered into the line of sight from the radiation propagating through the corona, and (≳ 30%) by X-rays reflected off the corona when i ≲ 45°. For observation angles i → 90°, the severe attenuation of the direct X-ray flux leads to burst characteristics not unlike those of the transient event detected in the coronal source 4U 2129+47. These include (1) an increase in the apparent rise time, (2) a peak flux considerably smaller than the presumed intrinsic one, and (3) a slow decay.
• Fatuzzo, M., & Melia, F. (1991). Transient particle acceleration in strongly magnetized neutron stars. II. Effects due to a dipole field geometry. Astrophysical Journal Letters, 382(2), 570-575.
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  Abstract: Sheared Alfvén waves generated by nonradial crustal disturbances above the polar cap of a stronngly magnetized neutron star (B ≲ 1012 gauss) induce an electric field component parallel to B. Earlier calculations assuming a uniform magnetic field showed that this mechanism could successfully accelerate particles to Lorentz factors γ ∼ (50 cm) m, where m (∼2-3 cm-1) characterizes the degree of shearing. However, magnetic field gradients in a realistic dipole field geometry cannot be ignored. Our goal here is to determine the manner in which the strong radial dependence of B affects the propagation of these sheared Alfvén waves, and whether this magnetohydrodynamic process is still an effective particle accelerator. By comparison to the field gradients, the field-line curvature is small within a stellar radius of the polar cap, and we therefore use an approximate form of the dipole field that ignores the component perpendicular to the z-axis. We find that although the general field equation is quite complicated, a simple wavelike solution can still be obtained under the conditions of interest (i.e., B ≲ 1012 G, and a particle number density ne ≲ 1026 cm-3) for which the Alfvén phase velocity → c and ne decouples from the wave equation. Our results may be applicable to γ-ray burst sources.
• Melia, F., & Fatuzzo, M. (1991). Computing the normalized dirac wave function for oblique electric and magnetic fields. Il Nuovo Cimento B, 106(3), 253-261.
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  Abstract: Interactions taking place in superstrong gravitational and magnetic fields can account for a wide variety of spectral emissions from compact objects such as neutron stars. A quantum treatment of the particle transport in these environments, e.g., needed to calculate the anisotropic electrical conductivity, depends on the evaluation of matrix elements with solutions to the Dirac equation in oblique electric (E) and magnetic (H) fields. However, due to the complexity of the bispinors, these overlap integrals are often extremely difficult to compute. Indeed, no normalized wave function yet exists. Here we show that, in the special case when E/H≲10-4, a simplification of the matrix elements permits an analytic integration that yields explicit expressions for the normalization constant and other overlap integrals. Physical quantities, such as the conductivity tensor, that depend directly on these matrix elements may then be computed in a straightforward way. © 1991 Società Italiana di Fisica.
• Melia, F., & Fatuzzo, M. (1991). Gamma-ray bursts from Sheared Alfvén waves. Astrophysical Journal Letters, 383(2 PART 2), L57-L60.
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  Abstract: Sheared Alfvén waves in strongly magnetized neutron stars are efficient particle accelerators by virtue of the large electric field components Ez they generate parallel to the unperturbed magnetic field B0 = B0 ẑ. For example, with an amplitude BA0 = 1011 G (=0.1B0) and a shear wavenumber m = 0.5 (corresponding to a crustal perturbation length scale of ≈10 cm), these waves can accelerate electrons to a Lorentz factor y ∼ 104-5 within 5 km of the stellar surface, even under the influence of a Compton drag due to the thermal emission of a warm (Tpc ≲ 1 keV) polar cap. Here, we apply this physical process to a study of gamma-ray bursts, taking into account both resonant and nonresonant scattering. We also discuss several very encouraging features of the model. (1) Although the field is oscillatory, virtually all the charges are ejected from the system, resulting in very little backheating of the stellar surface. (2) The particle number density (and hence the burst energy) is accounted for naturally in terms of BA0 and m, which in principle are known from the physical manifestation of the agent causing the crustal disturbance (e.g., a thermonuclear explosion or a starquake). (3) The resulting y-ray spectrum compares very favorably with the observations. And (4) this model restricts the geometry of the emission region, in the sense that only the Compton upscattering of soft photons from a warm polar cap (as opposed, e.g., to an isotropic source) will produce the correct spectral shape.
• Melia, F., & Fatuzzo, M. (1991). Relativistic charge currents in oblique electric and magnetic fields. Journal of Plasma Physics, 45(pt 3), 415-425.
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  Abstract: Runaway processes on neutron stars leading to the sudden release of large quantities of energy (up to of order 1040 erg) on time scales as short as a fraction of a second involve plasma heating and particle acceleration in superstrong magnetic fields H (of order 1012 G). These transient events are interesting from a theoretical standpoint because they require knowledge of particle transport properties in low-density plasmas (ne ≲ 1025 cm-3) threaded by both electric (E) and magnetic fields. The evaluation of matrix elements involving solutions to the Dirac equation for such a field configuration is often difficult and sometimes impossible, since no completely normalized wave function has yet been found. Here it is shown that, in the special case of E/H ≲ 10-4, a simplification of the overlap integrals permits an analytical integration that yields explicit expressions for the relativistic charge currents needed in the computation of the anisotropic conductivity tensor when E.H ≠ 0. The application of these results to the evaluation of the conductivity is briefly discussed. Among other things, this work is relevant to a theory of resistive magnetic tearing instabilities in a quantizing field.
• Melia, F., & Fatuzzo, M. (1991). Transient particle acceleration in strongly magnetized neutron stars. Astrophysical Journal Letters, 376(2), 673-677.
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  Abstract: Although several reasonable physical processes have been proposed to account for the initial energy release in transient events on strongly magnetized neutron stars, and although the manifestation of this activity as a burst of X- or γ-radiation evidently results from the Compton upscattering of soft photons by relativistic electrons, a self-consistent picture for the particle acceleration itself is lacking. Here, we develop a detailed model in which sheared Alfvén waves generated by the energized stellar crust (e.g., due to a starquake) accelerate the charges to large Lorentz factors. We show that for canonical stellar parameters and a burst energy ∼1037 ergs, the maximum attainable Lorentz factor is γmax ∼ 50m, where m(∼2-3 cm-1) characterizes the shear of the magnetic perturbation. This value of γmax is consistent with the particle energy required to produce the most energetic y-rays (∼10-100 MeV) observed in y-ray bursts.
• Melia, F., & Fatuzzo, M. (1991). Transverse conductivity of a relativistic plasma in oblique electric and magnetic fields. Astrophysical Journal Letters, 373(1), 198-207.
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  Abstract: Resistive tearing is a primary candidate for flares occurring in stressed magnetic fields. Its possible application to the strongly magnetized environments (Hz ∼ 1012 G) near the surface of neutron stars, particularly as a mechanism for generating the plasma heating and particle acceleration leading to gamma-ray bursts, has motivated a quantum treatment of this process, which requires knowledge of the electrical conductivity σ of a relativistic gas in a new domain, i.e., that of a low-density (ne) plasma in oblique electric [E = (0, Ey, Ez)] and magnetic fields. We discuss the mathematical formalism for calculating σ and present numerical results for the range of parameter values 109 ≤ Hz ≤ 1012 G, Ez/Hz ≲ 10-4, Ey ≲ 10-4Hz2/£2, and 1020 ≤ ne ≤ 1025 cm-3. Our results indicate that σ depends very strongly on both the applied electric and magnetic fields, and that σ ∼ Ez2 Ez/Hz2 over this range.
• Melia, F., & Fatuzzo, M. (1991). Transverse conductivity of a relativistic plasma in oblique electric and magnetic fields. Physical Review D, 43(2), 319-331.
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  Abstract: Resistive tearing is a primary candidate for flares occurring in stressed magnetic fields. Its possible application to the strongly magnetized environments (Hz1012 G) near the surface of neutron stars motivates a quantum treatment of this process, which requires knowledge of the electrical conductivity of a relativistic gas in a new domain, i.e., that of a low-density (ne) plasma in oblique electric [E=(0, Ey, Ez)] and magnetic fields. We derive the mathematical formalism for calculating and present numerical results for the range of parameter values 109Hz1012 G, EzHz10-4, Ey10-4Hz2Ez, and 1020ne1025 cm-3. We find that Ey2Ezne2Hz2 over this range. © 1991 The American Physical Society.
• Melia, F., & Zylstra, G. J. (1991). Flux-limited diffusion in a scattering medium. Astrophysical Journal Letters, 374(2), 732-740.
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  Abstract: We present a diffusion equation (FDT) with a coefficient that reduces to the appropriate limiting form in the streaming and near thermodynamic limits for a moving fluid in which the dominant source of opacity is Thomson scattering. This expression, derived from the general diffusion theory of Levermore & Pomraning and Pomraning, is fully flux-limited, and is useful for the radiative-hydrodynamical simulation of scattering media such as accretion-disk coronae. We compare our results to those obtained with the corresponding equations for an absorptive medium, and we show that the flux can differ by a large factor (≫ 1) between the two, depending on the temperature profile within the atmosphere. A direct comparison of our equations with exact solutions of the transfer equation suggests that FDT for a scattering medium is accurate to better than ≲ 17% over the range of optical depths 0 ≲τ ≲ 3.
• Melia, F., Zylstra, G. J., & Fryxell, B. (1991). Radiative-hydrodynamical simulations of accretion disk coronae. Astrophysical Journal Letters, 377(2 PART 2), L101-L104.
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  Abstract: We have developed an algorithm that will carry out detailed, two-dimensional, fully self-consistent radiative-hydrodynamical simulations of accretion disk coronae in X-ray-luminous compact sources. The calculation reported here, for an accreting neutron star radiating at 0.5 times the Eddington luminosity, reveals several striking features. (1) The corona is comprised of two main regions - an inner (r ≲1 × 108 cm) highly dynamic portion whose vertical structure varies cyclically on a dynamical time scale (∼0.2 s), and an outer, more stable zone in which the evaporated plasma rises to form a "sheath" that gradually merges into a wind at large radii. (2) The coronal structure shows a density inversion (due to the ram pressure support of the ionized flow) that contrasts sharply with the Gaussian profiles of earlier hydrostatic models. (3) Interestingly, flow velocities as high as a few × 109 cm s-1 are not uncommon in portions of the corona. We discuss the relevance of this point to the large measured width of the emission features seen in low-mass X-ray binaries and the Galactic black hole candidates.
• Melia, F. (1990). Gamma-ray bursts from magnetospheric plasma oscillations. Astrophysical Journal, 351(2), 601-605.
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  Abstract: We show that relativistic, (neutron star) magnetospheric plasma oscillations, damped by inverse Compton scatterings with the ambient radiation field, can account for the energetics, decay time scale, and spectra of typical gamma-ray bursts (GRBs). We find that the dissipation time scale τ depends on the energy of the perturbation (the "burst" energy Wburst), and that τ ~∼ 1 s for Wburst ≈ 1036 ergs. We discuss several plausible mechanisms for ""loading" the magnetosphere, all of which might be contributing to the GRB phenomenon. The soft photon source is likely due to backwarming of the reprocessing boundary (i.e., the stellar surface and possibly an accretion disk) by the incipient γ-rays, which is constistent with the Ginga observation of a slowly decaying, low-energy X-ray component in some bursts. The fractional power emitted in γ-rays depends on the maximum Lorentz factor γmax of the oscillating particles. Moderately relativistic oscillations with γmax ≲ 103 can account for some burst spectra with a single upscattered component, but possibly not those that exhibit cyclotron features. Oscillations with γmax ≫ 103 produce very hard γ-ray spectra that are deficient below several hundred keV, as required by the two component (i.e., incipient plus reflected) model for GRBs with absorption lines.
• Melia, F. (1990). Gamma-ray bursts from magnetospheric plasma oscillations. II. model spectra. Astrophysical Journal, 357(1), 161-174.
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  Abstract: Several mechanisms for the primary release of energy in gamma-ray bursts (GRBs) may result in the excitation of relativistic, magnetospheric plasma oscillations above the polar cap of a neutron star. In this paper, we report on detailed calculations of the inverse Compton scattering interaction between the sinusoidally accelerated particles in the plasma and the thermal radiation from the stellar surface, which we determine self-consistently by means of a Monte Carlo simulation for the upscattered photons and the deposition of γ-ray energy below the photosphere. In order to avoid complications arising from the quantizing effects of a teragauss magnetic field on the Compton scattering cross section, we restrict the value of the surface magnetic field such that Bs ≲ 1012(γmax/100)(losc/3R*) 3 G where R* is the stellar radius, γmax is the particle Lorentz factor, and los is the scale length of the oscillating region above the polar cap. We find that the overall spectrum is comprised of essentially four main components: (1) once-scattered radiation, which is generally highly beamed in the direction of the magnetic field B0 within the oscillating region because γmax must lie in the range 10 ≲ γmax≲ 102-3 in order to account for the observation of 1-100 MeV photons in some bursts; (2) multiply-scattered radiation (mostly second-order), which is also (at least partially) beamed. This component extends to higher frequencies than the first because photons gain energy with each subsequent scattering; (3) photons that are upscattered within the oscillating region and are then reflected by the stellar surface. Due to frequency redistribution within the star, this component contributes primarily in the ∼ 10-100 keV energy range and is generally more isotropic than components (1) and (2), and (4) thermal emission from the polar cap region due to the reprocessing of γ-rays scattered toward the star. Although we do not optimize our parameters to model individual GRBs, the qualitative agreement between the theoretical spectra and those observed during bursts such as GB720427 lead us to the following interpretation: (1) the turn-up at photon energy ≳1 MeV is probably due to multiply-scattered radiation, (2) the central, broad hump near ∼0.1-1 MeV arises from first-order scatterings and/or the reflected γ-rays, and (3) the component below ∼10 keV is likely due to thermal emission. We also show that the viability of this model may be tested through several of its robust predictions. First, γ-ray bursts should also be associated with emission at UV/soft X-ray energies, but the power in this component relative to that in γ-rays is a function of the aspect angle θ measured with respect to B0. Bursts from distant sources, which because of beaming would presumably be of the small-θ variety in order to have fluxes above the detection limit, should generally show a slight excess above the extrapolation of the spectrum from higher frequencies. Bursts originating nearby should be detectable over a wide range of θ, and the UV/soft X-ray emission in some of these might be more prominent relative to the γ-rays. Second, the low-energy absorption features should be evident only in bursts detected at θ ≳ 30○40○, for which the reflected component dominates the spectrum in the ∼ 10-100 keV range. Detailed calculations of the cyclotron line formation should therefore deduce large viewing angles (i.e., θ ≳ 30○-40○) with respect to B0. One might also expect that the presence of absorption lines might be correlated to the strength of the UV/soft X-ray excess and that spectra of bursts that display these lines should generally have lower values of the high-energy cutoff than those of the GRB population as a whole because the high-frequency component is preferentially beamed along B0, Third, the low-energy features should be evident in ≳ 18% of all bursts if the burst energy function is flat and γmax is distributed uniformly over the range 10-200. This seems to be consistent with the KONUS data.
• Ulmer, M. P., & Melia, F. (1990). X-ray and gamma-ray bursts. Nuclear Physics B (Proceedings Supplements), 14(2), 129-146.
• Zylstra, G. J., & Melia, F. (1990). Flux-Limited Diffusion Theory. Bulletin of the American Astronomical Society, 22.
• Melia, F. (1989). Geminga and the search for optical counterparts of γ-ray-burst sources. Nature, 338(6213), 322-324.
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  Abstract: THE nature of gamma-ray-burst (GRB) sources has remained a mystery, in part because of the lack of any optical identification. Recently, deep CCD photometry has identified the optical counterpart G″ to the γ-ray source Geminga, whose location on a colour-magnitude diagram is unique. Although the X-rays from G″ are probably due to thermal emission from the neutron star, the implied column density (≳ 5 × 1020) is inconsistent with the distance (D < 100 pc) required to fit the optical flux to the same spectral component. Here I show that the optical emission could be due instead to a cold accretion disk with accretion rate Ṁ ≈ 1011 g s-1. This has promising implications for the search for the optical counterparts to GRB sources whose basic physical parameters may resemble those of radio pulsars and related objects such as Geminga. I argue that a similar search in the field of a GRB location should produce candidates similar to G″ if 20 pc ≲ DGRB ≲ 500 pc, as predicted by the disk-reprocessing model for the associated optical transients. This possibility is discussed in the light of the fact that the first optical counterpart to a GRB source may already have been found, which, if it is confirmed, may be used to test the accretion-disk model. © 1989 Nature Publishing Group.
• Melia, F., & Fatuzzo, M. (1989). Electron Transport and Tearing Mode Instabilities in a Quantizing Magnetic Field. Bulletin of the American Astronomical Society, 21, 1175.
• Melia, F. (1988). A new interpretation of emission-like features in γ-ray burst spectra. Nature, 336(6200), 658-660.
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  Abstract: The spectra of γ-ray bursts, which are generally thought to result from a transient phenomenon on a neutron star, extend from the optical up to γ-ray energies. It has recently been suggested1 that the whole spectrum may be due to irradiation of a reprocessing and reflecting boundary near a source of power-law γ-rays with a low-energy cutoff E0 ≈ 300-500 keV. In this picture, the emission at intermediate energy (10 keV≲E≲300-500 keV) results from the reflection of incipient high-energy γ-rays by the surface layers of the underlying neutron star and/or its accretion disk. Some bursts display high-energy (∼400-keV) features, which may be gravitationally redshifted photons produced from the annihilation of electron-positron pairs. This explanation, however, has been open to criticism. I show that an alternative explanation for the high-energy features may be the superposition of the incipient and reflected spectral components near E0. © 1988 Nature Publishing Group.
• Melia, F. (1988). A new technique for solving parker-type wind equations. Journal of Computational Physics, 74(2), 488-492.
• Melia, F. (1988). Do Gamma-Ray Burst Sources Evolve from Be/X-Ray Binaries?. The Astrophysical Journal, 335, 965. doi:10.1086/166982
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  Conditions under which neutron star/cold disk systems may result from the evolution of a close binary consisting of a neutron star and a massive white dwarf are investigated. It is found that the expected gamma-ray burst source distribution is consistent with the observed angular distribution. The predicted source density implies repetition time scales of less that about 6-25 years for gamma-ray bursts. 45 references.
• Melia, F. (1988). Gamma-Ray Burst Spectra. Bulletin of the American Astronomical Society, 20, 1055.
• Melia, F. (1988). Gamma-ray burst reprocessing. Advances in Space Research, 8(2-3), 641-652.
• Melia, F., & Fatuzzo, M. (1988). Gamma-ray Emission in Radio Pulsars. Bulletin of the American Astronomical Society, 20, 1047.
• Melia, F., & Konigl, A. (1988). A New Interpretation of the Hard X-Ray Component in BL Lac Objects: Inverse Compton Scattering of Disk Radiation by a Relativistic Jet. Bulletin of the American Astronomical Society, 20, 646.
• Lamb, D. O., & Melia, F. (1987). Evolution of magnetic cataclysmic binaries. Astrophysics and Space Science, 131(1-2), 511-547.
• Lamb, D. Q., Melia, F., & Lamb, D. Q. (1987). Dynamical mass transfer in cataclysmic binaries. The Astrophysical Journal, 321, L139. doi:10.1086/185021
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  When a binary comes into contact and mass transfer begins, orbital angular momentum is stored in the accretion disk until the disk couples tidally to the binary system. Taam and McDermott (1987) have suggested that this leads to unstable dynamical mass transfer in many cataclysmic variables in which mass transfer would otherwise be stable, and that it explains the gap between 2 and 3 h in the orbital period distribution of these systems. Here the consequences of this hypothesis for the evolution of cataclysmic binaries are explored. It is found that systems coming into contact longward of the period gap undergo one or more episodes of dynamical mass transfer. 16 references.
• Melia, F. (1987). Cosmic High-Energy Physics. (Book Reviews: High Energy Phenomena around Collapsed Stars). Science, 238(4829). doi:10.1126/science.238.4829.970
• Melia, F. (1987). Cosmic high-energy physics: high energy phenomena around collapsed stars.. Science (New York, N.Y.), 238(4829), 970. doi:10.1126/science.238.4829.970-a
• Lamb, D. Q., Melia, F., & Lamb, D. Q. (1986). A New Mechanism for Producing Ultra-Short Period Binaries. Bulletin of the American Astronomical Society, 18, 974.
• Lamb, D. Q., Melia, F., & Lamb, D. Q. (1986). Evolution of Magnetic CVs. Bulletin of the American Astronomical Society, 18, 682.
• Frisch, D., & Melia, F. (1983). Siblings for SETI. Icarus, 55(3), 432-438.
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  Abstract: We on Earth and intelligent beings elsewhere in our galaxy can single out each other as special targets for communication if our stars have common properties, and can thus perhaps be stimulated to broadcast and listen directionally to try to give the coincidence between sending and listening that is necessary. The SAO Catalogue of roughly 260,000 stars was culled for such "sibling" candidates using their spectra, location, and motion. Only a few of the roughly 50,000 tabulated G-stars proved interesting, in part because the information about almost all of them is imprecise, and in part perhaps because our criteria were overselective. However, a set of mostly untabulated sibling candidate stars can be defined, given a direction and small solid angle that are mutually interesting to members of that set, so that overlapping broadcast/receiving cones can be selected on the basis of this commonality. We believe that the double cone about the direction to the galactic center, with half angle α = e2/ h {combining short stroke overlay}c = 1 137 radian, is an almost inevitable choice. With current technology, sending as well as receiving within this small solid angle can reach to about 1 kpsc, making available an estimated 103 G-star sibling candidates. © 1983.
• Lo, S., & Melia, F. (1979). Static quark potentials. Il Nuovo Cimento A, 50(1), 90-96.
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  Abstract: We present a classical, static solution of the Yang-Mills equations. It has a Coulomb plus linear behaviour for small r and grows like ln r at large distance © 1979 Società Italiana di Fisica.
• Melia, F., & Lo, S. (1978). Linear plane wave solutions of the Yang-Mills theory. Physics Letters B, 77(1), 71-72.
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  Abstract: A subset of solutions to the Yang-Mills equations is found to consist of plane waves that can be linearly superposed. This indicates that a sourceless Yang-Mills gauge particle can have some of the usual properties of a photon. © 1978.

Proceedings Publications

• Melia, F., Khalil, S., & Abdelqader, M. (2009, Spring). Decaying Dark Matter and the Deficit of Dwarf Halos. In AIP Conference Proceedings, 1115, 175-179.
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  The hierarchical clustering inherent in Lambda‐Cold Dark Matter cosmology (ΛCDM) seems to produce many of the observed characteristics of large‐scale structure. But some glaring problems still remain, including the over‐prediction (by a factor 10) of the number of dwarf galaxies within the virialized population of the local group. We carry out a detailed calculation of the dwarf halo evolution incorporating the effects of a hypothesized dark‐matter decay, D→D′+l, where D is the unstable particle, D′ is the more massive daughter particle and l is the other, lighter (or possibly massless) daughter particle. This process preferentially heats the smaller halos, expanding them during their evolution and reducing their present‐day circular velocity. We find that this mechanism can account very well for the factor 4 deficit in the observed number of systems with velocity 10–20 km s−1 compared to those predicted by the numerical simulations, if Δm/mD′∼5–7×10−5, where Δm is the mass difference between the initial ...
• Misra, R., & Melia, F. (2008, Spring). Evolution of the optically thick disk in Nova Muscae. In AIP Conference Proceedings, 410, 1000-1004.
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  We here model the soft X-ray flux from the black hole X-ray nova, GS 1124-68 (Nova Muscae, GRS 1124-68) as emission from an optically-thick accretion disk with Comptonization. We demonstrate that examining the disk’s radial dependence on the accretion rate separately in the various spectral states allows for a variation in the location of the disk’s inner edge. By extension, we infer the presence of a hot inner region, which can account for the hard component and thus produce a consistency with currently viable models. Our results suggest that the disk’s inner edge has a radius whose dependence on the accretion rate is a power-law, and that the slope of this function changes between the different spectral states. We show, moreover, that the accretion rate does not appear to be a direct measure of the total luminosity. This implies that a certain fraction of the released gravitational energy is probably advected inwards-through the event horizon.
• Nayakshin, S., & Melia, F. (2008). Big Blue Bump and transient active regions in Seyfert Galaxies. In AIP Conference Proceedings, 410, 1318-1322.
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  An important feature of the EUV spectrum (known as the Big Blue Bump, hereafter BBB) in Seyfert Galaxies is the narrow range in its cutoff energy Ec from source to source, even though the luminosity changes by 4 orders of magnitude. Here we show that if the BBB is due to accretion disk emission, then in order to account for this “universality” in the value of Ec, the emission mechanism is probably optically thin bremsstrahlung. In addition, we demonstrate that the two-phase model with active regions localized on the surface of the cold disk is consistent with this constraint if the active regions are very compact and are highly transient, i.e., they evolve faster than one dynamical time scale.
• Nayakshin, S., & Melia, F. (1997, Spring). Big Blue Bump and Transient Active Regions in Seyfert Galaxies. In The Astrophysical Journal Letters, 484, L103-L106.
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  An important feature of the EUV spectrum (known as the Big Blue Bump, hereafter BBB) in Seyfert galaxies is the narrow range in its cutoff energy Ec from source to source, even though the luminosity changes by 4 orders of magnitude. Here we show that if the BBB is due to accretion disk emission, then in order to account for this "universality" in the value of Ec, the emission mechanism is probably optically thin bremsstrahlung. In addition, we demonstrate that the two-phase model with active regions localized on the surface of the cold disk is consistent with this constraint if the active regions are very compact and are highly transient, i.e., they evolve faster than one dynamical timescale.
• Melia, F., & Fatuzzo, M. (1994, Spring). Focusing of Alfvénic power in neutron star magnetospheres. In AIP Conference Proceedings, 307, 515-519.
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  Highly dynamic magnetospheric perturbations in neutron star environments can naturally account for the features observed in gamma‐ray burst spectra. However, if GRB’s have an extragalactic origin, as is implied by the uniform yet spatially truncated distribution observed by the BATSE experiment, then noncatastrophic isotropic emission mechanisms may be ruled out on energetic and timing arguments. As such, we consider MHD processes which can produce strongly anisotropic γ‐rays with an observable flux out to distances of ∼1–2 Gpc. In particular, we show that sheared Alfven waves propagating along open magnetospheric field lines at the poles of magnetized neutron stars transfer their energy dissipationally to the charges generating the current which sustains the field misalignment, and thereby focus their power into a spatial region (i.e., the shear) that can be many times smaller than that of the crustal disturbance. This produces a strong (observable) flux enhancement along certain directions.
• Melia, F., & Fatuzzo, M. (1993, Spring). Gamma‐ray bursts from sheared Alfvén waves in the magnetospheres of extragalactic radio pulsars. In AIP Conference Proceedings > Volume 280, Issue 1, 280, 1030-1034.
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  The distribution of gamma‐ray bursts (GRBs) detected by BATSE, in combination with the spectral data gathered over the past two decades, argue for a cosmological population of neutron‐star sources. We demonstrate that GRB spectra may be understood in the context of the Compton upscattering of typical radio pulsar spectra, providing a natural interpretation to the spectral break ebreak at ∼0.2–3 MeV often exhibited by GRBs. This model also predicts a γ‐ray power spectral index −1≲μ≲2 above the break, though the prevalence of steeper radio spectra in the brightest pulsars implies a biasing of μ toward the bottom of this range. We find that the probability of detecting a burst in progress from an given source is ≊5×10−11, implying and individual stellar burst rate of about 1 every 50 years if all active pulsars are involved, or about 1 every 2–3 years for the very young members of this class. In addition, the energy released per burst coincides with those pertaining to the macro‐ and micro‐glitches seen in t...

Presentations

• Melia, F. (2019, June). Evidence of a Truncated Spectrum in the CMB Fluctuations. ICRAnet. Pescara, Italy: ICRAnet.
• Melia, F. (2019, June). New Constraints on Inflation. Astrophysics Institute, Tenerife. Astrophysics Institute, Tenerife, Spain: ESA.
• Melia, F. (2019, May). Evidence of a Truncated Spectrum in the CMB Fluctuations. European Research Council. ERC Headquarters, Bruxels Belgium: ERC.
• Melia, F. (2018, Fall 2018). The Cosmic Spacetime. Colloquium, Lowell Observatory, Flagstaff. Lowell Observatory, Flagstaff: NOAO.
• Melia, F. (2018, Summer 2018). A Truncated Spectrum in the Angular Correlation Function of the CMB. Colloquium, National Optical Observatory, Beijing China. National Optical Observatory, Beijing China: Chinese Academy of Sciences.
• Melia, F. (2018, Summer 2018). The Hubble Bubble. Astronomy Colloquium, Beijing Normal University. Astronomy Department, Beijing Normal University, Beijing China: Chinese Academy of Sciences.
• Melia, F. (2018, Summer 2018). The Standard Model in Transition. Physics Colloquium, Beijing Normal University. Physics Department, Beijing Normal University, Beijing China: Chinese Academy of Sciences.
• Melia, F. (2018, Summer 2018). The Standard Model in Transition. Physics Colloquium, Dezhou University, Dezhou China. Physics Department, Dezhou University, Dezhou China: Chinese Academy of Sciences.
• Melia, F. (2017, June). The Emergence of a Non-Inflationary Signature in the Angular Correlation Function of the CMB. Astrophysics Institute, Tenerife. Astrophysics Institute, Tenerife, Spain: ESA.
• Melia, F. (2017, June). The Emergence of a Non-Inflationary Signature in the Angular Correlation Function of the CMB. Physics Department, La Sapienza. La Sapienza, Rome Italy: La Sapienza.
• Melia, F. (2017, June). Why LCDM is not the Final Answer. ICRAnet. Pescara, Italy: ICRAnet.
• Melia, F. (2017, May). A Non-inflationary Signature in the CMB Angular Correlation Function. Very Large Array (VLA). Socorro, NM: VLA.
• Melia, F. (2017, May). Cosmology Today. European Research Council. ERC Headquarters, Bruxels Belgium: ERC.
• Melia, F. (2016, Summer). Cracking the Einstein Code. Swedish Royal Academy, Crafoord Banquet, Stockholm Sweden. Swedish Royal Academy, Stockholm Sweden: Swedish Royal Academy.
• Melia, F. (2016, Summer). Why LCDM is not the Final Answer. Astrophysics Institute, Tenerife Spain. Tenerife, Spain: Astrophysics Institute, Tenerife Spain.
• Melia, F. (2016, Summer). Why LCDM is not the Final Answer. Department of Physics and Astronomy, Aarhus University. Aarhus University, Denmark: Aarhus University.
• Melia, F. (2015, January 2015). Zero Active Mass condition in Friedmann-Robertson-Walker Cosmologies. Physics and Astronomy Department, UNLV. UNLV: The Chinese Academy of Science.
• Melia, F. (2015, January 2015). Zero Active Mass condition in Friedmann-Robertson-Walker Cosmologies. Sun-Yat Sen University, Guangzhou, China. Sun-Yat Sen University, Guangzhou, China: The Chinese Academy of Science.
• Melia, F. (2015, June 2015). The Cosmological Landscape. ERC, Bruxels, Belgium. Bruxels, Belgium: European Research Council.
• Melia, F. (2015, June 2015). Zero Active Mass condition in Friedmann-Robertson-Walker Cosmologies. Astrophysics Institute, Tenerife, Spain. Tenerife, Spain: Astrophysics Institute, Tenerife.
• Melia, F. (2015, June 2015). Zero Active Mass condition in Friedmann-Robertson-Walker Cosmologies. EWASS2015, Tenerife, Spain. Tenerife, Spain: EWASS2015, Tenerife.
• Melia, F. (2015, June 2015). Zero Active Mass condition in Friedmann-Robertson-Walker Cosmologies. Institute of Astronomy, Cambridge University. Cambridge, England: Cambridge University.
• Melia, F. (2014, June). Zero Active Mass Condition in FRW Cosmologies. Invited Talk. The Swinburne University of Technology: Swinburne University.
• Melia, F. (2014, May). Cracking the Einstein Code. Invited Public Lecture. The University of Sydney: The Walter Stubbs Foundation.
• Melia, F. (2014, May). Zero Active Mass Condition in FRW Cosmologies. Invited Talk. The School of Physics, Sydney University: Sydney University.
• Melia, F. (2014, November). Zero Active Mass Condition in FRW Cosmologies. Invited Talk. Purple Mountain Observatory, Nanjing China: Chinese Academy of Science.

Poster Presentations

• Melia, F. (2013, December). Zero Active Mass Condition in FRW Cosmologies. Invited Talk at the Astroparticle and Cosmology Center. Paris, France.
• Melia, F. (2013, June). Particle Acceleration at the Galactic Center. Invited Talk at the International Space Science Institute. Bern, Switzerland.
• Melia, F. (2013, November). The Supermassive Black Hole at the Galactic Center. Invited Talk at Xiamen University. China.
• Melia, F. (2013, November). Zero Active Mass Condition in FRW Cosmologies. Invited Talk at Nanjing University. China.
• Melia, F. (2013, November). Zero Active Mass Condition in FRW Cosmologies. Invited Talk at the Beijing National Observatory. China.
• Melia, F. (2013, October). The Cosmic Landscape. Invited Talk at Nanjing University. Nanjing, China.
• Melia, F. (2013, September). Cosmic-Ray Diffusion at the Galactic Center. Invited Talk at the Purple Mountain Observatory. Nanjing, China.
• Melia, F. (2012, July). Accreting Black Holes. Invited Talk at the University of Paris. Paris, France.
• Melia, F. (2012, June). The Cosmic Spacetime. Invited Talk at the Lebedev Institute. Moscow, Russia.
• Melia, F. (2012, May). A Supernova Origin for the X-ray Fluorescence in Sgr B2. Invited Talk at the International Space Science Institute. Bern, Switzerland.
• Melia, F. (2012, May). The Galactic Center Fermi-HESS Source. Invited Talk at the International Space Science Institute. Bern, Switzerland.
• Melia, F. (2011, August). The Cosmic Spacetime. Invited Talk at the School of Physics, Melbourne University.
• Melia, F. (2011, July). The Cosmic Spacetime. Invited Talk at the International Space Science Institute. Bern, Switzerland.
• Melia, F. (2011, October). The Cosmic Spacetime. Invited Talk at the Department of Physics, University of Cincinnati.