Dimitrios Psaltis

Interests

Research

General Relativity, High Energy Astrophysics, Computational Astrophysics

Teaching

Computational methods in physics, General Relativity, Classical Mechanics, Statistical Methods in Physics and Astronomy

Courses

Scholarly Contributions

Journals/Publications

• Baub{\"ock}, M., Psaltis, D., , F. (2019). Atmospheric Structure and Radiation Pattern for Neutron-star Polar Caps Heated by Magnetospheric Return Currents. \apj, 872(2), 162.
• Bogdanov, S., Guillot, S., Ray, P. S., Wolff, M. T., Chakrabarty, D., Ho, W. C., Kerr, M., Lamb, F. K., Lommen, A., Ludlam, R. M., Milburn, R., Montano, S., Miller, M. C., Baub{\"ock}, M., {\"Ozel}, F., Psaltis, D., Remillard, R. A., Riley, T. E., Steiner, J. F., , Strohmayer, T. E., et al. (2019). Constraining the Neutron Star Mass─Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-Ray Data Set. \apjl, 887(1), L25.
• Christian, P., & Psaltis, D. (2019). Interferometric Closure Phase Uncertainties in the Low Signal-to-Noise Ratio Regime. arXiv e-prints, arXiv:1909.04681.
• Collaboration, E., Akiyama, K., Alberdi, A., Alef, W., Asada, K., Azulay, R., Baczko, A., Ball, D., Balokovi{\'c}, M., Barrett, J., Bintley, D., Blackburn, L., Boland, W., Bouman, K. L., Bower, G. C., Bremer, M., Brinkerink, C. D., Brissenden, R., Britzen, S., , Broderick, A. E., et al. (2019). First M87 Event Horizon Telescope Results. II. Array and Instrumentation. \apjl, 875(1), L2.
• Collaboration, E., Akiyama, K., Alberdi, A., Alef, W., Asada, K., Azulay, R., Baczko, A., Ball, D., Balokovi{\'c}, M., Barrett, J., Bintley, D., Blackburn, L., Boland, W., Bouman, K. L., Bower, G. C., Bremer, M., Brinkerink, C. D., Brissenden, R., Britzen, S., , Broderick, A. E., et al. (2019). First M87 Event Horizon Telescope Results. III. Data Processing and Calibration. \apjl, 875(1), L3.
• Collaboration, E., Akiyama, K., Alberdi, A., Alef, W., Asada, K., Azulay, R., Baczko, A., Ball, D., Balokovi{\'c}, M., Barrett, J., Bintley, D., Blackburn, L., Boland, W., Bouman, K. L., Bower, G. C., Bremer, M., Brinkerink, C. D., Brissenden, R., Britzen, S., , Broderick, A. E., et al. (2019). First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole. \apjl, 875(1), L4.
• Collaboration, E., Akiyama, K., Alberdi, A., Alef, W., Asada, K., Azulay, R., Baczko, A., Ball, D., Balokovi{\'c}, M., Barrett, J., Bintley, D., Blackburn, L., Boland, W., Bouman, K. L., Bower, G. C., Bremer, M., Brinkerink, C. D., Brissenden, R., Britzen, S., , Broderick, A. E., et al. (2019). First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring. \apjl, 875(1), L5.
• Lockhart, W., Gralla, S. E., {\"Ozel}, F., & Psaltis, D. (2019). X-ray light curves from realistic polar cap models: inclined pulsar magnetospheres and multipole fields. \mnras, 490(2), 1774-1783.
• Lundquist, M., Paterson, K., Fong, W., Sand, D., Andrews, J., Shivaei, I., Daly, P., Valenti, S., Yang, S., Christensen, E., Gibbs, A., Shelly, F., Wyatt, S., Eskandari, O., Kuhn, O., Amaro, R., Arcavi, I., Behroozi, P., Butler, N., , Chomiuk, L., et al. (2019). Searches after Gravitational Waves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo\textquoterights Third Observing Run. \apjl, 881(2), L26.
• Medeiros, L., Psaltis, D., , F. (2019). A Parametric model for the shapes of black-hole shadows in non-Kerr spacetimes. arXiv e-prints, arXiv:1907.12575.
• Porth}, O., Chatterjee, K., Narayan, R., Gammie, C. F., Mizuno, Y., Anninos, P., Baker, J. G., Bugli, M., Chan, C., Davelaar, J., Del, Z. L., Etienne, Z. B., Fragile, P. C., Kelly, B. J., Liska, M., Markoff, S., McKinney, J. C., Mishra, B., Noble, S. C., , Olivares, H., et al. (2019). The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project. \apjs, 243(2), 26.
• Psaltis, D. (2019). Testing general relativity with the Event Horizon Telescope. General Relativity and Gravitation, 51(10), 137.
• Raithel, C. A., {\"Ozel}, F., & Psaltis, D. (2019). Finite-temperature Extension for Cold Neutron Star Equations of State. \apj, 875(1), 12.
• Ray, P. S., Arzoumanian, Z., Ballantyne, D., Bozzo, E., Brandt, S., Brenneman, L., Chakrabarty, D., Christophersen, M., DeRosa, A. r., Feroci, M., Gendreau, K., Goldstein, A., Hartmann, D., Hernanz, M., Jenke, P., Kara, E., Maccarone, T., McDonald, M., Nowak, M., , Phlips, B., et al. (2019). STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years. arXiv e-prints, arXiv:1903.03035.
• Ball, D., {"Ozel}, F., Psaltis, D., Chan, C., & Sironi, L. (2018). The Properties of Reconnection Current Sheets in GRMHD Simulations of Radiatively Inefficient Accretion Flows. apj, 853, 184.
• Ball, D., {\"Ozel}, F., Psaltis, D., Chan, C., & Sironi, L. (2018). The Properties of Reconnection Current Sheets in GRMHD Simulations of Radiatively Inefficient Accretion Flows. \apj, 853(2), 184.
• Chan, C., Medeiros, L., {"Ozel}, F., & Psaltis, D. (2018). GRay2: A General Purpose Geodesic Integrator for Kerr Spacetimes. apj, 867, 59.
• Chan, C., Medeiros, L., {\"Ozel}, F., & Psaltis, D. (2018). GRay2: A General Purpose Geodesic Integrator for Kerr Spacetimes. \apj, 867(1), 59.
• Giddings, S. B., & Psaltis, D. (2018). Event Horizon Telescope observations as probes for quantum structure of astrophysical black holes. \prd, 97(8), 084035.
• Johnson, M. D., Narayan, R., Psaltis, D., Blackburn, L., Kovalev, Y. Y., Gwinn, C. R., Zhao, G., Bower, G. C., Moran, J. M., Kino, M., Kramer, M., Akiyama, K., Dexter, J., Broderick, A. E., & Sironi, L. (2018). The Scattering and Intrinsic Structure of Sagittarius A* at Radio Wavelengths. \apj, 865(2), 104.
• Johnson, M., Narayan, R., Psaltis, D., Blackburn, L., Kovalev, Y., Gwinn, C., Zhao, G., Bower, G., Moran, J., Kino, M., Kramer, M., Akiyama, K., Dexter, J., Broderick, A., & Sironi, L. (2018). The Scattering and Intrinsic Structure of Sagittarius A* at Radio Wavelengths. apj, 865, 104.
• Lu, R., Krichbaum, T. P., Roy, A. L., Fish, V. L., Doeleman, S. S., Johnson, M. D., Akiyama, K., Psaltis, D., Alef, W., Asada, K., Beaudoin, C., Bertarini, A., Blackburn, L., Blundell, R., Bower, G. C., Brinkerink, C., Broderick, A. E., Cappallo, R., Crew, G. B., , Dexter, J., et al. (2018). Detection of Intrinsic Source Structure at ̃3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*. \apj, 859(1), 60.
• Lu, R., Krichbaum, T., Roy, A., Fish, V., Doeleman, S., Johnson, M., Akiyama, K., Psaltis, D., Alef, W., Asada, K., Beaudoin, C., Bertarini, A., Blackburn, L., Blundell, R., Bower, G., Brinkerink, C., Broderick, A., Cappallo, R., Crew, G., , Dexter, J., et al. (2018). Detection of Intrinsic Source Structure at tilde3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*. apj, 859, 60.
• Medeiros, L., Chan, C., {"Ozel}, F., Psaltis, D., Kim, J., Marrone, D., & Sa{c, d. A. (2018). GRMHD Simulations of Visibility Amplitude Variability for Event Horizon Telescope Images of Sgr A*. apj, 856, 163.
• Medeiros, L., Chan, C., {\"Ozel}, F., Psaltis, D., Kim, J., Marrone, D. P., & Saḑowski, A. (2018). GRMHD Simulations of Visibility Amplitude Variability for Event Horizon Telescope Images of Sgr A*. \apj, 856(2), 163.
• Medeiros, L., Lauer, T. R., Psaltis, D., , F. (2018). Principal Component Analysis as a Tool for Characterizing Black Hole Images and Variability. \apj, 864(1), 7.
• Medeiros, L., Lauer, T., Psaltis, D., , F. (2018). Principal Component Analysis as a Tool for Characterizing Black Hole Images and Variability. apj, 864, 7.
• Psaltis, D. (2018). Testing General Relativity with the Event Horizon Telescope. arXiv e-prints.
• Psaltis, D., Johnson, M., Narayan, R., Medeiros, L., Blackburn, L., & Bower, G. (2018). A Model for Anisotropic Interstellar Scattering and its Application to Sgr A*. arXiv e-prints, arXiv:1805.01242.
• Psaltis, D., Johnson, M., Narayan, R., Medeiros, L., Blackburn, L., & Bower, G. (2018). A Model for Anisotropic Interstellar Scattering and its Application to Sgr A*. arXiv e-prints.
• Raithel, C. A., {\"Ozel}, F., & Psaltis, D. (2018). Tidal Deformability from GW170817 as a Direct Probe of the Neutron Star Radius. \apjl, 857(2), L23.
• Raithel, C., {"Ozel}, F., & Psaltis, D. (2018). Tidal Deformability from GW170817 as a Direct Probe of the Neutron Star Radius. apjl, 857, L23.
• Chan, C., Medeiros, L., Ozel, F., & Psaltis, D. (2017). GRay2: A General Purpose Geodesic Integrator for Kerr Spacetimes. ArXiv e-prints.
• Dexter, J., Deller, A., Bower, G., Demorest, P., Kramer, M., Stappers, B., Lyne, A., Kerr, M., Spitler, L., Psaltis, D., Johnson, M., & Narayan, R. (2017). Locating the intense interstellar scattering towards the inner Galaxy. \mnras, 471(3), 3563-3576.
• Medeiros, L., Chan, C., {\"Ozel}, F., Psaltis, D., Kim, J., Marrone, D. P., & Saḑowski, A. (2017). Variability in GRMHD Simulations of Sgr \{\{\textbackslashrm\{A\}\}\}\^\{* \}: Implications for EHT Closure Phase Observations. \apj, 844(1), 35.
• Medeiros, L., Chan, C., {\"Ozel}, F., Psaltis, D., Kim, J., Marrone, D., & Sa{\c, d. A. (2017). Variability in GRMHD Simulations of Sgr $\$$\{$$\backslash$rm$\{$A$\}$$\}$$\}$\^{$\{$* $\}$: Implications for EHT Closure Phase Observations. \apj, 844, 35.
• Raithel, C. A., {\"Ozel}, F., & Psaltis, D. (2017). From Neutron Star Observables to the Equation of State. II. Bayesian Inference of Equation of State Pressures. \apj, 844(2), 156.
• Ball, D., {\"Ozel}, F., Psaltis, D., & Chan, C. (2016). Particle Acceleration and the Origin of X-Ray Flares in GRMHD Simulations of SGR A. \apj, 826, 77.
• Fish, V., Johnson, M., Doeleman, S., Broderick, A., Psaltis, D., Lu, R., Akiyama, K., Alef, W., Algaba, J., Asada, K., Beaudoin, C., Bertarini, A., Blackburn, L., Blundell, R., Bower, G., Brinkerink, C., Cappallo, R., Chael, A., Chamberlin, R., , Chan, C., et al. (2016). Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales. \apj, 820, 90.
• Fish, V., Johnson, M., Doeleman, S., Broderick, A., Psaltis, D., Lu, R., Akiyama, K., Alef, W., Algaba, J., Asada, K., Beaudoin, C., Bertarini, A., Blackburn, L., Blundell, R., Bower, G., Brinkerink, C., Cappallo, R., Chael, A., Chamberlin, R., , Chan, C., et al. (2016). VizieR Online Data Catalog: 4yr 1.3mm VLBI observations of SgrA* with EHT (Fish+, 2016). VizieR Online Data Catalog, 182.
• Giddings, S., & Psaltis, D. (2016). Event Horizon Telescope Observations as Probes for Quantum Structure of Astrophysical Black Holes. \apj.
• G{\"uver}, T., {\"Ozel}, F., Marshall, H., Psaltis, D., Guainazzi, M., & D{\'{\i}az-Trigo}, M. (2016). Systematic Uncertainties in the Spectroscopic Measurements of Neutron-star Masses and Radii from Thermonuclear X-Ray Bursts. III. Absolute Flux Calibration. \apj, 829, 48.
• Johannsen, T., Wang, C., Broderick, A., Doeleman, S., Fish, V., Loeb, A., & Psaltis, D. (2016). Testing General Relativity with Accretion-Flow Imaging of Sgr A$^*$. Physical Review Letters, 117(9), 091101.
• Kim, J., Marrone, D., Chan, C., Medeiros, L., {\"Ozel}, F., & Psaltis, D. (2016). Bayesian Techniques for Comparing Time-dependent GRMHD Simulations to Variable Event Horizon Telescope Observations. \apj, 832, 156.
• Krawczynski, H., Stern, D., Harrison, F., Kislat, F., Zajczyk, A., Beilicke, M., Hoormann, J., Guo, Q., Endsley, R., Ingram, A., Miyasaka, H., Madsen, K., Aaron, K., Amini, R., Baring, M., Beheshtipour, B., Bodaghee, A., Booth, J., Borden, C., , B{\"ottcher}, M., et al. (2016). X-ray polarimetry with the Polarization Spectroscopic Telescope Array (PolSTAR). Astroparticle Physics, 75, 8-28.
• Medeiros, L., Chan, C., Ozel, F., Psaltis, D., Kim, J., Marrone, D., & Sadowski, A. (2016). GRMHD simulations of visibility amplitude variability for Event Horizon Telescope images of Sgr A*. \apj.
• Medeiros, L., Chan, C., {\"Ozel}, F., Psaltis, D., Kim, J., Marrone, D., & S{\c, a. A. (2016). Variability in GRMHD simulations of Sgr A$^*$: Implications for EHT closure phase observations. \apj.
• Narayan, R., Zhu, Y., Psaltis, D., & Sa{\c, d. A. (2016). HEROIC: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs. \mnras, 457, 608-628.
• Psaltis, D., Wex, N., & Kramer, M. (2016). A Quantitative Test of the No-hair Theorem with Sgr A* Using Stars, Pulsars, and the Event Horizon Telescope. \apj, 818, 121.
• Raithel, C., {\"Ozel}, F., & Psaltis, D. (2016). From Neutron Star Observables to the Equation of State. I. An Optimal Parametrization. \apj, 831, 44.
• Raithel, C., {\"Ozel}, F., & Psaltis, D. (2016). Model-independent inference of neutron star radii from moment of inertia measurements. \prc, 93(3), 032801.
• Watts, A., Andersson, N., Chakrabarty, D., Feroci, M., Hebeler, K., Israel, G., Lamb, F., Miller, M., Morsink, S., {\"Ozel}, F., Patruno, A., Poutanen, J., Psaltis, D., Schwenk, A., Steiner, A., Stella, L., Tolos, L., & Klis, M. (2016). Colloquium: Measuring the neutron star equation of state using x-ray timing. Reviews of Modern Physics, 88(2), 021001.
• {Narayan}, R., {Zhu}, Y., {Psaltis}, D., dowski}, A. (2016). HEROIC: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs. MNRAS, 457, 608-628.
• {\"Ozel}, F., Psaltis, D., Arzoumanian, Z., Morsink, S., & Baub{\"ock}, M. (2016). Measuring Neutron Star Radii via Pulse Profile Modeling with NICER. \apj, 832, 92.
• {\"Ozel}, F., Psaltis, D., G{\"uver}, T., Baym, G., Heinke, C., & Guillot, S. (2016). The Dense Matter Equation of State from Neutron Star Radius and Mass Measurements. \apj, 820, 28.
• {{\"O}zel}, F., {Psaltis}, D., {G{\"u}ver}, T., {Baym}, G., {Heinke}, C., , S. (2016). The Dense Matter Equation of State from Neutron Star Radius and Mass Measurements. Astrophys. J., 820, 28.
• {Baker}, T., {Psaltis}, D., , C. (2015). Linking Tests of Gravity on All Scales: from the Strong-field Regime to Cosmology. Astrophys. J., 802, 63.
• {Baub{\"o}ck}, M., {Psaltis}, D., zel}, F. (2015). Effects of Spot Size on Neutron-Star Radius Measurements from Pulse Profiles. Astrophys. J., 811, 144.
• {Baub{\"o}ck}, M., {{\"O}zel}, F., {Psaltis}, D., , S. (2015). Rotational Corrections to Neutron-star Radius Measurements from Thermal Spectra. Astrophys. J., 799, 22.
• {Chan}, C., {Psaltis}, D., {{\"O}zel}, F., {Medeiros}, L., {Marrone}, D., {Sa{\c d}owski}, A., , R. (2015). Fast Variability and Millimeter/IR Flares in GRMHD Models of Sgr A* from Strong-field Gravitational Lensing. Astrophys. J., 812, 103.
• {Chan}, C., {Psaltis}, D., {{\"O}zel}, F., {Narayan}, R., owski}, A. (2015). The Power of Imaging: Constraining the Plasma Properties of GRMHD Simulations using EHT Observations of Sgr A*. Astrophys. J., 799, 1.
• {Johnson}, M., {Fish}, V., {Doeleman}, S., {Marrone}, D., {Plambeck}, R., {Wardle}, J., {Akiyama}, K., {Asada}, K., {Beaudoin}, C., {Blackburn}, L., {Blundell}, R., {Bower}, G., {Brinkerink}, C., {Broderick}, A., {Cappallo}, R., {Chael}, A., {Crew}, G., {Dexter}, J., {Dexter}, M., , {Freund}, R., et al. (2015). Resolved magnetic-field structure and variability near the event horizon of Sagittarius A*. Science, 350, 1242-1245.
• {Psaltis}, D., {Narayan}, R., {Fish}, V., {Broderick}, A., {Loeb}, A., , S. (2015). Event Horizon Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way with the Inner Stellar Disk. Astrophys. J., 798, 15.
• {Psaltis}, D., {{\"O}zel}, F., {Chan}, C., , D. (2015). A General Relativistic Null Hypothesis Test with Event Horizon Telescope Observations of the Black Hole Shadow in Sgr A*. Astrophys. J., 814, 115.
• {Zhu}, Y., {Narayan}, R., {Sadowski}, A., , D. (2015). HERO - A 3D general relativistic radiative post-processor for accretion discs around black holes. MNRAS, 451, 1661-1681.
• {{\"O}zel}, F., , D. (2015). Statistics of Measuring Neutron Star Radii: Assessing a Frequentist and a Bayesian Approach. Astrophys. J., 810, 135.
• Broderick, A. E., Johannsen, T., Loeb, A., & Psaltis, D. (2014). Testing the no-hair theorem with event horizon telescope observations of sagittarius A*. Astrophysical Journal, 784(1).
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  Abstract: The advent of the Event Horizon Telescope (EHT), a millimeter-wave very long baseline interferometric array, has enabled spatially resolved studies of the subhorizon-scale structure for a handful of supermassive black holes. Among these, the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), presents the largest angular cross section. Thus far, these studies have focused on measurements of the black hole spin and the validation of low-luminosity accretion models. However, a critical input in the analysis of EHT data is the structure of the black hole spacetime, and thus these observations provide the novel opportunity to test the applicability of the Kerr metric to astrophysical black holes. Here we present the first simulated images of a radiatively inefficient accretion flow (RIAF) around Sgr A* employing a quasi-Kerr metric that contains an independent quadrupole moment in addition to the mass and spin that fully characterize a black hole in general relativity. We show that these images can be significantly different from the images of an RIAF around a Kerr black hole with the same spin and demonstrate the feasibility of testing the no-hair theorem by constraining the quadrupolar deviation from the Kerr metric with existing EHT data. Equally important, we find that the disk inclination and spin orientation angles are robust to the inclusion of additional parameters, providing confidence in previous estimations assuming the Kerr metric based on EHT observations. However, at present, the limits on potential modifications of the Kerr metric remain weak. © 2014. The American Astronomical Society. All rights reserved.
• Wellons, S., Zhu, Y., Psaltis, D., Narayan, R., & McClintock, J. E. (2014). A high-frequency doppler feature in the power spectra of simulated grmhd black hole accretion disks. Astrophysical Journal, 785(2).
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  Abstract: Black hole binaries exhibit a wide range of variability phenomena, from large-scale state changes to broadband noise and quasi-periodic oscillations, but the physical nature of much of this variability is poorly understood. We examine the variability properties of three GRMHD simulations of thin accretion disks around black holes of varying spin, producing light curves and power spectra as would be seen by observers. We find that the simulated power spectra show a broad feature at high frequency, which increases in amplitude with the inclination of the observer. We show that this high-frequency feature is a product of the Doppler effect and that its location is a function of the mass and spin of the black hole. This Doppler feature demonstrates that power spectral properties of the accretion disk can be tied to, and potentially used to determine, physical properties of the black hole. © 2014. The American Astronomical Society. All rights reserved..
• {Chakrabarty}, D., {Tomsick}, J., {Grefenstette}, B., {Psaltis}, D., {Bachetti}, M., {Barret}, D., {Boggs}, S., {Christensen}, F., {Craig}, W., {F{\"u}rst}, F., {Hailey}, C., {Harrison}, F., {Kaspi}, V., {Miller}, J., {Nowak}, M., {Rana}, V., {Stern}, D., {Wik}, D., {Wilms}, J., , W. (2014). A Hard X-Ray Power-law Spectral Cutoff in Centaurus X-4. Astrophys. J., 797, 92.
• {Fish}, V., {Johnson}, M., {Lu}, R., {Doeleman}, S., {Bouman}, K., {Zoran}, D., {Freeman}, W., {Psaltis}, D., {Narayan}, R., {Pankratius}, V., {Broderick}, A., {Gwinn}, C., , L. (2014). Imaging an Event Horizon: Mitigation of Scattering toward Sagittarius A*. Astrophys. J., 795, 134.
• {Psaltis}, D., zel}, F. (2014). Pulse Profiles from Spinning Neutron Stars in the Hartle-Thorne Approximation. Astrophys. J., 792, 87.
• {Psaltis}, D., {{\"O}zel}, F., , D. (2014). Prospects for Measuring Neutron-star Masses and Radii with X-Ray Pulse Profile Modeling. Astrophys. J., 787, 136.
• Bauböck, M., Berti, E., Psaltis, D., & Özel, F. (2013). Relations between neutron-star parameters in the Hartle-Thorne approximation. Astrophysical Journal, 777(1).
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  Abstract: Using stellar structure calculations in the Hartle-Thorne approximation, we derive analytic expressions connecting the ellipticity of the stellar surface to the compactness, the spin angular momentum, and the quadrupole moment of the spacetime. We also obtain empirical relations between the compactness, the spin angular momentum, and the spacetime quadrupole. Our formulae reproduce the results of numerical calculations to within a few percent and help reduce the number of parameters necessary to model the observational appearance of moderately spinning neutron stars. This is sufficient for comparing theoretical spectroscopic and timing models to observations that aim to measure the masses and radii of neutron stars and to determine the equation of state prevailing in their interiors. © 2013. The American Astronomical Society. All rights reserved.
• Bauböck, M., Psaltis, D., & Özel, F. (2013). Narrow atomic features from rapidly spinning neutron stars. Astrophysical Journal, 766(2).
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  Abstract: Neutron stars spinning at moderate rates (∼300-600 Hz) become oblate in shape and acquire a nonzero quadrupole moment. In this paper, we calculate the profiles of atomic features from such neutron stars using a ray-tracing algorithm in the Hartle-Thorne approximation. We show that line profiles acquire cores that are much narrower than the widths expected from pure Doppler effects for a large range of observer inclinations. As a result, the effects of both the oblateness and the quadrupole moments of neutron stars need to be taken into account when aiming to measure neutron-star radii from rotationally broadened lines. Moreover, the presence of these narrow cores substantially increases the likelihood of detecting atomic lines from rapidly spinning neutron stars. © 2013. The American Astronomical Society. All rights reserved.
• Chan, C., Psaltis, D., & Özel, F. (2013). GRay: A massively parallel gpu-based code for ray tracing in relativistic spacetimes. Astrophysical Journal, 777(1).
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  Abstract: We introduce GRay, a massively parallel integrator designed to trace the trajectories of billions of photons in a curved spacetime. This graphics-processing-unit (GPU)-based integrator employs the stream processing paradigm, is implemented in CUDA C/C++, and runs on nVidia graphics cards. The peak performance of GRay using single-precision floating-point arithmetic on a single GPU exceeds 300 GFLOP (or 1 ns per photon per time step). For a realistic problem, where the peak performance cannot be reached, GRay is two orders of magnitude faster than existing central-processing-unit-based ray-tracing codes. This performance enhancement allows more effective searches of large parameter spaces when comparing theoretical predictions of images, spectra, and light curves from the vicinities of compact objects to observations. GRay can also perform on-the-fly ray tracing within general relativistic magnetohydrodynamic algorithms that simulate accretion flows around compact objects. Making use of this algorithm, we calculate the properties of the shadows of Kerr black holes and the photon rings that surround them. We also provide accurate fitting formulae of their dependencies on black hole spin and observer inclination, which can be used to interpret upcoming observations of the black holes at the center of the Milky Way, as well as M87, with the Event Horizon Telescope. © 2013. The American Astronomical Society. All rights reserved..
• Johannsen, T., & Psaltis, D. (2013). Testing the no-hair theorem with observations in the electromagnetic spectrum. IV. Relativistically broadened iron lines. Astrophysical Journal, 773(1).
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  Abstract: According to the no-hair theorem, astrophysical black holes are fully characterized by their masses and spins and are described by the Kerr metric. This theorem can be tested observationally by measuring (at least) three different multipole moments of the spacetimes of black holes. In this paper, we calculate the profiles of fluorescent iron lines emitted from the accretion flows around black hole candidates within a framework that allows us to perform the calculation as a function of its mass and spin as well as of a free parameter that measures potential deviations from the Kerr metric. We show that such deviations lead to line profiles that are significantly altered and may exhibit a modified flux ratio of the two peaks in their characteristic double-peaked shape. We also show that the disk inclination can be measured independently of the spin and the deviation parameter at low to intermediate inclination angles, as in the case of Kerr black holes. We estimate the precision that near-future X-ray missions such as Astro-H and ATHENA+ are required to achieve in order to resolve deviations from the Kerr metric in iron line profiles and show that constraints on such deviations will be strongest for rapidly spinning black holes. More generally, we show that measuring the line profile with a precision of ∼5% at disk inclinations of 30° or 60°constrains the deviation parameter to order unity for values of the spin a ≳ 0.5M. © 2013. The American Astronomical Society. All rights reserved.
• Psaltis, D., Gongjie, L. i., & Loeb, A. (2013). Deviation of stellar orbits from test particle trajectories around SGr A* due to tides and winds. Astrophysical Journal, 777(1).
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  Abstract: Monitoring the orbits of stars around Sgr A* offers the possibility of detecting the precession of their orbital planes due to frame dragging, of measuring the spin and quadrupole moment of the black hole, and of testing the no-hair theorem. Here we investigate whether the deviations of stellar orbits from test-particle trajectories due to wind mass loss and tidal dissipation of the orbital energy compromise such measurements. We find that the effects of stellar winds are, in general, negligible. On the other hand, for the most eccentric orbits (e > 0.96) for which an optical interferometer, such as GRAVITY, will detect orbital plane precession due to frame dragging, the tidal dissipation of orbital energy occurs at timescales comparable to the timescale of precession due to the quadrupole moment of the black hole. As a result, this non-conservative effect is a potential source of systematic uncertainty in testing the no-hair theorem with stellar orbits. © 2013. The American Astronomical Society. All rights reserved.
• Bauböck, M., Psaltis, D., Özel, F., & Johannsen, T. (2012). A ray-tracing algorithm for spinning compact object spacetimes with arbitrary quadrupole moments. II. Neutron stars. Astrophysical Journal, 753(2).
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  Abstract: A moderately spinning neutron star acquires an oblate shape and a spacetime with a significant quadrupole moment. These two properties affect its apparent surface area for an observer at infinity, as well as the light curve arising from a hot spot on its surface. In this paper, we develop a ray-tracing algorithm to calculate the apparent surface areas of moderately spinning neutron stars making use of the Hartle-Thorne metric. This analytic metric allows us to calculate various observables of the neutron star in a way that depends only on its macroscopic properties and not on the details of its equation of state. We use this algorithm to calculate the changes in the apparent surface area, which could play a role in measurements of neutron-star radii and, therefore, in constraining their equation of state. We show that whether a spinning neutron star appears larger or smaller than its non-rotating counterpart depends primarily on its equatorial radius. For neutron stars with radii 10 km, the corrections to the Schwarzschild spacetime cause the apparent surface area to increase with spin frequency. In contrast, for neutron stars with radii 15 km, the oblateness of the star dominates the spacetime corrections and causes the apparent surface area to decrease with increasing spin frequency. In all cases, the change in the apparent geometric surface area for the range of observed spin frequencies is ≲5% and hence only a small source of error in the measurement of neutron-star radii. © 2012. The American Astronomical Society. All rights reserved..
• Feroci, M., Herder, J. D., Bozzo, E., Barret, D., Brandt, S., Hernanz, M., Van, M., Pohl, M., Santangelo, A., Stella, L., Watts, A., Wilms, J., Zane, S., Ahangarianabhari, M., Alpar, A., Altamirano, D., Alvarez, L., Amati, L., Amoros, C., , Andersson, N., et al. (2012). LOFT - The large observatory for x-ray timing. Proceedings of SPIE - The International Society for Optical Engineering, 8443.
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  Abstract: The LOFT mission concept is one of four candidates selected by ESA for the M3 launch opportunity as Medium Size missions of the Cosmic Vision programme. The launch window is currently planned for between 2022 and 2024. LOFT is designed to exploit the diagnostics of rapid X-ray flux and spectral variability that directly probe the motion of matter down to distances very close to black holes and neutron stars, as well as the physical state of ultradense matter. These primary science goals will be addressed by a payload composed of a Large Area Detector (LAD) and a Wide Field Monitor (WFM). The LAD is a collimated (
• Feroci, M., Stella, L., van, M., Courvoisier, T. J., Hernanz, M., Hudec, R., Santangelo, A., Walton, D., Zdziarski, A., Barret, D., Belloni, T., Braga, J., Brandt, S., Budtz-Jørgensen, C., Campana, S., Herder, J. d., Huovelin, J., Israel, G. L., Pohl, M., , Ray, P., et al. (2012). The Large Observatory for X-ray Timing (LOFT). Experimental Astronomy, 34(2), 415-444.
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  Abstract: High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultradense matter and to black hole masses and spins. A 10 m 2-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme "Matter under extreme conditions", namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of the four Cosmic Vision M3 candidate missions to undergo an assessment phase, will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. Thanks to an innovative design and the development of large-area monolithic silicon drift detectors, the Large Area Detector (LAD) on board LOFT will achieve an effective area of ~12 m 2 (more than an order of magnitude larger than any spaceborne predecessor) in the 2-30 keV range (up to 50 keV in expanded mode), yet still fits a conventional platform and small/medium-class launcher. With this large area and a spectral resolution of
• Güver, T., Psaltis, D., & Zel, F. (2012). Systematic uncertainties in the spectroscopic measurements of neutron-star masses and radii from thermonuclear X-ray bursts. I. Apparent radii. Astrophysical Journal, 747(1).
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  Abstract: The masses and radii of low-magnetic field neutron stars can be measured by combining different observable quantities obtained from their X-ray spectra during thermonuclear X-ray bursts. One of these quantities is the apparent radius of each neutron star as inferred from the X-ray flux and spectral temperature measured during the cooling tails of bursts, when the thermonuclear flash is believed to have engulfed the entire star. In this paper, we analyze 13,095 X-ray spectra of 446 X-ray bursts observed from 12 sources in order to assess possible systematic effects in the measurements of the apparent radii of neutron stars. We first show that the vast majority of the observed X-ray spectra are consistent with blackbody functions to within a few percent. We find that most X-ray bursts follow a very well determined correlation between X-ray flux and temperature, which is consistent with the whole neutron-star surface emitting uniformly during the cooling tails. We develop a Bayesian Gaussian mixture algorithm to measure the apparent radii of the neutron stars in these sources, while detecting and excluding a small number of X-ray bursts that show irregular cooling behavior. This algorithm also provides us with a quantitative measure of the systematic uncertainties in the measurements. We find that those errors in the spectroscopic determination of neutron-star radii that are introduced by systematic effects in the cooling tails of X-ray bursts are in the range ≃ 3%-8%. Such small errors are adequate to distinguish between different equations of state provided that uncertainties in the distance to each source and the absolute calibration of X-ray detectors do not dominate the error budget. © 2012. The American Astronomical Society. All rights reserved.
• Johannsen, T., Psaltis, D., Gillessen, S., Marrone, D. P., Özel, F., Doeleman, S. S., & Fish, V. L. (2012). Masses of nearby supermassive black holes with very long baseline interferometry. Astrophysical Journal, 758(1).
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  Abstract: Dynamical mass measurements to date have allowed determinations of the mass M and the distance D of a number of nearby supermassive black holes. In the case of Sgr A*, these measurements are limited by a strong correlation between the mass and distance scaling roughly as M ∼ D 2. Future very long baseline interferometric (VLBI) observations will image a bright and narrow ring surrounding the shadow of a supermassive black hole, if its accretion flow is optically thin. In this paper, we explore the prospects of reducing the correlation between mass and distance with the combination of dynamical measurements and VLBI imaging of the ring of Sgr A*. We estimate the signal-to-noise ratio of near-future VLBI arrays that consist of five to six stations, and we simulate measurements of the mass and distance of Sgr A* using the expected size of the ring image and existing stellar ephemerides. We demonstrate that, in this best-case scenario, VLBI observations at 1mm can improve the error on the mass by a factor of about two compared to the results from the monitoring of stellar orbits alone. We identify the additional sources of uncertainty that such imaging observations have to take into account. In addition, we calculate the angular diameters of the bright rings of other nearby supermassive black holes and identify the optimal targets besides Sgr A* that could be imaged by a ground-based VLBI array or future space-VLBI missions allowing for refined mass measurements. © 2012. The American Astronomical Society. All rights reserved.
• Psaltis, D. (2012). The influence of gas dynamics on measuring the properties of the black hole in the center of the milky way with stellar orbits and pulsars. Astrophysical Journal, 759(2).
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  Abstract: Observations of stars and pulsars orbiting the black hole in the center of the Milky Way offer the potential of measuring not only the mass of the black hole but also its spin and quadrupole moment, thereby providing observational verification of the no-hair theorem. The relativistic effects that will allow us to measure these higher moments of the gravitational field, however, are very small and may be masked by drag forces that stars and pulsars experience orbiting within the hot, tenuous plasma that surrounds the black hole. The properties of this plasma at large distances from the central object have been measured using observations of the extended X-ray emission that surrounds the point source. At distances comparable to the black hole event horizon, the properties of the accretion flow have been constrained using observations of its long-wavelength emission and polarization, as well as of the size of the emitting region at 1.3mm. I use models of the plasma density and temperature at various distances from the black hole to investigate the effect of hydrodynamic drag forces on future measurements of the higher moments of its gravitational field. I find that hydrodynamic drag does not preclude measurements of the black hole spin and quadrupole moment using high-resolution observations of stars and pulsars that orbit within a few thousand gravitational radii from its horizon. © 2012. The American Astronomical Society. All rights reserved.
• Psaltis, D., & Johannsen, T. (2012). A ray-tracing algorithm for spinning compact object spacetimes with arbitrary quadrupole moments. I. Quasi-kerr black holes. Astrophysical Journal, 745(1).
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  Abstract: We describe a new numerical algorithm for ray tracing in the external spacetimes of spinning compact objects characterized by arbitrary quadrupole moments. Such spacetimes describe non-Kerr vacuum solutions that can be used to test the no-hair theorem in conjunction with observations of accreting black holes. They are also appropriate for neutron stars with spin frequencies in the ≃ 300-600 Hz range, which are typical of the bursting sources in low-mass X-ray binaries. We use our algorithm to show that allowing for the quadrupole moment of the spacetime to take arbitrary values leads to observable effects in the profiles of relativistic broadened fluorescent iron lines from geometrically thin accretion disks. © 2012. The American Astronomical Society. All rights reserved.
• Özel, F., Psaltis, D., Narayan, R., & Villarreal, A. S. (2012). On the mass distribution and birth masses of neutron stars. Astrophysical Journal, 757(1).
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  Abstract: We investigate the distribution of neutron star masses in different populations of binaries, employing Bayesian statistical techniques. In particular, we explore the differences in neutron star masses between sources that have experienced distinct evolutionary paths and accretion episodes. We find that the distribution of neutron star masses in non-recycled eclipsing high-mass binaries as well as of slow pulsars, which are all believed to be near their birth masses, has a mean of 1.28 M Ȯ and a dispersion of 0.24 M Ȯ. These values are consistent with expectations for neutron star formation in core-collapse supernovae. On the other hand, double neutron stars, which are also believed to be near their birth masses, have a much narrower mass distribution, peaking at 1.33 M Ȯ, but with a dispersion of only 0.05 M Ȯ. Such a small dispersion cannot easily be understood and perhaps points to a particular and rare formation channel. The mass distribution of neutron stars that have been recycled has a mean of 1.48 M Ȯ and a dispersion of 0.2 M Ȯ, consistent with the expectation that they have experienced extended mass accretion episodes. The fact that only a very small fraction of recycled neutron stars in the inferred distribution have masses that exceed 2 M Ȯ suggests that only a few of these neutron stars cross the mass threshold to form low-mass black holes. © 2012. The American Astronomical Society. All rights reserved.
• Johannsen, T., & Psaltis, D. (2011). Metric for rapidly spinning black holes suitable for strong-field tests of the no-hair theorem. Physical Review D - Particles, Fields, Gravitation and Cosmology, 83(12).
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  Abstract: According to the no-hair theorem, astrophysical black holes are uniquely characterized by their masses and spins and are described by the Kerr metric. Several parametric deviations from the Kerr metric have been suggested to study observational signatures in both the electromagnetic and gravitational-wave spectra that differ from the expected Kerr signals. Because of the no-hair theorem, however, such spacetimes cannot be regular everywhere outside the event horizons, if they are solutions to the Einstein field equations; they are often characterized by naked singularities or closed timelike loops in the regions of the spacetime that are accessible to an external observer. For observational tests of the no-hair theorem that involve phenomena in the vicinity of the circular photon orbit or the innermost stable circular orbit around a black hole, these pathologies limit the applicability of the metrics only to compact objects that do not spin rapidly. In this paper, we construct a Kerr-like metric which depends on a set of free parameters in addition to its mass and spin and which is regular everywhere outside of the event horizon. We derive expressions for the energy and angular momentum of a particle on a circular equatorial orbit around the black hole and compute the locations of the innermost stable circular orbit and the circular photon orbit. We demonstrate that these orbits change significantly for even moderate deviations from the Kerr metric. The properties of our metric make it an ideally suited spacetime to carry out strong-field tests of the no-hair theorem in the electromagnetic spectrum using the properties of accretion flows around astrophysical black holes of arbitrary spin. © 2011 American Physical Society.
• Johannsen, T., & Psaltis, D. (2011). Testing the no-hair theorem with observations in the electromagnetic spectrum. III. Quasi-periodic variability. Astrophysical Journal, 726(1).
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  Abstract: According to the no-hair theorem, astrophysical black holes are uniquely described by their masses and spins. An observational test of the no-hair theorem can be performed by measuring at least three different multipole moments of the spacetime of a black hole and verifying whether their values are consistent with the unique combinations of the Kerr solution. In this paper, we study quasi-periodic variability observed in the emission from black holes across the electromagnetic spectrum as a test of the no-hair theorem. We derive expressions for the Keplerian and epicyclic frequencies in a quasi-Kerr spacetime, in which the quadrupole moment is a free parameter in addition to mass and spin. We show that, for moderate spins, the Keplerian frequency is practically independent of small deviations of the quadrupole moment from the Kerr value, while the epicyclic frequencies exhibit significant variations. We apply this framework to quasi-periodic oscillations (QPOs) in black hole X-ray binaries in two different scenarios. In the case that a pair of QPOs can be identified as the fundamental g-and c-modes in the accretion disk, we show that the no-hair theorem can be tested in conjunction with an independent mass measurement. If pairs of oscillations are identified with non-parametric resonance of dynamical frequencies in the accretion disk, then testing the no-hair theorem also requires an independent measurement of the black hole spin. In addition, we argue that VLBI observations of Sgr A* may test the no-hair theorem through a combination of imaging observations and the detection of quasi-periodic variability. © 2011. The American Astronomical Society. All rights reserved. Printedin the U.S.A.
• Johannsen, T., & Psaltis, D. (2011). Testing the no-hair theorem with observations of black holes in the electromagnetic spectrum. Advances in Space Research, 47(3), 528-532.
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  Abstract: According to the no-hair theorem, astrophysical black holes are uniquely described by their mass and spin. In this paper, we review a new framework for testing the no-hair hypothesis with observations in the electromagnetic spectrum. The approach is formulated in terms of a Kerr-like spacetime containing a quadrupole moment that is independent of both mass and spin. If the no-hair theorem is correct, then any deviation from the Kerr metric quadrupole has to be zero. We show how upcoming VLBI imaging observations of Sgr A as well as spectroscopic observations of iron lines from accreting black holes with IXO may lead to the first astrophysical test of the no-hair theorem. © 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.
• Psaltis, D., & Johannsen, T. (2011). Sgr A*: The optimal testbed of strong-field gravity. Journal of Physics: Conference Series, 283(1).
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  Abstract: The black hole in the center of the Milky Way has been observed and modeled intensely during the last decades. It is also the prime target of a number of new experiments that aim to zoom into the vicinity of its horizon and reveal the inner working of its spacetime. In this review we discuss our current understanding of the gravitational field of Sgr A* and the prospects of testing the Kerr nature of its spacetime via imaging, astrometric, and timing observations.
• Cooney, A., Dedeo, S., & Psaltis, D. (2010). Neutron stars in f(R) gravity with perturbative constraints. Physical Review D - Particles, Fields, Gravitation and Cosmology, 82(6).
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  Abstract: We study the structure of neutron stars in f(R) gravity theories with perturbative constraints. We derive the modified Tolman-Oppenheimer-Volkov equations and solve them for a polytropic equation of state. We investigate the resulting modifications to the masses and radii of neutron stars and show that observations of surface phenomena alone cannot break the degeneracy between altering the theory of gravity versus choosing a different equation of state of neutron-star matter. On the other hand, observations of neutron-star cooling, which depends on the density of matter at the stellar interior, can place significant constraints on the parameters of the theory. © 2010 The American Physical Society.
• Johannsen, T., & Psaltis, D. (2010). Testing the no-hair theorem with observations in the electromagnetic spectrum. I. Properties of a Quasi-Kerr spacetime. Astrophysical Journal Letters, 716(1), 187-197.
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  Abstract: According to the no-hair theorem, an astrophysical black hole is uniquely described by only two quantities, the mass and the spin. In this series of papers, we investigate a framework for testing the no-hair theorem with observations of black holes in the electromagnetic spectrum. We formulate our approach in terms of a parametric spacetime which contains a quadrupole moment that is independent of both mass and spin. If the no-hair theorem is correct, then any deviation of the black hole quadrupole moment from its Kerr value has to be zero. We analyze in detail the properties of this quasi-Kerr spacetime that are critical to interpreting observations of black holes and demonstrate their dependence on the spin and quadrupole moment. In particular, we show that the location of the innermost stable circular orbit and the gravitational lensing experienced by photons are affected significantly at even modest deviations of the quadrupole moment from the value predicted by the no-hair theorem. We argue that observations of black hole images, of relativistically broadened iron lines, as well as of thermal X-ray spectra from accreting black holes will lead in the near future to an experimental test of the no-hair theorem. © 2010 The American Astronomical Society.
• Lin, J., Özel, F., Chakrabarty, D., & Psaltis, D. (2010). The incompatibility of rapid rotation with narrow photospheric X-ray lines in EXO 0748-676. Astrophysical Journal Letters, 723(2), 1053-1056.
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  Abstract: X-ray observations of EXO 0748-676 during thermonuclear bursts revealed a set of narrow (Δλ/λ = 0.018) absorption lines that potentially originate from the stellar photosphere. The identification of these lines with particular atomic transitions led to the measurement of the surface gravitational redshift of the neutron star and to constraints on its mass and radius. However, the recent detection of 552 Hz oscillations at 15% rms amplitude revealed the spin frequency of the neutron star and brought into question the consistency of such a rapid spin with the narrow width of the absorption lines. Here, we calculate the amplitudes of burst oscillations and the width of absorption lines emerging from the surface of a rapidly rotating neutron star for a wide range of model parameters. We show that no combination of neutron star and geometric parameters can simultaneously reproduce the narrowness of the absorption lines, the high amplitude of the oscillations, and the observed flux at the time the oscillations were detected. We, therefore, conclude that the observed absorption lines are unlikely to originate from the surface of this neutron star. © 2010. The American Astronomical Society.
• Muleri, F., Feroci, M., Belloni, T., Braga, J., Campana, S., Courvousier, T., Hernanz, M., Hudec, R., Israel, G. L., Ray, P. S., Santangelo, A., Stella, L., Vacchi, A., Van, M., Walton, D., Zdziarski, A., Alvarez, J. M., Argan, A., Baldazzi, G., , Barbera, M., et al. (2010). LOFT: A large observatory for x-ray timing. Proceedings of Science.
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  Abstract: LOFT (Large area Observatory For x-ray Timing) is an innovative mission submitted in response to the Cosmic Vision "Call for a Medium-size mission opportunity for a launch in 2022" recently issued by ESA. LOFT is an ideal candidate for the next generation of (extremely) large experiments for X-ray timing dedicated to the study of the physics of compact objects and to the understanding of the behavior of matter in strong gravitational fields. Recent developments in the field of large area monolithic silicon detectors allowed us to reach an effective area ∼12 m2 (15 m2 goal), more than a order of magnitude larger that RXTE/PCA, in the energy range 2-30 keV (1-40 keV goal). This Large Area Detector (LAD) will have both high timing resolution (
• Yunes, N., Psaltis, D., Özel, F., & Loeb, A. (2010). Constraining parity violation in gravity with measurements of neutron-star moments of inertia. Physical Review D - Particles, Fields, Gravitation and Cosmology, 81(6).
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  Abstract: Neutron stars are sensitive laboratories for testing general relativity, especially when considering deviations where velocities are relativistic and gravitational fields are strong. One such deviation is described by dynamical, Chern-Simons modified gravity, where the Einstein-Hilbert action is modified through the addition of the gravitational parity-violating Pontryagin density coupled to a field. This four-dimensional effective theory arises naturally both in perturbative and nonperturbative string theory, loop quantum gravity, and generic effective field theory expansions. We calculate here Chern-Simons modifications to the properties and gravitational fields of slowly spinning neutron stars. We find that the Chern-Simons correction affects only the gravitomagnetic sector of the metric to leading order, thus introducing modifications to the moment-of-inertia but not to the mass-radius relation. We show that an observational determination of the moment-of-inertia to an accuracy of 10%, as is expected from near-future observations of the double pulsar, will place a constraint on the Chern-Simons coupling constant of ξ1/45km, which is at least three-orders of magnitude stronger than the previous strongest bound. © 2010 The American Physical Society.
• Özel, F., Psaltis, D., Narayan, R., & McClintock, J. E. (2010). The black hole mass distribution in the galaxy. Astrophysical Journal Letters, 725(2), 1918-1927.
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  Abstract: We use dynamical mass measurements of 16 black holes in transient low-mass X-ray binaries to infer the stellar black hole mass distribution in the parent population. We find that the observations are best described by a narrow mass distribution at 7.8±1.2M⊙.We identify a selection effect related to the choice of targets for optical follow-ups that results in a flux-limited sample.We demonstrate, however, that this selection effect does not introduce a bias in the observed distribution and cannot explain the absence of black holes in the 2-5M⊙ mass range. On the high-mass end, we argue that the rapid decline in the inferred distribution may be the result of the particular evolutionary channel followed by low-mass X-ray binaries. This is consistent with the presence of high-mass black holes in the persistent, high-mass X-ray binary sources. If the paucity of low-mass black holes is caused by a sudden decrease of the supernova explosion energy with increasing progenitor mass, this would have observable implications for ongoing transient surveys that target core-collapse supernovae. Our results also have significant implications for the calculation of event rates from the coalescence of black hole binaries for gravitational wave detectors. © 2010. The American Astronomical Society. All rights reserved.
• Özel, F., Psaltis, D., Ransom, S., Demorest, P., & Alford, M. (2010). The massive pulsar PSR J1614-2230: Linking quantum chromodynamics, gamma-ray bursts, and gravitational wave astronomy. Astrophysical Journal Letters, 724(2 PART 2), L199-L202.
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  Abstract: The recent measurement of the Shapiro delay in the radio pulsar PSR J1614-2230 yielded amass of 1.97±0.04M⊙, making it the most massive pulsar known to date. Its mass is high enough that, even without an accompanying measurement of the stellar radius, it has a strong impact on our understanding of nuclear matter, gamma-ray bursts (GRBs), and the generation of gravitational waves from coalescing neutron stars. This single high-mass value indicates that a transition to quark matter in neutron-star cores can occur at densities comparable to the nuclear saturation density only if the quarks are strongly interacting and are color superconducting. We further show that a high maximum neutron-star mass is required if short-duration GRBs are powered by coalescing neutron stars and, therefore, this mechanism becomes viable in the light of the recent measurement. Finally, we argue that the low-frequency (≤500 Hz) gravitational waves emitted during the final stages of neutron-star coalescence encode the properties of the equation of state because neutron stars consistent with this measurement cannot be centrally condensed. This will facilitate the measurement of the neutron star equation of state with Advanced LIGO/Virgo. © 2010. The American Astronomical Society. All rights reserved.
• Cooney, A., Dedeo, S., & Psaltis, D. (2009). Gravity with perturbative constraints: Dark energy without new degrees of freedom. Physical Review D - Particles, Fields, Gravitation and Cosmology, 79(4).
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  Abstract: Major observational efforts in the coming decade are designed to probe the equation of state of dark energy. Measuring a deviation of the equation-of-state parameter w from -1 would indicate a dark energy that cannot be represented solely by a cosmological constant. While it is commonly assumed that any implied modification to the ΛCDM model amounts to the addition of new dynamical fields, we propose here a framework for investigating whether or not such new fields are required when cosmological observations are combined with a set of minimal assumptions about the nature of gravitational physics. In our approach, we treat the additional degrees of freedom as perturbatively constrained and calculate a number of observable quantities, such as the Hubble expansion rate and the cosmic acceleration, for a homogeneous universe. We show that current observations place our universe within the perturbative validity of our framework and allow for the presence of nondynamical gravitational degrees of freedom at cosmological scales. © 2009 The American Physical Society.
• Gnedin, O. Y., MacCarone, T. J., Psaltis, D., & Zepf, S. E. (2009). Shrinking the braneworld: Black hole in a globular cluster. Astrophysical Journal Letters, 705(2 PART 2), L168-L171.
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  Abstract: Large extra dimensions have been proposed as a possible solution to the hierarchy problem in physics. In one of the suggested models, the RS2 braneworld model, black holes may evaporate by Hawking radiation faster than in general relativity, on a timescale that depends on the black hole mass and on the asymptotic radius of curvature of the extra dimensions. Thus the size of the extra dimensions can be constrained by astrophysical observations. Here we point out that the black hole, recently discovered in an extragalactic globular cluster, places the strongest upper limit on the size of the extra dimensions in the RS2 model, L ≲ 0.003mm. This black hole has the virtues of old age and relatively small mass. The derived upper limit is within an order of magnitude of the absolute limit afforded by astrophysical observations of black holes. © 2009. The American Astronomical Society.
• Johannsen, T., Psaltis, D., & McClintock, J. E. (2009). Constraints on the size of extra dimensions from the orbital evolution of black-hole X-ray binaries. Astrophysical Journal, 691(2), 997-1004.
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  Abstract: One of the plausible unification schemes in physics considers the observable universe to be a four-dimensional surface (the "brane") embedded in a higher dimensional curved spacetime (the "bulk"). In such braneworld gravity models with infinitely large extra dimensions, black holes evaporate fast through the emission of the additional gravitational degrees of freedom, resulting in lifetimes of stellar-mass black holes that are significantly smaller than the Hubble time. We show that the predicted evaporation rate leads to a change in the orbital period of X-ray binaries harboring black holes that is observable with current instruments. We obtain an upper limit on the rate of change of the orbital period of the binary A0620-00 and use it to constrain the asymptotic curvature radius of the extra dimension to a value comparable to the one obtained by table-top experiments. Furthermore, we argue that any measurement of a period increase for low-mass X-ray binaries with a high mass ratio is evidence for new physics beyond general relativity and the standard model. © 2009. The American Astronomical Society. All rights reserved..
• Mao, S. A., Psaltis, D., & Milsom, J. A. (2009). Super-keplerian frequencies in accretion disks. implications for mass and spin measurements of compact objects from x-ray variability studies. Astrophysical Journal Letters, 703(1), 717-720.
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  Abstract: The detection of fast quasi-periodic variability from accreting black holes and neutron stars has been used to constrain their masses, radii, and spins. If the observed oscillations are linear modes in the accretion disks, then bounds can be placed on the properties of the central objects by assuming that these modes are locally sub-Keplerian. If, on the other hand, the observed oscillations correspond to nonlinear resonances between disk modes, then the properties of the central objects can be measured by assuming that the resonant modes are excited at the same radial annulus in the disk. In this paper, we use numerical simulations of vertically integrated, axisymmetric hydrodynamic accretion disks to provide examples of situations in which the assumptions implicit in both methods are not satisfied. We then discuss our results for the robustness of the mass and spin measurements of compact objects from variability studies. © 2009 The American Astronomical Society. All rights reserved.
• Psaltis, D. (2009). Two approaches to testing general relativity in the strong-field regime. Journal of Physics: Conference Series, 189.
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  Abstract: Observations of compact objects in the electromagnetic spectrum and the detection of gravitational waves from them can lead to quantitative tests of the theory of general relativity in the strong-field regime following two very different approaches. In the first approach, the general relativistic field equations are modified at a fundamental level and the magnitudes of the potential deviations are constrained by comparison with observations. In the second approach, the exterior spacetimes of compact objects are parametrized in a phenomenological way, the various parameters are measured observationally, and the results are finally compared against the general relativistic predictions. In this article, I discuss the current status of both approaches, focusing on the lessons learned from a large number of recent investigations. © 2009 IOP Publishing Ltd.
• Özel, F., & Psaltis, D. (2009). Reconstructing the neutron-star equation of state from astrophysical measurements. Physical Review D - Particles, Fields, Gravitation and Cosmology, 80(10).
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  Abstract: The properties of matter at ultrahigh densities, low temperatures, and with a significant asymmetry between protons and neutrons can be studied exclusively through astrophysical observations of neutron stars. We show that measurements of the masses and radii of neutron stars can lead to tight constraints on the pressure of matter at three fiducial densities, from 1.85 to 7.4 times the density of nuclear saturation, in a manner that is largely model independent and that captures the key characteristics of the equation of state. We demonstrate that observations with 10% uncertainties of at least three neutron stars can lead to measurements of the pressure at these fiducial densities with an accuracy of 0.11 dex or □30%. Observations of three neutron stars with 5% uncertainties are sufficient to distinguish at a better than 3σ confidence level between currently proposed equations of state. In the electromagnetic spectrum, such accurate measurements will become possible for weakly magnetic neutron stars during thermonuclear flashes and in quiescence with future missions such as the International X-ray Observatory. © 2009 The American Physical Society.
• Özel, F., Güver, T., & Psaltis, D. (2009). The mass and radius of the neutron star in EXO 1745-248. Astrophysical Journal, 693(2), 1775-1779.
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  Abstract: Bursting X-ray binaries in globular clusters are ideal sources for measuring neutron star masses and radii, and hence, for determining the equation of state of cold, ultradense matter. We use time-resolved spectroscopic data from EXO 1745-248 during thermonuclear bursts that show strong evidence for photospheric radius expansion to measure the Eddington flux and the apparent surface area of the neutron star. We combine this with the recent measurement of the distance to the globular cluster Terzan 5, where this source resides, to measure the neutron star mass and radius. We find tightly constrained pairs of values for the mass and radius, which are centered around M = 1.4 M and R = 11 km or around M = 1.7 M and R = 9 km. These values favor nucleonic equations of state with symmetry energy that is relatively low and has a weak dependence on density. © 2009. The American Astronomical Society. All rights reserved..
• Alpar, M. A., & Psaltis, D. (2008). The highest dynamical frequency in the inner region of an accretion disc. Monthly Notices of the Royal Astronomical Society, 391(3), 1472-1476.
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  Abstract: In the inner regions of accretion discs around compact objects, the orbital frequency of the gas deviates from the local Keplerian value. For long-wavelength modes in this region, the radial epicyclic frequency κ is higher than the azimuthal frequency Ω. This has significant implications for models of the twin kilohertz quasi-periodic oscillations (QPOs) observed in many neutron star sources that traditionally identify the frequencies of the 2 kHz QPOs with dynamical frequencies in the accretion disc. The recognition that the highest frequency in the transition or boundary region of the disc may be the epicyclic frequency also significantly affects the constraints imposed by the observation of high-frequency QPOs on the mass and radius of the compact objects. © 2008 RAS.
• DeDeo, S., & Psaltis, D. (2008). Stable, accelerating universes in modified-gravity theories. Physical Review D - Particles, Fields, Gravitation and Cosmology, 78(6).
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  Abstract: Modifications to gravity that add additional functions of the Ricci curvature to the Einstein-Hilbert action-collectively known as f(R) theories-have been studied in great detail. When considered as complete theories of gravity they can generate nonperturbative deviations from the general relativistic predictions in the solar system, and the simplest models show instabilities on cosmological scales. Here, we show that it is possible to treat f(R)=R±μ4/R gravity in a perturbative fashion such that it shows no instabilities on cosmological scales and, in the solar system, is consistent with measurements of the parametrized post-Newtonian parameters. We show that such a theory produces a spatially flat, accelerating universe, even in the absence of dark energy and when the matter density is too small to close the Universe in the general relativistic case. © 2008 The American Physical Society.
• Erkut, M. H., Psaltis, D., & Alpar, M. A. (2008). Quasi-periodic oscillations as global hydrodynamic modes in the boundary layers of viscous accretion disks. Astrophysical Journal, 687(2), 1220-1229.
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  Abstract: The observational characteristics of quasi-periodic oscillations (QPOs) from accreting neutron stars strongly indicate the oscillatory modes in the innermost regions of accretion disks as a likely source of the QPOs. The inner regions of accretion disks around neutron stars can harbor very high frequency modes related to the radial epicyclic frequency κ. The degeneracy of κ with the orbital frequency Ω is removed in a non-Keplerian boundary or transition zone near themagnetopause between the disk and the compact object.We show, by analyzing the global hydrodynamic modes of long wavelength in the boundary layers of viscous accretion disks, that the fastest growing mode frequencies are associated with frequency bands around κ and κ ± Ω. The maximum growth rates are achieved near the radius where the orbital frequency Ω is maximum. The global hydrodynamic parameters such as the surface density profile and the radial drift velocity determine which modes of free oscillations will grow at a given particular radius in the boundary layer. In accordance with the peak separation between kHz QPOs observed in neutron star sources, the difference frequency between two consecutive bands of the fastest growing modes is always related to the spin frequency of the neutron star. This is a natural outcome of the boundary condition imposed by the rotating magnetosphere on the boundary region of the inner disk. © 2008. The American Astronomical Society. All rights reserved.
• Galloway, D. K., Muno, M. P., Hartman, J. M., Psaltis, D., & Chakrabarty, D. (2008). Thermonuclear (type i) X-ray bursts observed by the rossi X-ray timing explorer. Astrophysical Journal, Supplement Series, 179(2), 360-422.
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  Abstract: We have assembled a sample of 1187 thermonuclear (type I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than 10 years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate. We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that the peak flux of such bursts varies typically by 13%. We classified sources into two main groups based on the burst properties: (1) both long and short bursts (indicating mixed H/He accretion), and (2) consistently short bursts (primarily He accretion), and we calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at >0.06MEdd ({suceeds or eeequivalent to}2 × 1037 ergs s-1) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He burning. We found many examples of bursts with recurrence times
• Galloway, D. K., Özel, F., & Psaltis, D. (2008). Biases for neutron star mass, radius and distance measurements from Eddington-limited X-ray bursts. Monthly Notices of the Royal Astronomical Society, 387(1), 268-272.
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  Abstract: Eddington-limited X-ray bursts from neutron stars can be used in conjunction with other spectroscopic observations to measure neutron star masses, radii and distances. In order to quantify some of the uncertainties in the determination of the Eddington limit, we analysed a large sample of photospheric radius-expansion thermonuclear bursts observed with the Rossi X-ray Timing Explorer. We identified the instant at which the expanded photosphere 'touches down' back on to the surface of the neutron star and compared the corresponding touchdown flux to the peak flux of each burst. We found that for the majority of sources, the ratio of these fluxes is smaller than ≃1.6, which is the maximum value expected from the changing gravitational redshift during the radius expansion episodes (for a 2 M⊙ neutron star). The only sources for which this ratio is larger than ≃1.6 are high-inclination sources that include dippers and Cyg X-2. We discuss two possible geometric interpretations of this effect and show that the inferred masses and radii of neutron stars are not affected by this bias. On the other hand, systematic uncertainties as large as ∼50 per cent may be introduced to the distance determination. © 2008 RAS.
• Pessah, M. E., Chan, C., & Psaltis, D. (2008). The fundamental difference between shear alpha viscosity and turbulent magnetorotational stresses. Monthly Notices of the Royal Astronomical Society, 383(2), 683-690.
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  Abstract: Numerical simulations of turbulent, magnetized, differentially rotating flows driven by the magnetorotational instability (MRI) are often used to calculate the effective values of alpha viscosity that is invoked in analytical models of accretion discs. In this paper, we use various dynamical models of turbulent magnetohydrodynamic stresses, as well as numerical simulations of shearing boxes, to show that angular momentum transport in MRI-driven accretion discs cannot be described by the standard model for shear viscosity. In particular, we demonstrate that turbulent magnetorotational stresses are not linearly proportional to the local shear and vanish identically for angular velocity profiles that increase outwards. © 2007 The Authors. Journal compilation © 2007 RAS.
• Psaltis, D. (2008). Constraining brans-dicke gravity with accreting millisecond pulsars in ultracompact binaries. Astrophysical Journal, 688(2), 1282-1287.
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  Abstract: The five accreting, millisecond X-ray pulsars in ultracompact binaries that were recently discovered with the Rossi X-Ray Timing Explorer provide excellent candidates for constraining the deviations from general relativity described by the Brans-Dicke parameter ωBD. I calculate the expected rate of change of the orbital periods of these binaries and discuss the prospects of constraining ωBD to values that can be as stringent as current solar system constraints. Finally, I show how measurements of the orbital period derivative in ultracompact binaries can be used to place lower bounds on their orbital inclinations. © 2008. The American Astronomical Society. All rights reserved.
• Psaltis, D. (2008). Probes and tests of strong-field gravity with observations in the electromagnetic spectrum. Living Reviews in Relativity, 11.
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  Abstract: Neutron stars and black holes are the astrophysical systems with the strongest gravitational fields in the universe. In this article, I review the prospect of using observations of such compact objects to probe some of the most intriguing general relativistic predictions in the strong-field regime: the absence of stable circular orbits near a compact object and the presence of event horizons around black-hole singularities. I discuss the need for a theoretical framework, within which future experiments will provide detailed, quantitative tests of gravity theories. Finally, I summarize the constraints imposed by current observations of neutron stars on potential deviations from general relativity.
• Psaltis, D. (2008). Testing general metric theories of gravity with bursting neutron stars. Physical Review D - Particles, Fields, Gravitation and Cosmology, 77(6).
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  Abstract: I show that several observable properties of bursting neutron stars in metric theories of gravity can be calculated using only conservation laws, Killing symmetries, and the Einstein equivalence principle, without requiring the validity of the general relativistic field equations. I calculate, in particular, the gravitational redshift of a surface atomic line, the touchdown luminosity of a radius-expansion burst, which is believed to be equal to the Eddington critical luminosity, and the apparent surface area of a neutron star as measured during the cooling tails of bursts. I show that, for a general metric theory of gravity, the apparent surface area of a neutron star depends on the coordinate radius of the stellar surface and on its gravitational redshift in the exact same way as in general relativity. On the other hand, the Eddington critical luminosity depends also on an additional parameter that measures the degree to which the general relativistic field equations are satisfied. These results can be used in conjunction with current and future high-energy observations of bursting neutron stars to test general relativity in the strong-field regime. © 2008 The American Physical Society.
• Ballantyne, D. R., Özel, F., & Psaltis, D. (2007). Constraining radiatively inefficient accretion flows with polarization. Astrophysical Journal, 663(1 II), L17-L20.
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  Abstract: The low-luminosity black hole Sgr A* provides a test bed for models of radiatively inefficient accretion flows (RIAFs). Recent submillimeter linear polarization measurements of Sgr A* have provided evidence that the electrons in the accretion flow are relativistic over a large range of radii. Here we show that these high temperatures result in elliptical plasma normal modes. Thus, polarized millimeter and submillimeter radiation emitted within RIAFs will undergo generalized Faraday rotation, a cyclic conversion between linear and circular polarization. This effect will not depolarize the radiation even if the rotation measure is extremely high. Rather, the beam will take on the linear and circular polarization properties of the plasma normal modes. As a result, polarization measurements of Sgr A* in this frequency regime will constrain the temperature, density, and magnetic profiles of RIAF models. © 2007. The American Astronomical Society. All rights reserved.
• Pessah, M. E., Chan, C., & Psaltis, D. (2007). Angular momentum transport in accretion disks: Scaling laws in MRI-driven turbulence. Astrophysical Journal, 668(1 PART 2), L51-L54.
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  Abstract: We present a scaling law that predicts the values of the stresses obtained in numerical simulations of saturated MRI-driven turbulence in nonstratified shearing boxes. It relates the turbulent stresses to the strength of the vertical magnetic field, the sound speed, the vertical size of the box, and the numerical resolution and predicts accurately the results of 35 numerical simulations performed for a wide variety of physical conditions. We use our result to show that the saturated stresses in simulations with zero net magnetic flux depend linearly on the numerical resolution and would become negligible if the resolution were set equal to the natural dissipation scale in astrophysical disks. We conclude that in order for MRI-driven turbulent angular momentum transport to be able to account for the large value of the effective alpha viscosity inferred observationally, the disk must be threaded by a significant vertical magnetic field and the turbulent magnetic energy must be in near equipartition with the thermal energy. This result has important implications for the spectra of accretion disks and their stability. © 2007, The American Astronomical Society.
• Psaltis, D. (2007). Constraints on braneworld gravity models from a kinematic limit on the age of the black hole XTE J1118+480. Physical Review Letters, 98(18).
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  Abstract: In braneworld gravity models with a finite anti-de Sitter space (AdS) curvature in the extra dimension, the AdS/conformal field theory correspondence leads to a prediction for the lifetime of astrophysical black holes that is significantly smaller than the Hubble time, for asymptotic curvatures that are consistent with current experiments. Using the recent measurements of the position, three-dimensional spatial velocity, and mass of the black hole XTE J1118+480, I calculate a lower limit on its kinematic age of 11Myr (95% confidence). This translates into an upper limit for the asymptotic AdS curvature in the extra dimensions of
• Psaltis, D., & Psaltis, D. -. (2007). Constraints on braneworld gravity models from a kinematic limit on the age of the black hole XTE J1118+480. Physical review letters, 98(18).
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  In braneworld gravity models with a finite anti-de Sitter space (AdS) curvature in the extra dimension, the AdS/conformal field theory correspondence leads to a prediction for the lifetime of astrophysical black holes that is significantly smaller than the Hubble time, for asymptotic curvatures that are consistent with current experiments. Using the recent measurements of the position, three-dimensional spatial velocity, and mass of the black hole XTE J1118+480, I calculate a lower limit on its kinematic age of > or =11 Myr (95% confidence). This translates into an upper limit for the asymptotic AdS curvature in the extra dimensions of
• Chan, C., Psaltis, D., & Özel, F. (2006). Spectral methods for time-dependent studies of accretion flows. II. Two-dimensional hydrodynamic disks with self-gravity. Astrophysical Journal Letters, 645(1 I), 506-518.
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  Abstract: Spectral methods are well suited for solving hydrodynamic problems in which the self-gravity of the flow needs to be considered. Because Poisson's equation is linear, the numerical solution for the gravitational potential for each individual mode of the density can be precomputed, thus reducing substantially the computational cost of the method. In this second paper, we describe two different approaches to computing the gravitational field of a two-dimensional flow with pseudospectral methods. For situations in which the density profile is independent of the third coordinate (i.e., an infinite cylinder), we use a standard Poisson solver in spectral space. On the other hand, for situations in which the density profile is a δ-function along the third coordinate (i.e., an infinitesimally thin disk), or any other function known a priori, we perform a direct integration of Poisson's equation using a Green's functions approach. We devise a number of test problems to verify the implementations of these two methods. Finally, we use our method to study the stability of polytropic, self-gravitating disks. We find that when the polytropic index Γ is ≤4/3, Toomre's criterion correctly describes the stability of the disk. However, when Γ > 4/3 and for large values of the polytropic constant K, the numerical solutions are always stable, even when the linear criterion predicts the contrary. We show that in the latter case, the minimum wavelength of the unstable modes is larger than the extent of the unstable region, and hence the local linear analysis is inapplicable. © 2006. The American Astronomical Society. All rights reserved.
• Galloway, D. K., Psaltis, D., Muno, M. P., & Chakrabarty, D. (2006). Eddington-limited X-ray bursts as distance indicators. II. Possible compositional effects in bursts from 4U 1636-536. Astrophysical Journal Letters, 639(2 I), 1033-1038.
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  Abstract: We analyzed 123 thermonuclear (type I) X-ray bursts observed by the Rossi X-Ray Timing Explorer (RXTE) from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius expansion bursts in this sample reached peak fluxes normally distributed about a mean of 6.4 × 10-8 ergs cm -2 s-1, with a standard deviation of 7.6%. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 ± 0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ≈2. This variation is comparable to the range of the Eddington limit between material with solar H fraction (X = 0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is either eliminated by steady hot CNO burning or expelled in a radiatively driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ≈2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius-expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X ≈ 0.7), and the brighter bursts the limit for pure He (X = 0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with MNS = 1.4 M⊙, RNS = 10 km) to be 6.0 ± 0.5 kpc, or for MNS = 2 M⊙ at most 7.1 kpc. © 2006. The American Astronomical Society. All rights reserved.
• Kylafis, N., Giannios, D., & Psaltis, D. (2006). Spectra and time variability of black-hole binaries in the low/hard state. Advances in Space Research, 38(12), 2810-2812.
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  Abstract: We propose a jet model for the low/hard state of galactic black-hole X-ray sources which explains the energy spectra from radio to X-rays and a number of timing properties in the X-ray domain such as the time lag spectra, the hardening of the power density spectra and the narrowing of the autocorrelation function with increasing photon energy. The model assumes that (i) there is a magnetic field along the axis of the jet, (ii) the electron density in the jet drops inversely proportional to distance, (iii) the jet is "hotter" near its center than at its periphery, and (iv) the electrons in the jet follow a power-law distribution function. We have performed Monte Carlo simulations of Compton upscattering of soft photons from the accretion disk and have found power-law high-energy spectra with photon-number index in the range 1.5-2 and cutoff at a few hundred keV, power-law time lags versus Fourier frequency with index ∼0.8, and an increase of the rms amplitude of variability and a narrowing of the autocorrelation function with increasing photon energy as they have been observed in Cygnus X-1. The spectrum at long wavelengths (radio, infrared, optical) is modeled to come from synchrotron radiation of the energetic electrons in the jet. We find flat to inverted radio spectra that extend from the radio up to about the optical band. For magnetic field strengths of the order 105-106 G at the base of the jet, the calculated spectra agree well in slope and flux with the observations. © 2005 COSPAR.
• Pessah, M. E., Chan, C., & Psaltis, D. (2006). Local model for angular-momentum transport in accretion disks driven by the magnetorotational instability. Physical Review Letters, 97(22).
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  Abstract: We develop a local model for the exponential growth and saturation of the Reynolds and Maxwell stresses in turbulent flows driven by the magnetorotational instability. We first derive equations that describe the effects of the instability on the growth and pumping of the stresses. We highlight the relevance of a new type of correlations that couples the dynamical evolution of the Reynolds and Maxwell stresses and plays a key role in developing and sustaining the magnetorotational turbulence. We then supplement these equations with a phenomenological description of the triple correlations that lead to a saturated turbulent state. We show that the steady-state limit of the model describes successfully the correlations among stresses found in numerical simulations of shearing boxes. © 2006 The American Physical Society.
• Pessah, M. E., Chan, C., & Psaltis, D. (2006). The signature of the magnetorotational instability in the Reynolds and Maxwell stress tensors in accretion discs. Monthly Notices of the Royal Astronomical Society, 372(1), 183-190.
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  Abstract: The magnetorotational instability is thought to be responsible for the generation of magnetohydrodynamic turbulence that leads to enhanced outward angular momentum transport in accretion discs. Here, we present the first formal analytical proof showing that, during the exponential growth of the instability, the mean (averaged over the disc scaleheight) Reynolds stress is always positive, the mean Maxwell stress is always negative, and hence the mean total stress is positive and leads to a net outward flux of angular momentum. More importantly, we show that the ratio of the Maxwell to the Reynolds stresses during the late times of the exponential growth of the instability is determined only by the local shear and does not depend on the initial spectrum of perturbations or the strength of the seed magnetic field. Even though we derived this property of the stress tensors for the exponential growth of the instability in incompressible flows, numerical simulations of shearing boxes show that this characteristic is qualitatively preserved under more general conditions, even during the saturated turbulent state generated by the instability. © 2006 RAS.
• Psaltis, D., Pessah, M. E., Chan, C., & Psaltis, D. -. (2006). Local model for angular-momentum transport in accretion disks driven by the magnetorotational instability. Physical review letters, 97(22).
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  We develop a local model for the exponential growth and saturation of the Reynolds and Maxwell stresses in turbulent flows driven by the magnetorotational instability. We first derive equations that describe the effects of the instability on the growth and pumping of the stresses. We highlight the relevance of a new type of correlations that couples the dynamical evolution of the Reynolds and Maxwell stresses and plays a key role in developing and sustaining the magnetorotational turbulence. We then supplement these equations with a phenomenological description of the triple correlations that lead to a saturated turbulent state. We show that the steady-state limit of the model describes successfully the correlations among stresses found in numerical simulations of shearing boxes.
• Chan, C., Psaltis, D., & Özel, F. (2005). Spectral methods for time-dependent studies of accretion flows. I. Two-dimensional, viscous, hydrodynamic disks. Astrophysical Journal Letters, 628(1 I), 353-367.
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  Abstract: We present a numerical method for studying the normal modes of accretion flows around black holes. In this first paper, we focus on two-dimensional, viscous, hydrodynamic disks, for which the linear modes have been calculated analytically in previous investigations. We use pseudospectral methods and low-storage Runge-Kutta methods to solve the continuity equation, the Navier-Stokes equation, and the energy equation. We devise a number of test problems to verify the implementation. These tests demonstrate the ability of spectral methods to handle accurately advection problems and to reproduce correctly the stability criteria for differentially rotating hydrodynamic flows. They also show that our implementation is able to handle sound waves correctly with nonreflective boundary conditions, to recover the standard solution for a viscous-spreading ring, and to produce correctly the Shakura-Sunyaev steady-disk solution. Finally, we have applied our algorithm to the problem of a nonaxisymmetric viscous-spreading ring and verify that such configuration is unstable to nonaxisymmetric perturbations. © 2005. The American Astronomical Society. All rights reserved.
• Pessah, M. E., & Psaltis, D. (2005). The stability of magnetized rotating plasmas with superthermal fields. Astrophysical Journal Letters, 628(2 I), 879-901.
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  Abstract: During the last decade it has become evident that the magnetorotational instability is at the heart of the enhanced angular momentum transport in weakly magnetized accretion disks around neutron stars and black holes. In this paper we investigate the local linear stability of differentially rotating, magnetized flows and the evolution of the magnetorotational instability beyond the weak-field limit. We show that, when superthermal toroidal fields are considered, the effects of both compressibility and magnetic tension forces, which are related to the curvature of toroidal field lines, should be taken fully into account. We demonstrate that the presence of a strong toroidal component in the magnetic field plays a nontrivial role. When strong fields are considered, the strength of the toroidal magnetic field not only modifies the growth rates of the unstable modes but also determines which modes are subject to instabilities. We find that, for rotating configurations with Keplerian laws, the magnetorotational instability is stabilized at low wave-numbers for toroidal Alfvén speeds exceeding the geometric mean of the sound speed and the rotational speed. For a broad range of magnetic field strengths, we also find that two additional distinct instabilities are present; they both appear as the result of coupling between the modes that become the Alfvén and the slow modes in the limit of no rotation. We discuss the significance of our findings for the stability of cold, magnetically dominated, rotating fluids and argue that, for these systems, the curvature of toroidal field lines cannot be neglected even when short-wavelength perturbations are considered. We also comment on the implications of our results for the validity of shearing box simulations in which superthermal toroidal fields are generated. © 2005. The American Astronomical Society. All rights reserved.
• Giannios, D., Kylafis, N. D., & Psaltis, D. (2004). Spectra and time variability of Galactic black-hole X-ray sources in the low/hard state. Astronomy and Astrophysics, 425(1), 163-169.
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  Abstract: We propose a jet model for the low/hard state of galactic black-hole X-ray sources which explains a) the X-ray spectra; b) the time-lag spectra; c) the increase of the variability amplitude (QPO and high frequency) with increasing photon energy; and d) the narrowing of the autocorrelation function with increasing photon energy. The model (in its simplest form) assumes that i) there is a uniform magnetic field along the axis of the jet; ii) the electron density in the jet is inversely proportional to distance; and iii) the jet is "hotter" near its center than at its periphery. We have performed Monte Carlo simulations of Compton upscattering of soft photons from the accretion disk and have found power-law high-energy spectra with photon-number index in the range 1.5-2, power-law time lags versus Fourier frequency with index ∼0.8, and an increase of the rms amplitude of variability and a narrowing of the autocorrelation function with photon energy as has been observed in Cygnus X-1. Similar energy spectra and time variability have been observed in neutron-star systems in the island state. It is therefore quite likely that a similar model holds for these sources as well.
• DeDeo, S., & Psaltis, D. (2003). Towards new tests of strong-field gravity with measurements of surface atomic line, redshifts from neutron stars. Physical Review Letters, 90(14), 141101/1-141101/4.
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  Abstract: To date, studies of neutron stars have focused on better determining the equation of state, whose details are still debated. In this paper, it is argued that the uncertainty in the behavior of strong-field gravity introduces much greater differences in neutron-star properties than do current uncertainties in the neutron-star equations of state. Furthermore, it is shown that new observational developments allow for the measurement of the gravitational redshift of surface atomic lines, which is the cleanest source of information about the mass, radius, and spin of neutron stars.
• Galloway, D. K., Psaltis, D., Chakrabarty, D., & Muno, M. P. (2003). Eddington-limited X-ray bursts as distance indicators. I. Systematic trends and spherical symmetry in bursts from 4U 1728-34. Astrophysical Journal Letters, 590(2 I), 999-1007.
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  Abstract: We investigate the limitations of thermonuclear X-ray bursts as a distance indicator for the weakly magnetized accreting neutron star 4 U 1728 - 34. We measured the unabsorbed peak flux of 81 bursts in public data from the Roxsi X-Ray Timing Explorer (RXTE). The distribution of peak fluxes was bimodal: 66 bursts exhibited photospherk; radius expansion (presumably reaching the local Eddington limit) and were distributed about a mean bolometric flux of 9.2 × 10-8 ergs cm-2 s-1, while the remaining (non-radius expansion) bursts reached 4.5 × 10-8 ergs cm-2 s-1, on average. The peak fluxes of the radius expansion bursts were not constant, exhibiting a standard deviation of 9.4% and a total variation of 46%. These bursts showed significant correlations between their peak flux and the X-ray colors of the persistent emission immediately prior to the burst. We also found evidence for quasi-periodic variation of the peak fluxes of radius expansion bursts, with a timescale of ≃40 days. The persistent flux observed with RXTE/ASM over 5.8 yr exhibited quasiperiodic variability on a similar timescale. We suggest that these variations may have a common origin in reflection from a warped accretion disk. Once the systematic variation of the peak burst fluxes is subtracted, the residual scatter is only ≃3%, roughly consistent with the measurement uncertainties. The narrowness of this distribution strongly suggests that (1) the radiation from the neutron star atmosphere during radius expansion episodes is nearly spherically symmetric and (2) the radius expansion bursts reach a common peak flux that may be interpreted as a standard candle intensity. Adopting the minimum peak flux for the radius expansion bursts as the Eddington flux limit, we derive a distance for the source of 4.4-4.8 kpc (assuming RNS = 10 km), with the uncertainty arising from the probable range of the neutron star mass M NS = 1.4-2 M⊙.
• Vignarca, F., Migliari, S., Belloni, T., Psaltis, D., & Van, M. (2003). Tracing the power-law component in the energy spectrum of black hole candidates as a function of the QPO frequency. Astronomy and Astrophysics, 397(2), 729-738.
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  Abstract: We investigated the relation between the centroid frequency of the quasi-periodic oscillation observed in the power density spectra of a sample of galactic black-hole candidates with the power-law photon index obtained from spectral fits. Our aim is to avoid inner accretion disk radius determination directly from spectral fits, given the uncertainties of the absolute values obtained in that way, but to base our analysis on the likely association of QPO frequency to a characteristic radius. We used archival RXTE data of GRS 1915+105 and published parameters for GRO 1655-40, XTE J1550-564, XTE J1748-288 and 4U 1630-47. While for low values of the QPO frequency, the two parameters are clearly correlated for each source, there is evidence for a turnoff in the correlation above a characteristic frequency, different for different sources. We discuss the possible nature of this turnoff.
• Özel, F., & Psaltis, D. (2003). Spectral lines from rotating neutron stars. Astrophysical Journal Letters, 582(1 II), L31-L34.
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  Abstract: The line profiles from rotating neutron stars are affected by a number of relativistic processes such as Doppler boosts, strong self-lensing, frame dragging, and the differential gravitational redshift arising from the stellar oblateness. In this Letter, we calculate line profiles, taking into account the first two effects; this approach is accurate for rotation rates less than the breakup frequency. We show that the line profiles are not only broadened and weakened but are also significantly asymmetric, and allow for an independent measurement of both the mass and the radius of the neutron star. Furthermore, we investigate the case when a fraction of the neutron star surface contributes to the emission and find that the line profiles are typically doubly peaked. We discuss the implications of our results for searches for line features in the spectra of isolated neutron stars and X-ray bursters. We finally assess the systematic uncertainties introduced by the line asymmetry in inferring the compactness of neutron stars from the detection of redshifted lines.
• Belloni, T., Psaltis, D., & Van, M. (2002). A unified description of the timing features of accreting X-ray binaries. Astrophysical Journal Letters, 572(1 I), 392-406.
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  Abstract: We study an empirical model for a unified description of the power spectra of accreting neutron stars and black holes. This description is based on a superposition of multiple Lorentzians and offers the advantage that all quasi-periodic oscillation and noise components are dealt with in the same way, without the need of deciding in advance the nature of each component. This approach also allows us to compare frequencies of features with high and low coherences in a consistent manner and greatly facilitates comparison of power spectra across a wide range of source types and states. We apply the model to six sources: the low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269, and GS 1826-24; the high-latitude transient XTE J1118+480; the bright system Cir X-1; and the Z source GX 17+2. We find that it provides a good description of the observed spectra without the need for a scale-free (1/f) component. We update previously reported correlations between characteristic frequencies of timing features in the light of this new approach and discuss similarities between different types of systems that may point toward similar underlying physics.
• Muno, M. P., Chakrabarty, D., Galloway, D. K., & Psaltis, D. (2002). The frequency stability of millisecond oscillations in thermonuclear X-ray bursts. Astrophysical Journal Letters, 580(2 I), 1048-1059.
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  Abstract: We analyze the frequency evolution of millisecond oscillations observed during type I X-ray bursts with the Rossi X-Ray Timing Explorer, in order to establish the stability of the mechanism underlying the oscillations. Our sample contains 68 pulse trains detected in a search of 159 bursts from eight accreting neutron stars. As a first step, we confirm that the oscillations usually drift upward in frequency by about 1% toward an apparent saturation frequency. Previously noted anomalies, such as drifts toward lower frequencies as the oscillations disappear ("spin-down" episodes) and instances of two signals present simultaneously at frequencies separated by a few Hz, occur in 5% of oscillations. Having verified the generally accepted description of burst oscillations, we proceed to study the coherence of the oscillations during individual bursts and the dispersion in the asymptotic frequencies in bursts observed over five years. On short timescales, we find that 30% of the oscillation trains do not appear to evolve smoothly in phase. This suggests either that two signals are present simultaneously with a frequency difference too small to resolve (≲ 1 Hz), that the frequency evolution is discontinuous, or that discrete phase jumps occur. On timescales of years, the maximum frequencies of the oscillations exhibit fractional dispersions of Δνmax/(〈νmax)〉 ≲4 × 10-3. In the case of 4U 1636-536, this dispersion is uncorrelated with the known orbital phase, which indicates that a mechanism besides orbital Doppler shifts prevents the oscillations from appearing perfectly stable. In the course of this analysis, we also search for connections between the properties of the oscillations and the underlying bursts. We find that the magnitudes of the observed frequency drifts are largest when the oscillations are first observed at the start of the burst, which suggests that their evolution begins when the burst is ignited. We also find that radius expansion appears to temporarily interrupt the oscillation trains. We interpret these results under the assumption that the oscillations originate from anisotropies in the emission from the surfaces of these rotating neutron stars.
• Psaltis, D. (2002). Radiative transfer in obliquely illuminated accretion disks. Astrophysical Journal Letters, 574(1 I), 306-314.
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  Abstract: The illumination of an accretion disk around a black hole or neutron star by the central compact object or the disk itself often determines its spectrum, stability, and dynamics. The transport of radiation within the disk is, in general, a multidimensional, nonaxisymmetric problem, which is challenging to solve. Here I present a method of decomposing the radiative transfer equation that describes absorption, emission, and Compton scattering in an obliquely illuminated disk into a set of four one-dimensional transfer equations. I show that the exact calculation of the ionization balance and radiation heating of the accretion disk requires the solution of only one of the one-dimensional equations, which can be solved using existing numerical methods. I present a variant of the Feautrier method for solving the full set of equations, which accounts for the fact that the scattering kernels in the individual transfer equations are not forward-backward symmetric. I then apply this method in calculating the albedo of a cold, geometrically thin accretion disk.
• Psaltis, D., & Miller, M. C. (2002). Implications of the narrow period distribution of anomalous X-ray pulsars and soft gamma-ray repeaters. Astrophysical Journal Letters, 578(1 I), 325-329.
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  Abstract: The spin periods of 10 observed anomalous X-ray pulsars and soft gamma-ray repeaters lie in the very narrow range of 6-12 s. We use a point-likelihood technique to show that the observed period clustering is not simply a statistical fluctuation, and we quantify the constraints it imposes on the birth period and on the final period of such systems. We consider a general law for their spin evolution described by a constant braking index. We find that, for positive values of the braking index, the observed clustering requires an upper cutoff period that is very close to the maximum observed period of ≃ 12 s. We also show that the constraint on the birth period depends very strongly on the assumed value of the braking index n, ranging from a few milliseconds for n ≳ 2 to a few seconds for n ≲ 2. We discuss possible ways of tightening these constraints based on similarities with the population of radio pulsars and by future observations of such sources with current X-ray observatories.
• Dedeo, S., Psaltis, D., & Narayan, R. (2001). General relativistic constraints on emission models of anomalous X-ray pulsars. Astrophysical Journal Letters, 559(1 PART 1), 346-352.
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  Abstract: Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray spectra and pulsations by means of radiation processes that occur on the surfaces of neutron stars. For any such model, general relativistic deflection of light severely suppresses the amplitude of the observed pulsations. We calculate the expected pulsation amplitudes of AXPs according to various models and compare the results with observations. We show that the high (≲ 70%) pulse amplitudes observed in some AXPs can be accounted for only if the surface emission is localized (spot radius < 40°) and strongly beamed (e.g., radially peaked with cosn θ′ and n ≳ 2, where θ′ is the angle to the normal). These constraints are incompatible with those cooling and magnetar models in which the observed X-rays originate as thermal emission from the neutron star surface. Accretion models, on the other hand, are compatible with observations for a wide range of parameters. Finally, definitive conclusions cannot be reached on magnetospheric models, since their localization and beaming properties are not well understood.
• Juett, A. M., Psaltis, D., & Chakrabarty, D. (2001). Ultracompact X-ray binaries with neon-rich degenerate donors. Astrophysical Journal Letters, 560(1 PART 2), L59-L63.
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  Abstract: There are three low-mass X-ray binaries (4U 0614+091, 2S 0918-549, and 4U 1543-624) for which broadline emission near 0.7 keV was previously reported. A recent high-resolution observation of 4U 0614+091 with the Chandra Low-Energy Transmission Grating Spectrometer (LETGS) found evidence of an unusually high Ne/O abundance ratio along the line of sight but failed to detect the previously reported 0.7 keV feature. We have made a search of archival ASCA spectra and identified a fourth source with the 0.7 keV feature, the 20 minute ultracompact binary 4U 1850-087. In all four of these sources, the 0.7 keV residual is eliminated from the ASCA spectra by allowing excess photoelectric absorption due to a nonsolar relative abundance of neon, just as in the LETGS spectrum of 4U 0614+091. The optical properties of these systems suggest that all four are ultracompact (Porb < 80 minutes) binaries. We propose that there is excess neon local to each of these sources, as also found in the ultracompact binary pulsar 4U 1626-67. We suggest that the mass donor in these systems is a low-mass, neon-rich degenerate dwarf and that the binaries are all ultracompact.
• Psaltis, D. (2001). Compton scattering in static and moving media. II. System-frame solutions for spherically symmetric flows. Astrophysical Journal Letters, 555(2 PART 1), 786-800.
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  Abstract: I study the formation of Comptonization spectra in spherically symmetric, fast-moving media in a flat spacetime. I analyze the mathematical character of the moments of the transfer equation in the system frame and describe a numerical method that provides fast solutions of the time-independent radiative transfer problem that are accurate in both the diffusion and free-streaming regimes. I show that even if the flows are mildly relativistic (V ∼ 0.1, where V is the electron bulk velocity in units of the speed of light), terms that are second order in V alter the emerging spectrum both quantitatively and qualitatively. In particular, terms that are second order in V produce power-law spectral tails, which are the dominant feature at high energies, and therefore cannot be neglected. I further show that photons from a static source are upscattered by the bulk motion of the medium even if the velocity field does not converge. Finally, I discuss these results in the context of radial accretion onto and outflows from compact objects.
• Psaltis, D. (2001). Models of quasi-periodic variability in neutron stars and black holes. Advances in Space Research, 28(2-3), 481-491.
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  Abstract: Black holes and weakly-magnetic accreting neutron stars show strong quasi-periodic variability over timescales that span a very wide range. This property offer us a unique opportunity to improve our understanding of basic physical processes in strong gravitational fields as it reveals, for the first time, phenomena that occur on the most fundamental timescales near accreting compact objects. In this article, I review our current understanding of the variability properties of accretion flows into neutron stars and black holes. I discuss a number of challenges faced by theoretical models, in order to identify open questions in accretion physics that need to be addressed. Finally, I discuss the relation to and implications for variability models of the detection of nearly coherent oscillations during type I X-ray bursts in several neutron-star sources. © 2001 Cospar. Published by Elsevier Science Ltd. All rights reserved.
• Özel, F., Psaltis, D., & Kaspi, V. M. (2001). Constraints on thermal emission models of anomalous X-ray pulsars. Astrophysical Journal Letters, 563(1 PART 1), 255-266.
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  Abstract: Thermal emission from the surface of an ultramagnetic neutron star is believed to contribute significantly to the soft X-ray flux of the anomalous X-ray pulsars (AXPs). We compare the detailed predictions of models of the surface emission from a magnetar to the observed spectral and variability properties of AXPs. In particular, we focus on the combination of their luminosities and energy-dependent pulsed fractions. We use the results of recent calculations for strongly magnetized atmospheres in radiative equilibrium to obtain the angle-and energy-dependence of the surface emission. We also include in our calculations the significant effects of general relativistic photon transport to an observer at infinity as well as the effects of interstellar extinction. We find that the combination of the large pulsed fractions and the high inferred luminosities of AXPs cannot be accounted for by surface emission from a magnetar with two antipodal hot regions or a temperature distribution characteristic of a magnetic dipole. This result is robust for reasonable neutron star radii, for the range of magnetic field strengths inferred from the observed spin down rates, and for surface temperatures consistent with the spectral properties of AXPs. Models with a single hot emitting region can reproduce the observations, provided that the distance to one of the sources is ∼30% less than the current best estimate, and allowing for systematic uncertainties in the spectral fit of a second source. Finally, the thermal emission models with antipodal emission geometry predict a characteristic strong increase of the pulsed fraction with photon energy, which is apparently inconsistent with the current data. The energy-dependence of the pulsed fraction in the models with one hot region shows a wider range of behavior and can be consistent with the existing data. Upcoming high-resolution observations with Chandra and XMM-Newton will provide a conclusive test.
• Kalogera, V., & Psaltis, D. (2000). Bounds on neutron-star moments of inertia and the evidence for general relativistic frame dragging. Physical Review D - Particles, Fields, Gravitation and Cosmology, 61(2), 1-8.
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  Abstract: Recent x-ray variability observations of accreting neutron stars may provide the first evidence for frame dragging effects around spinning relativistic objects. Motivated by this possibility and its implications for neutron-star structural properties, we calculate new optimal bounds on the masses, radii, and moments of inertia of slowly rotating neutron stars that show kilohertz quasi-periodic oscillations (QPOs). These bounds are derived under minimal assumptions about the properties of matter at high densities and therefore are largely independent of the unknown equation of state. We further derive a semi-analytical upper bound on the neutron-star moment of inertia without making any assumptions about the equation of state of matter at any density. We use this upper bound to show that the maximum possible nodal precession frequency of an inclined circular orbit around a slowly spinning neutron star is νNP≃45.2(νs/300 Hz) Hz, where νs is the spin frequency of the neutron star. We conclude that the nodal-precession interpretation of low-frequency QPOs in accreting neutron stars is inconsistent with the beat-frequency interpretation of the kHz QPOs or the identification of the highest-frequency QPO with that of a circular Keplerian orbit in the accretion disk. ©1999 The American Physical Society.
• Psaltis, D., Özel, F., & DeDeo, S. (2000). Photon propagation around compact objects and the inferred properties of thermally emitting neutron stars. Astrophysical Journal Letters, 544(1 PART 1), 390-396.
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  Abstract: Anomalous X-ray pulsars, compact nonpulsing X-ray sources in supernova remnants, and X-ray bursters are three distinct types of sources for which there are viable models that attribute their X-ray spectra to thermal emission from the surface of a neutron star. Inferring the surface area of the emitting regions in such systems is crucial in assessing the viability of different models and in providing bounds on the radii of neutron stars. We show that the spectroscopically inferred areas of the emitting regions may be over- or underestimated by a factor of ≲2, because of the three-dimensional geometry of the system and general relativistic light deflection, combined with the effects of phase averaging. Such effects make the determination of neutron star radii uncertain, especially when compared to the ∼5% level required for constraining the equation of state of neutron star matter. We also note that, for a given spectral shape, the inferred source luminosities and pulse fractions are anticorrelated because they depend on the same properties of the emitting regions, namely, their sizes and orientations. As a result, brighter sources have on average weaker pulsation amplitudes than fainter sources. We argue that this property can be used as a diagnostic tool in distinguishing between different spectral models. As an example, we show that the high inferred pulse fraction and brightness of the pulsar RXS J1708-40 are inconsistent with isotropic thermal emission from a neutron star surface. Finally, we discuss the implication of our results for surveys in the soft X-rays for young, cooling neutron stars in supernova remnants and show that the absence of detectable pulsations from the compact source at the center of Cas A (at a level of ≳ 30%) is not a strong argument against its identification with a spinning neutron star.
• Özel, F., Psaltis, D., & Narayan, R. (2000). Hybrid thermal-nonthermal synchrotron emission from hot accretion flows. Astrophysical Journal Letters, 541(1 PART 1), 234-249.
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  Abstract: We investigate the effect of a hybrid electron population, consisting of both thermal and nonthermal particles, on the synchrotron spectrum, image size, and image shape of a hot accretion flow onto a supermassive black hole. We find two universal features in the emitted synchrotron spectrum: (1) a prominent shoulder at low (≲1011 Hz) frequencies that is weakly dependent on the shape of the electron energy distribution, and (2) an extended tail of emission at high (≳1013 Hz) frequencies whose spectral slope depends on the slope of the power-law energy distribution of the electrons. In the low-frequency shoulder, the luminosity can be up to 2 orders of magnitude greater than with a purely thermal plasma even if only a small fraction (
• Lamb, F. K., Miller, M. C., & Psaltis, D. (1999). Rapid X-ray variability of neutron stars in low-mass binary systems. Nuclear Physics B - Proceedings Supplements, 69(1-3), 113-122.
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  Abstract: The dramatic discovery with the Rossi X-Ray Timing Explorer satellite of remarkably coherent ∼300-1200 Hz oscillations in the X-ray brightness of some sixteen neutron stars in low-mass binary systems has spurred theoretical modeling of these oscillations and investigation of their implications for the neutron stars and accretion flows in these systems. High-frequency oscillations are observed both during thermonuclear X-ray bursts and during intervals of accretion-powered emission and appear to be a characteristic feature of disk-accreting neutron stars with weak magnetic fields. In this review we focus on the high-frequency quasi-periodic oscillations (QPOs) seen in the accretion-powered emission. We first summarize the key properties of these kilohertz QPOs and then describe briefly the models that have been proposed to explain them. The existing evidence strongly favors beat-frequency models. We mention several of the difficulties encountered in applying the magnetospheric beat-frequency model to the kilohertz QPOs. The most fully developed and successful model is the sonic-point beat-frequency model. We describe the work on this model in some detail. We then discuss observations that could help to distinguish between models. We conclude by noting some of the ways in which study of the kilohertz QPOs may advance our understanding of dense matter and strong gravitational fields.
• Matteo, T. D., & Psaltis, D. (1999). Quasi-periodic variability and the inner radii of thin accretion disks in galactic black hole systems. Astrophysical Journal Letters, 526(2 PART 2), L101-L104.
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  Abstract: We calculate upper bounds on the inner radii of geometrically thin accretion disks in galactic black hole systems by relating their rapid variability properties to those of neutron stars. We infer that the inner disk radii do not exhibit large excursions between different spectral states, in contrast with the concept that the disk retreats significantly during the soft-to-hard-state transition. We find that, in the hard state, the accretion disks extend down to radii ≲6-25 GM/c2 and discuss the implications of our results for models of black hole X-ray spectra.
• Miller, M. C., Lamb, F. K., & Psaltis, D. (1999). Constraints on the equation of state of neutron star matter from observations of kilohertz QPOs. Nuclear Physics B - Proceedings Supplements, 69(1-3), 123-128.
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  Abstract: The frequencies of the highest-frequency kilohertz QPOs recently discovered in some sixteen neutron stars in low-mass X-ray binary systems are most likely the orbital frequencies of gas in Keplerian orbit around these neutron stars. If so, these QPOs provide tight upper bounds on the masses and radii of these neutron stars and important new constraints on the equation of state of neutron star matter. If the frequency of a kilohertz QPO can be established as the orbital frequency of gas at the innermost stable circular orbit, this would confirm one of the key predictions of general relativity in the strong-gravity regime. If the spin frequency of the neutron star can also be determined, the frequency of the QPO would fix the mass of the neutron star for each assumed equation of state. Here we show how to derive mass and radius bounds, using the kilohertz QPOs, for nonrotating and slowly rotating stars, and discuss how these bounds are affected by rapid stellar rotation and radial radiation forces. We also describe observational results that would be strong evidence for the presence of an innermost stable circular orbit. No such strong evidence is present in current data, but future prospects are excellent.
• Psaltis, D., & Chakrabarty, D. (1999). The disk-magnetosphere interaction in the accretion-powered millisecond pulsar SAX J1808.4-3658. Astrophysical Journal Letters, 521(1 PART 1), 332-340.
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  Abstract: The recent discovery of the first known accretion-powered millisecond pulsar with the Rossi X-Ray Timing Explorer provides the first direct probe of the interaction of an accretion disk with the magnetic field of a weakly magnetic (B ≲ 1010 G) neutron star. We demonstrate that the presence of coherent pulsations from a weakly magnetic neutron star over a wide range of accretion rates places strong constraints on models of the disk-magnetosphere interaction. We argue that the simple Ṁ3/7 scaling law for the Keplerian frequency at the magnetic interaction radius, widely used to model disk accretion onto magnetic stars, is not consistent with observations of SAX J1808.4 - 3658 for most proposed equations of state for stable neutron stars. We show that the usually neglected effects of multipole magnetic moments, radiation drag forces, and general relativity must be considered when modeling such weakly magnetic systems. Using only very general assumptions, we obtain a robust estimate of μ ≃ (1-10) × 1026 G cm3 for the dipole magnetic moment of SAX J1808.4-3658, implying a surface dipole field of ∼ 108-109 G at the stellar equator. We therefore infer that after the end of its accretion phase, this source will become a normal millisecond radio pulsar. Finally, we compare the physical properties of this pulsar with those of the nonpulsing, weakly magnetic neutron stars in low-mass X-ray binaries and argue that the absence of coherent pulsations from the latter does not necessarily imply that these neutron stars have significantly different magnetic field strengths from SAX J1808.4-3658.
• Psaltis, D., Belloni, T., & Van, M. (1999). Correlations in quasi-periodic oscillation and noise frequencies among neutron star and black hole X-ray binaries. Astrophysical Journal Letters, 520(1 PART 1), 262-270.
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  Abstract: We study systematically the ≃0.1-1200 Hz quasi-periodic oscillations (QPOs) and broad noise components observed in the power spectra of nonpulsing neutron star and black hole low-mass X-ray binaries. We show that among these components we can identify two, occurring over a wide range of source types and luminosities, whose frequencies follow a tight correlation. The variability components involved in this correlation include neutron star kilohertz QPOs and horizontal-branch oscillations, as well as black hole QPOs and noise components. Our results suggest that the same types of variability may occur in both neutron star and black hole systems over 3 orders of magnitude in frequency and with coherences that vary widely but systematically. Confirmation of this hypothesis will strongly constrain theoretical models of these phenomena and provide additional clues to understanding their nature.
• Psaltis, D., Wunands, R., Homan, J., Jonker, P. G., Van, M., Miller, M. C., Lamb, F. K., Kuulkers, E., Paradijs, J. V., & H., W. (1999). On the magnetospheric beat-frequency and lense-thirring interpretations of the horizontal-branch oscillation in the Z sources. Astrophysical Journal Letters, 520(2 PART 1), 763-775.
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  Abstract: Three types of quasi-periodic oscillations (QPOs) have been discovered so far in the persistent emission of the most luminous neutron star low-mass X-ray binaries, the Z sources: ∼10-60 Hz horizontal-branch and ∼6-20 Hz normal/flaring-branch oscillations and ∼200-1200 Hz kilohertz QPOs, which usually occur in pairs. Here we study the horizontal-branch oscillations and the two simultaneous kilohertz QPOs, which were discovered using the Rossi X-Ray Timing Explorer, comparing their properties in five Z sources with the predictions of the magnetospheric beat-frequency and Lense-Thirring precession models. We find that the variation of the horizontal-branch oscillation frequency with accretion rate predicted by the magnetospheric beat-frequency model for a purely dipolar stellar magnetic field and a radiation-pressure-dominated inner accretion disk is consistent with the observed variation. The model predicts a universal relation between the horizontal-branch oscillation, stellar spin, and upper kilohertz QPO frequencies that agrees with the data on five Z sources. The model implies that the neutron stars in the Z sources are near magnetic spin equilibrium, that their magnetic field strengths are ∼109-1010 G, and that the critical fastness parameter for these sources is ≳0.8. If the frequency of the upper kilohertz QPO is an orbital frequency in the accretion disk, the magnetospheric beat-frequency model requires that a small fraction of the gas in the disk does not couple strongly to the stellar magnetic field at 3-4 stellar radii but instead drifts slowly inward in nearly circular orbits until it is within a few kilometers of the neutron star surface. The Lense-Thirring precession model is consistent with the observed magnitudes of the horizontal-branch oscillation frequencies only if the moments of inertia of the neutron stars in the Z sources are ∼4-5 times larger than the largest values predicted by realistic neutron star equations of state. If instead the moments of inertia of neutron stars have the size expected and their spin frequencies in the Z sources are approximately equal to the frequency separation of the kilohertz QPOs, Lense-Thirring precession can account for the magnitudes of the horizontal-branch oscillation frequencies only if the fundamental frequency of the horizontal-branch oscillation is at least 4 times the precession frequency. We argue that the change in the slope of the correlation between the frequency of the horizontal-branch oscillation and the frequency of the upper kilohertz QPO, when the latter is greater than 850 Hz, is directly related to the varying frequency separation of the kilohertz QPOs.
• Jonker, P. G., Wijnands, R., Van, M., Psaltis, D., Kuulkers, E., & Lamb, F. K. (1998). Discovery of kilohertz quasi-periodic oscillations in the Z source GX 340+0. Astrophysical Journal Letters, 499(2 PART II), L191-L194.
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  Abstract: We have discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in the Z source GX 340+0 with the Rossi X-Ray Timing Explorer. The X-ray hardness-intensity and color-color diagrams each show a full Z track, with an extra limb branching off the flaring branch of the Z. Both peaks moved to higher frequencies when the mass accretion rate increased. The two peaks moved from 247 ±6 and 567 ±39 Hz at the left end of the horizontal branch to 625 ±18 and 820± 19 Hz at its right end. The higher frequency peak's rms amplitude (5-60 keV) and FWHM decreased from ~5% and 383 ±135 Hz to 2% and 145 ±62 Hz, respectively. The rms amplitude and FWHM of the lower peak were consistent with being constant near 2.5% and 100 Hz. The kHz QPO separation was consistent with being constant at 325 ±10 Hz. Simultaneous with the kHz QPOs, we detected the horizontal-branch oscillation (HBO) and its second harmonic, at frequencies between 20 and 50 Hz, and 38 and 69 Hz, respectively. The normal-branch oscillations were only detected on the upper and middle normal branch and became undetectable on the lower normal branch. The HBO frequencies do not fall within the range predicted for Lense-Thirring precession, unless either the ratio of the neutron star moment of inertia to neutron star mass is at least 4, 1045g cm2 M⊙ , the frequencies of the HBOs are in fact the second harmonic oscillations, or the observed kHz peak difference is half the spin frequency and not the spin frequency. During a 1.2 day gap between two observations, the Z track in the hardness-intensity diagram moved to higher count rates by about 3.5%. Comparing data before and after this shift, we find that the HBO properties are determined by position on the Z track and not directly by count rate or X-ray colors. © 1998. The American Astronomical Society. All rights reserved.
• Miller, M. C., Lamb, F. K., & Psaltis, D. (1998). Sonic-point model of kilohertz quasi-periodic brightness oscillations in low-mass X-ray binaries. Astrophysical Journal Letters, 508(2 PART II), 791-830.
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  Abstract: Quasi-periodic brightness oscillations (QPOs) with frequencies ranging from ∼300 to ∼1200 Hz have been discovered in the X-ray emission from 14 neutron stars in low-mass binary systems and from another neutron star in the direction of the Galactic center. These kilohertz QPOs are very strong, with rms relative amplitudes ranging up to ∼15% of the total X-ray count rate, and are remarkably coherent, with frequency-to-FWHM ratios as large as ∼200. Two simultaneous kilohertz QPOs differing in frequency by ∼250-350 Hz have been detected in 12 of the 15 sources. Here we propose a model for these QPOs. In this model, the X-ray source is a neutron star with a surface magnetic field ∼107-1010 G and a spin frequency of a few hundred hertz, accreting gas via a Keplerian disk. Some of the accreting gas is channeled by the stellar magnetic field but some remains in a Keplerian disk flow that penetrates to within a few kilometers of the stellar surface. The frequency of the higher frequency QPO in a kilohertz QPO pair is the Keplerian frequency at a radius near the sonic point at the inner edge of the Keplerian flow, whereas the frequency of the lower frequency QPO is the difference between the Keplerian frequency at a radius near the sonic point and the fundamental or first overtone of the stellar spin frequency. The difference between the frequencies of the pair of QPOs is therefore close to (but not necessarily equal to) the stellar spin frequency. The amplitudes of the QPOs at the sonic-point Keplerian frequency and at the beat frequency depend on the strength of the neutron star's magnetic field and the accretion rate, and hence one or both of these QPOs may sometimes be undetectable. Oscillations at the stellar spin frequency and its overtones are expected to be weak but may sometimes be detectable. This model is consistent with the magnetic field strengths, accretion rates, and scattering optical depths inferred from previous modeling of the X-ray spectra and rapid X-ray variability of the atoll and Z sources. It explains naturally the frequencies of the kilohertz QPOs and the similarity of these frequencies in sources with different accretion rates and magnetic fields. The model also explains the high coherence and large amplitudes of the kilohertz QPOs and the steep increase of QPO amplitude with photon energy. The increase in QPO frequency with inferred accretion rate seen in many sources is also understandable in this model. We show that if the frequency of the higher frequency QPO in a pair is an orbital frequency, as in the sonic-point model, the frequencies of these QPOs place interesting upper bounds on the masses and radii of the neutron stars in the kilohertz QPO sources and provide new constraints on the equation of state of matter at high densities. Further observations of these QPOs may provide compelling evidence for the existence of a marginally stable orbit, confirming a key prediction of general relativity in the strong-field regime. © 1998. The American Astronomical Society. All rights reserved.
• Méndez, M., Van, M., Paradijs, J. V., Lewin, W. H., Vaughan, B. A., Kuulkers, E., Zhang, W., Lamb, F. K., & Psaltis, D. (1998). Discovery of a second kilohertz quasi-periodic oscillation peak in 4U 1608-52. Astrophysical Journal Letters, 494(1 PART II), L65-L69.
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  Abstract: Using a new technique to improve the sensitivity to weak quasi-periodic oscillations (QPOs), we discovered a new QPO peak at about 1100 Hz in the March 1996 outburst observations of 4U 1608-52, simultaneous with the ∼600-900 Hz peak previously reported from these data. The frequency separation between the upper and the lower QPO peaks varied significantly from 232.7 ± 11.5 Hz on March 3 to 293.1 ± 6.6 Hz on March 6. This is the first case of a variable kilohertz peak separation in an atoll source. We discuss to what extent this result could be accommodated in beat-frequency models such as proposed for the kilohertz QPOs. We measured the rms fractional amplitude of both QPOs as a function of energy, and we found that the relation is steeper for the lower than for the upper frequency peak. This is the first source where such a difference between the energy spectrum of the two kilohertz QPOs could be measured. © 1998. The American Astronomical Society. All rights reserved.
• Wijnands, R., Homan, J., Van, M., Kuulkers, E., Paradijs, J. V., Lewin, W. H., Lamb, F. K., Psaltis, D., & Vaughan, B. (1998). Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2. Astrophysical Journal Letters, 493(2 PART II), L87-L90.
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  Abstract: During observations with the Rossi X-Ray Timing Explorer from 1997 June 31 to July 3 we discovered two simultaneous kHz quasi-periodic oscillations (QPOs) near 500 and 860 Hz in the low-mass X-ray binary and Z source Cygnus X-2. In the X-ray color-color diagram and hardness-intensity diagram (HID), a clear Z track was traced out, which shifted in the HID within 1 day to higher count rates at the end of the observation. Z track shifts are well known to occur in Cyg X-2; our observation for the first time catches the source in the act. A single kHz QPO peak was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731 ± 20 Hz and an amplitude of 4.7-0.6+0.8% rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532 ± 43 Hz was detected with an rms amplitude of 3.0-0.7+1.0%. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher frequency QPO increased to 839 ± 13 Hz, and its amplitude decreased to 3.5-0.3+0.4% rms. The higher frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8-0.4+0.6% and a frequency of 1007 ± 15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower frequency QPO was most of the time undetectable, with typical upper limits of 2% rms; no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in some beat-frequency models), then the neutron star spin period is 2.9 ± 0.2 ms (346 ± 29 Hz). This discovery makes Cyg X-2 the fourth Z source that displays kHz QPOs. The properties of the kHz QPOs in Cyg X-2 are similar to those of other Z sources. Simultaneous with the kHz QPOs, the well-known horizontal-branch QPOs (HBOs) were visible in the power spectra. At the left end of the HB, the second harmonic of the HBO was also detected. We also detected six small X-ray bursts. No periodic oscillations or QPOs were detected in any of them, with typical upper limits of 6%-8% rms. © 1998. The American Astronomical Society. All rights reserved.
• Wijnands, R., Méndez, M., Der, M., Psaltis, D., Kuulkers, E., & Lamb, F. K. (1998). Discovery of kilohertz quasi-periodic oscillations in the Z source GX 5-1. Astrophysical Journal Letters, 504(1 PART II), L35-L38.
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  Abstract: We discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in the bright low-mass X-ray binary and Z source GX 5-1 with the Rossi X-ray Timing Explorer. In the X-ray color-color and hardness-intensity diagram a clear Z track is traced out, which shifted between observations. The frequencies of the two kHz QPOs increased from ∼215 and ∼500 Hz on the left part of the horizontal branch to ∼700 and ∼890 Hz, respectively, on the upper part of the normal branch. With increasing frequency the FWHM and rms amplitude (8.6-60 keV) of the higher frequency kHz QPO decreased from 300 to 30 Hz, and from 6.6% to 2.4%, respectively. The FWHM and amplitude of the lower frequency kHz QPO (50-100 Hz and 3%-4% rms) did not correlate with the position of the source on the Z track. The kHz QPO separation was consistent with being constant at 298 ± 11 Hz. Simultaneously with the kHz QPOs horizontal branch oscillations were detected with frequencies between 18 and 56 Hz. © 199S. The American Astronomical Society. All rights reserved.
• Méndez, M., Van, M., Paradijs, J. V., Lewin, W. H., Lamb, F. K., Vaughan, B. A., Kuulkers, E., & Psaltis, D. (1997). Kilohertz quasi-periodic oscillation and atoll source states in 4U 0614+09. Astrophysical Journal Letters, 485(1 PART II), L37-L40.
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  Abstract: We report three RXTE/PCA observations of the low-mass X-ray binary 4U 0614+09. They show strong (∼30% rms) band-limited noise with a cutoff frequency varying between 0.7 and 15 Hz in correlation with the X-ray flux, fx. We observe two nonsimultaneous 11%-15% (rms) kHz peaks near 728 and 629 Hz in the power spectra of two of our observations when fx ∼ 10-9 ergs cm-2 s-1 (2-10 keV) but find no quasi-periodic oscillations (QPOs;
• Psaltis, D., & Lamb, F. K. (1997). Compton scattering by static and moving media. I. The transfer equation and its moments. Astrophysical Journal Letters, 488(2 PART I), 881-894.
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  Abstract: Compton scattering of photons by nonrelativistic particles is thought to play an important role in forming the radiation spectrum of many astrophysical systems. Here we derive the time-dependent photon kinetic equation that describes spontaneous and induced Compton scattering, as well as absorption and emission by static and moving media, the corresponding radiative transfer equation, and their zeroth and first angular moments, both in the system frame and in the frame comoving with the medium. We show that it is necessary to use the correct relativistic differential scattering cross section in order to obtain a photon kinetic equation that is correct to first order in ε/me, Te/me, and V, where ε is the photon energy, Te and me are the electron temperature and rest mass, and V is the electron bulk velocity in units of the speed of light. We also demonstrate that the terms in the radiative transfer equation that are second order in V should usually be retained, because if the radiation energy density is sufficiently large, compared to the radiation flux, the effects of bulk Comptonization described by the terms that are second order in V can be as important as the effects described by the terms that are first order in V, even when V is small. The system- and fluid-frame equations that we derive are correct to first order in ε/me. Our system-frame equations, which are correct to second order in V, may be used when V is not too large. Our fluid-frame equations, which are exact in V, may be used when V → 1. Both sets of equations are valid for systems of arbitrary optical depth and can therefore be used in both the free-streaming and diffusion regimes. We demonstrate that Comptonization by the electron bulk motion occurs whether or not the radiation field is isotropic or the bulk flow converges and that it is more important than thermal Comptonization if V2 > 3Te/me. © 1997. The American Astronomical Society. All rights reserved.
• Vaughan, B. A., Van, M., Medez, M., Paradijs, J. V., Wljnands, R. A., Lewin, W. H., Lamb, F. K., Psaltis, D., Kuulkers, E., & Oosterbroek, T. (1997). Discovery of microsecond time lags in kilohertz QPOs. Astrophysical Journal Letters, 483(2 PART II), L115-L118.
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  Abstract: Using the Rossi X-Ray Timing Explorer, we have measured ~27 /us time delays in 830 Hz quasi-periodic oscillations (QPOs) between 4-6 and 11-17 keV in 4U 1608-52, with high-energy photons lagging low-energy photons, and found upper limits to the time delays of 45 jus between 2-6.5 and 6.5-67 keV in ~730 Hz QPOs in 4U 0614+091 and 30 /is between 8.7-12.4 and 12.4-67 keV in ∼870 Hz QPOs in 4U 1636-53. We also find that the cross-coherence function between QPOs at different energies is greater than 0.85 with 95% confidence in 4U 1608-52 and 4U 1636-53. If Compton upscattering of low-energy X-rays in a region with an optical depth of a few is responsible for the delays, then the Compton upscattering region is between a few kilometers and a few tens of kilometers in size. © 1997. The American Astronomical Society. All rights reserved.
• Wijnands, R., Homan, J., Van, M., Méndez, M., Kuulkers, E., Paradijs, J. V., H., W., Lamb, F. K., Psaltis, D., & Vaughan, B. (1997). Discovery of kilohertz quasi-periodic oscillations in GX 17+2. Astrophysical Journal Letters, 490(2 PART II), L157-L160.
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  Abstract: We observed the low-mass X-ray binary and Z source GX 17+2 with the Rossi X-Ray Timing Explorer during 1997 February 6-8, April 1-4, and July 26-27. The X-ray color-color diagram shows a clear Z track. Two simultaneous kHz quasi-periodic oscillations (QPOs) are present in each observation, whose frequencies are well correlated with the position of the source on the Z track. At the left end of the horizontal branch (HB), only the higher frequency peak is observed, at 645 ± 9 Hz, with an rms amplitude of 5.7% ± 0.5% and an FWHM of 183 ± 35 Hz. When the source moves down the Z track to the upper normal branch, the frequency of the kHz QPO increases to 1087 ± 12 Hz, and the rms amplitude and FWHM decrease by a factor of 2. Farther down the Z track, the QPO becomes undetectable, with rms upper limits typically of 2.0%. Halfway down the HB, a second QPO appears in the power spectra with a frequency of 480 ± 23 Hz. The frequency of this QPO also increases when the source moves along the Z track, up to 781 ± 11 Hz halfway down the normal branch, while the rms amplitude and FWHM stay approximately constant at 2.5% and 70 Hz. The QPO frequency difference is constant at 293.5 ± 7.5 Hz. Simultaneously with the kHz QPOs, we detect HB QPOs (HBOs). The simultaneous presence of HBOs and kHz QPOs excludes the magnetospheric beat-frequency model as the explanation for at least one of these two phenomena. © 1997. The American Astronomical Society. All rights reserved.
• Daumerie, P., Kalogera, V., Lamb, F. K., & Psaltis, D. (1996). A strongly magnetic neutron star in a nearly face-on binary system. Nature, 382(6587), 141-143.
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  Abstract: THE first example of a new type of transient X-ray source recently appeared in the direction of the Galactic Centre. During the peak of its outburst, the new source, GRO J1744-28, was very bright in X-rays and produced both 2.1-Hz periodic pulsations and intense bursts lasting tens of seconds. Before the discovery of this source, it was thought that X-ray stars could not display these different types of activity simultaneously. Here we discuss the nature of the source, which seems to be a strongly magnetic neutron star accreting matter from a low-mass companion star in a low- inclination orbit. The dipole component of its magnetic field is λ 1011 G. When the source was at its brightest, its X-ray luminosity between bursts was close to the Eddington critical luminosity, at which the outward force of the escaping radiation balances the inward force of gravity. The X-ray bursts probably occur when matter that has accumulated in the inner part of the accretion disk briefly overcomes the forces that oppose its inflow, and the gas falls onto the neutron star.
• Psaltis, D., Lamb, F. K., & Zylstra, G. J. (1996). Structure of the magnetic field near weakly-magnetic neutron stars accreting from disks. Astrophysical Letters and Communications, 34(1-6), 377-382.
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  Abstract: Many of the X-ray stars in low-mass binary systems are thought to be weakly magnetic neutron stars accreting from Keplerian disks. If the stellar magnetic field is ∼108 G, magnetic stresses terminate the Keplerian flow close to the stellar surface. Here we report the results of detailed numerical modeling of the magnetic field structure near such neutron stars, taking into account the electrical currents flowing in the disk and magnetosphere as well as in the star and the twisting of the magnetic field caused by the motion of the disk plasma relative to the neutron star. © 1996 OPA (Overseas Publishers Association) Amsterdam B.V. Published in The Netherlands under license by Gordon and Breach Science Publishers SA.
• Wijnands, R. A., Van, M., Psaltis, D., Lamb, F. K., Kuulkers, E., Dieters, S., Paradijs, J. V., & Lewin, W. H. (1996). Discovery of a variable-frequency 50-60 Hz quasi-periodic oscillation on the normal branch of GX 17+2. Astrophysical Journal Letters, 469(1 PART II), L5-L8.
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  Abstract: We report the discovery, with the Rossi X-Ray Timing Explorer, of a 50-60 Hz quasi-periodic oscillation (QPO) in GX 17+2. The QPO is seen when GX 17+2 is on the normal branch in the X-ray color-color diagram. Its frequency initially increases from 59 to 62 Hz as the source moves down the normal branch, but below the middle of the normal branch it decreases to ∼50 Hz. Together with this frequency decrease, the QPO peak becomes much broader, from ∼4 Hz in the upper part of the normal branch to ∼15 Hz in the lower normal branch. The rms amplitude remains approximately constant between 1% and 2% along the entire normal branch. From a comparison of the properties of this QPO with those of QPOs-previously observed along the normal branch in other Z sources, we conclude that it is most likely the horizontal-branch QPO (HBO). However, this QPO displays a number of unusual characteristics. The decrease in the QPO frequency along the lower normal branch is not in agreement with the predictions of the beat-frequency model for the HBO unless the mass flux through the inner disk decreases as the source moves down the lower normal branch. We tentatively suggest that the required decrease in the mass flux through the inner disk is caused by an unusually rapid increase in the mass flux in the radial inflow as GX 17+2 moves down the normal branch. Assuming that this explanation is correct, we can derive an upper bound on the dipole component of the star's magnetic field at the magnetic equator of 5 × 109 G for a 1.4 M⊙ neutron star with a radius of 106 cm. © 1996. The American Astronomical Society. All rights reserved.
• Psaltis, D., Lamb, F. K., & Miller, G. S. (1995). X-ray spectra of Z sources. Astrophysical Journal Letters, 454(2 PART 2), L137-L140.
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  Abstract: A physically consistent model has been proposed that seeks to explain in a unified way the X-ray spectra and rapid variability of the Z sources and other weakly magnetic neutron stars in low-mass systems. Here we describe a simple, four-parameter spectral model derived from the unified model that accurately reproduces the X-ray colors, spectra, and count rates of the Z sources. In this model, photons are produced primarily by electron cyclotron emission in the neutron star magnetosphere and are then Comptonized in the magnetosphere, hot central corona, and inward radial flow. In addition to explaining their Z tracks, the model explains several other previously unexplained properties of the Z sources.

Proceedings Publications

• Ray, P., Arzoumanian, Z., Ballantyne, D., Bozzo, E., Brandt, S., Brenneman, L., Chakrabarty, D., Christophersen, M., DeRosa, A. r., Feroci, M., Gendreau, K., Goldstein, A., Hartmann, D., Hernanz, M., Jenke, P., Kara, E., Maccarone, T., McDonald, M., Martindale, A., , Nowak, M., et al. (2019, sep). STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years. In \baas, 51.
• Feroci, M., Bozzo, E., Brandt, S., Hernanz, M., Klis, M., Liu, L., Orleanski, P., Pohl, M., Santangelo, A., Schanne, S., & al., e. (2016, jul). The LOFT mission concept: a status update. In Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray, 9905.
• Benkevitch, L., Fish, V. L., Johannsen, T., Akiyama, K., Broderick, A. E., Psaltis, D., Doeleman, S., Monnier, J. D., & Baron, F. (2013, jan). Testing General Relativity with the Event Horizon Telescope. In American Astronomical Society Meeting Abstracts, 221, #143.10.
• Chakrabarty, D., Tomsick, J., Grefenstette, B., Barret, D., Boggs, S. E., Christensen, F., Craig, W. W., Hailey, C. J., Harrison, F., Psaltis, D., Stern, D., Wik, D. R., & Zhang, W. (2014, jan). NuSTAR/XMM-Newton Detection of a Hard Cut-Off in Cen X-4. In American Astronomical Society Meeting Abstracts, 223, #438.19.
• Gaspar, A., Rieke, G., Psaltis, D., & Ozel, F. (2013, jan). Debris Disk Time Evolution - Connecting Observations with Theory. In American Astronomical Society Meeting Abstracts, 221, #403.01.
• Johannsen, T., Psaltis, D., & Steiner, J. F. (2013, apr). Constraints on Deviations from the Kerr Metric by XTE J1550-564. In AAS/High Energy Astrophysics Division, 13, #126.25.
• Feroci, M., Herder, J. W., Bozzo, E., Barret, D., Brandt, S., Hernanz, M., Klis, M., Pohl, M., Santangelo, A., Stella, L., & al., e. (2012, sep). LOFT: the Large Observatory For X-ray Timing. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 8443.