Chi-kwan Chan

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Journals/Publications

• Christian, P., & Chan, C. (2021). FANTASY: User-friendly Symplectic Geodesic Integrator for Arbitrary Metrics with Automatic Differentiation. ApJ, 909(1), 67.
• Collaboration, E., Akiyama, K., Algaba, J. C., Alberdi, A., Alef, W., Anantua, R., Asada, K., Azulay, R., Baczko, A., Ball, D., Balokovi{\'c}, M., Barrett, J., Benson, B. A., Bintley, D., Blackburn, L., Blundell, R., Boland, W., Bouman, K. L., Bower, G. C., , Boyce, H., et al. (2021). First M87 Event Horizon Telescope Results. VII. Polarization of the Ring. ApJL, 910(1), L12.
• Collaboration, E., Akiyama, K., Algaba, J. C., Alberdi, A., Alef, W., Anantua, R., Asada, K., Azulay, R., Baczko, A., Ball, D., Balokovi{\'c}, M., Barrett, J., Benson, B. A., Bintley, D., Blackburn, L., Blundell, R., Boland, W., Bouman, K. L., Bower, G. C., , Boyce, H., et al. (2021). First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon. ApJL, 910(1), L13.
• Group, E., Algaba, J., Anczarski, J., Asada, K., Balokovi{\'c}, M., Chandra, S., Cui, Y. -., Falcone, A., Giroletti, M., Goddi, C., Hada, K., Haggard, D., Jorstad, S., Kaur, A., Kawashima, T., Keating, G., Kim, J. -., Kino, M., Komossa, S., , Kravchenko, E., et al. (2021). Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign. ApJL, 911(1), L11.
• Walls, R. L., Tyle, K. R., Teal, T. K., Tarin, T., Swetnam, T. L., Sutherland, W., Sahneh, F. D., Rushforth, R. R., Roy, S., Rose, P. W., Ribes, D., Quinn, D. P., Papes, M., Nord, B., Nearing, G., Merchant, N., Matheson, T., Lyons, E., Lushbough, C., , Lee, T., et al. (2021). Ten simple rules to cultivate transdisciplinary collaboration in data science.. PLoS computational biology, 17(5), e1008879. doi:10.1371/journal.pcbi.1008879
• Zhao, S., Zhao, G., Zensus, J. A., Yuan, Y., Yuan, F., Younsi, Z., Young, K., Young, A., Yoon, D., Wu, Q., Wong, G. N., Wielgus, M., Wharton, R., Wex, N., Weintroub, J., Wardle, J. F., Ward-thompson, D., Wagner, J., Trippe, S., , Trent, T., et al. (2021). The Polarized Image of a Synchrotron-emitting Ring of Gas Orbiting a Black Hole. The Astrophysical Journal, 912(1). doi:10.3847/1538-4357/abf117
• Zhao, S., Zhao, G., Zensus, J. A., Yuan, Y., Yuan, F., Younsi, Z., Young, K., Young, A., Yoon, D., Wu, Q., Wong, G. N., Wielgus, M., Wharton, R., Wex, N., Weintroub, J., Wardle, J. F., Ward-thompson, D., Wagner, J., Valtonen, M., , Trippe, S., et al. (2021). Polarimetric Properties of Event Horizon Telescope Targets from ALMA. The Astrophysical Journal, 910(1), L14. doi:10.3847/2041-8213/abee6a
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  We present the results from a full polarization study carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) during the first Very Long Baseline Interferometry (VLBI) campaign, which was conducted in 2017 April in the λ3 mm and λ1.3 mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively. We determine the polarization and Faraday properties of all VLBI targets, including Sgr A*, M87, and a dozen radio-loud active galactic nuclei (AGNs), in the two bands at several epochs in a time window of 10 days. We detect high linear polarization fractions (2%–15%) and large rotation measures (RM > 103.3–105.5 rad m−2), confirming the trends of previous AGN studies at millimeter wavelengths. We find that blazars are more strongly polarized than other AGNs in the sample, while exhibiting (on average) order-of-magnitude lower RM values, consistent with the AGN viewing angle unification scheme. For Sgr A* we report a mean RM of (−4.2 ± 0.3) × 105 rad m−2 at 1.3 mm, consistent with measurements over the past decade and, for the first time, an RM of (–2.1 ± 0.1) × 105 rad m−2 at 3 mm, suggesting that about half of the Faraday rotation at 1.3 mm may occur between the 3 mm photosphere and the 1.3 mm source. We also report the first unambiguous measurement of RM toward the M87 nucleus at millimeter wavelengths, which undergoes significant changes in magnitude and sign reversals on a one year timescale, spanning the range from −1.2 to 0.3 × 105 rad m−2 at 3 mm and −4.1 to 1.5 × 105 rad m−2 at 1.3 mm. Given this time variability, we argue that, unlike the case of Sgr A*, the RM in M87 does not provide an accurate estimate of the mass accretion rate onto the black hole. We put forward a two-component model, comprised of a variable compact region and a static extended region, that can simultaneously explain the polarimetric properties observed by both the EHT (on horizon scales) and ALMA (which observes the combined emission from both components). These measurements provide critical constraints for the calibration, analysis, and interpretation of simultaneously obtained VLBI data with the EHT and GMVA.
• Zhao, S., Zhao, G., Zensus, J. A., Yuan, Y., Yuan, F., Younsi, Z., Young, K., Young, A., Yoon, D., Wu, Q., Wong, G. N., Wielgus, M., Wharton, R., Wex, N., Weintroub, J., Wardle, J., Ward-thompson, D., Wagner, J., Trippe, S., , Trent, T., et al. (2021). Constraints on black-hole charges with the 2017 EHT observations of M87*. Physical Review D, 103(10). doi:10.1103/physrevd.103.104047
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  Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes.
• Azulay, R., Zhao, G. Y., Yuan, F., Sanchez, S., Psaltis, D., Marrone, D. P., Lico, R., Fromm, C. M., Chan, C., Britzen, S., Torne, P., Toma, K., Titus, M., Tilanus, R. P., Tiede, P., Tazaki, F., Soohoo, J., Sohn, B. W., Small, D., , Shen, Z., et al. (2020). SYMBA: An end-to-end VLBI synthetic data generation pipeline: Simulating Event Horizon Telescope observations of M 87. Astronomy and Astrophysics, 636, 1-19. doi:10.1051/0004-6361/201936622
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  Context. Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are the most important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a direct comparison with observational data. Aims. We present the SYnthetic Measurement creator for long Baseline Arrays (SYMBA), a novel synthetic data generation pipeline for Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. Methods. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a millimetre VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects, compared to the addition of thermal noise only. Using synthetic data based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M 87, we performed case studies to assess the image quality that can be obtained with the current and future EHT array for different weather conditions. Results. Our synthetic observations show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of our GRMHD source models can be recovered robustly with the EHT2017 array after performing calibration steps, which include fringe fitting, a priori amplitude and network calibration, and self-calibration. With the planned addition of new stations to the EHT array in the coming years, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images.
• Ball, D., {\"Ozel}, F., Christian, P., Chan, C., & Psaltis, D. (2020). A Plasmoid Model for the Sgr A* Flares Observed with GRAVITY and Chandra. arXiv e-prints, arXiv:2005.14251.
• Gold}, R., Broderick, A. E., Younsi, Z., Fromm, C. M., Gammie, C. F., Mo{\'scibrodzka}, M., Pu, H., Bronzwaer, T., Davelaar, J., Dexter, J., Ball, D., Chan, C., Kawashima, T., Mizuno, Y., Ripperda, B., Akiyama, K., Alberdi, A., Alef, W., Asada, K., , Azulay, R., et al. (2020). Verification of Radiative Transfer Schemes for the EHT. ApJ, 897(2), 148.
• Kim}, J., Krichbaum, T. P., Broderick, A. E., Wielgus, M., Blackburn, L., G{\'omez}, J. L., Johnson, M. D., Bouman, K. L., Chael, A., Akiyama, K., Jorstad, S., Marscher, A. P., Issaoun, S., Janssen, M., Chan, C., Savolainen, T., Pesce, D. W., {\"Ozel}, F., Alberdi, A., , Alef, W., et al. (2020). Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution. A&A, 640, A69.
• Psaltis, D., Medeiros, L., Lauer, T. R., Chan, C., & Ozel, F. (2020). Discretization and Filtering Effects on Black Hole Images Obtained with the Event Horizon Telescope. arXiv e-prints, arXiv:2004.06210.
• Psaltis, D., Ozel, F., Medeiros, L., Christian, P., Kim, J., Chan, C., Conway, L. J., Raithel, C. A., Marrone, D., & Lauer, T. R. (2020). Markov Chains for Horizons (MARCH). I. Identifying Biases in Fitting Theoretical Models to Event Horizon Telescope Observations. arXiv e-prints, arXiv:2005.09632.
• Psaltis}, D., Medeiros, L., Christian, P., {\"Ozel}, F., Akiyama, K., Alberdi, A., Alef, W., Asada, K., Azulay, R., Ball, D., Balokovi{\'c}, M., Barrett, J., Bintley, D., Blackburn, L., Boland, W., Bower, G. C., Bremer, M., Brinkerink, C. D., Brissenden, R., , Britzen, S., et al. (2020). Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole. PRL, 125(14), 141104.
• Wielgus, M., Akiyama, K., Blackburn, L., Chan, C., Dexter, J., Doeleman, S. S., Fish, V. L., Issaoun, S., Johnson, M. D., Krichbaum, T. P., Lu, R., Pesce, D. W., Wong, G. N., Bower, G. C., Broderick, A. E., Chael, A., Chatterjee, K., Gammie, C. F., Georgiev, B., , Hada, K., et al. (2020). Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope. ApJ, 901(1), 67.
• Behroozi, P., Chan, C., Morrison, S. J., Naoz, S., Ramirez-ruiz, E., Bailin, J., Bogdanovic, T., Bromm, V., Eifler, T. F., Faucher-giguere, C., Gabriel, T., Johnston, K. V., Juneau, S., Lidz, A., Offner, S., Paschalidis, V., Weltman, A., Zingale, M., Aykutalp, A., , Gandolfi, S., et al. (2019). Training the Future Generation of Computational Researchers. Bulletin of the American Astronomical Society, 51(7).
• Blackburn, L., Chan, C., Crew, G. B., Fish, V. L., Issaoun, S., Johnson, M. D., Wielgus, M., Akiyama, K., Barrett, J., Bouman, K. L., Cappallo, R., Chael, A. A., Janssen, M., Lonsdale, C. J., & Doeleman, S. S. (2019). EHT-HOPS Pipeline for Millimeter VLBI Data Reduction. apj, 882(1), 23.
• 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. I. The Shadow of the Supermassive Black Hole. apjl, 875(1), L1.
• 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.
• 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. VI. The Shadow and Mass of the Central Black Hole. apjl, 875(1), L6.
• Gill, A., Blackburn, L., Roshanineshat, A., Chan, C., Doeleman, S. S., Johnson, M. D., Raymond, A. W., & Weintroub, J. (2019). Prospects for Wideband VLBI Correlation in the Cloud. pasp, 131(1006), 124501.
• 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.
• Zensus, J. A., Younsi, Z., Wielgus, M., Torne, P., Tilanus, R. P., Shao, L., Rygl, K. L., Roy, A. L., Rottmann, H., Ros, E., Roelofs, F., Rezzolla, L., Porth, O., Natarajan, I., Moscibrodzka, M., Mizuno, Y., Messias, H., Matthews, L. D., Marti-vidal, I., , Liuzzo, E., et al. (2019). First M87 Event Horizon Telescope Results and the Role of ALMA. The Messenger, 177(25), 25-35. doi:10.18727/0722-6691/5150
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  In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate super- massive black hole (SMBH) at the centre of the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image shows a ring of glowing plasma with a dark patch at the centre, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein’s theory of gravity, or general relativity, was made possible by assembling a global network of radio telescopes operating at millimetre wavelengths that for the first time included the Atacama Large Millimeter/submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical reality. The expansion of the array over the next few years will include new stations on different continents — and eventually satellites in space. This will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on the processes of black hole accretion and jet formation on event-horizon scales, thereby enabling more precise tests of general relativity in the truly strong field regime.
• Psaltis, D., Chan, C., Psaltis, D., Ozel, F., Medeiros, L., & Chan, C. (2018). GRay2: A General Purpose Geodesic Integrator for Kerr Spacetimes. The Astrophysical Journal, 867(1), 59. doi:10.3847/1538-4357/aadfe5
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  Fast and accurate integration of geodesics in Kerr spacetimes is an important tool in modeling the orbits of stars and the transport of radiation in the vicinities of black holes. Most existing integration algorithms employ Boyer-Lindquist coordinates, which have coordinate singularities at the event horizon and along the poles. Handling the singularities requires special numerical treatment in these regions, often slows down the calculations, and may lead to inaccurate geodesics. We present here a new general-purpose geodesic integrator, GRay2, that overcomes these issues by employing the Cartesian form of Kerr-Schild coordinates. By performing particular mathematical manipulations of the geodesic equations and several optimizations, we develop an implementation of the Cartesian Kerr-Schild coordinates that outperforms calculations that use the seemingly simpler equations in Boyer-Lindquist coordinates. We also employ the OpenCL framework, which allows GRay2 to run on multi-core CPUs as well as on a wide range of GPU hardware accelerators, making the algorithm more versatile. We report numerous convergence tests and benchmark results for GRay2 for both time-like (particle) and null (photon) geodesics.
• Psaltis, D., Chan, C., Sironi, L., Psaltis, D., Ozel, F., Chan, C., & Ball, D. (2018). The Properties of Reconnection Current Sheets in GRMHD Simulations of Radiatively Inefficient Accretion Flows. The Astrophysical Journal, 853(2), 184. doi:10.3847/1538-4357/aaa42f
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  Chandra Award [TM6-17006X]; NASA TCAN award [NNX14AB48G]; Radcliffe Institute for Advanced Study at Harvard University; John Simon Guggenheim Memorial Foundation; DoE [DE-SC0016542]; NASA Fermi [NNX-16AR75G]; NASA ATP [NNX-17AG21G]; NSF [ACI-1657507, AST-1716567]
• Psaltis, D., Marrone, D. P., Chan, C., Sadowski, A., Psaltis, D., Ozel, F., Medeiros, L., Marrone, D. P., Kim, J., & Chan, C. (2018). GRMHD Simulations of Visibility Amplitude Variability for Event Horizon Telescope Images of Sgr A. The Astrophysical Journal, 856(2), 163. doi:10.3847/1538-4357/aab204
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  NFS GRFP [DGE 1144085]; NASA/NSF TCAN [NNX14AB48G]; NSF [AST 1108753, AST 1312034, AST-1207752, AST-1440254, 1228509]
• Chan, C., Psaltis, D., Marrone, D. P., Sadowski, A., Psaltis, D., Ozel, F., Medeiros, L., Marrone, D. P., Kim, J., & Chan, C. (2017). Variability in GRMHD Simulations of Sgr A∗: Implications for EHT Closure Phase Observations. The Astrophysical Journal, 844(1), 35. doi:10.3847/1538-4357/aa7751
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  NSF GRFP [DGE 1144085]; NASA/NSF TCAN [NNX14AB48G]; NSF [TM6-17006X, AST 1312034, AST-1207752, 1228509]; John Simon Guggenheim Memorial Foundation; Radcliffe Institute for Advanced Study at Harvard University
• Psaltis, D., Chan, C., Psaltis, D., Ozel, F., Chan, C., & Ball, D. (2016). PARTICLE ACCELERATION AND THE ORIGIN OF X-RAY FLARES IN GRMHD SIMULATIONS OF SGR A*. The Astrophysical Journal, 826(1), 77. doi:10.3847/0004-637x/826/1/77
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  Significant X-ray variability and flaring has been observed from Sgr A* but is poorly understood from a theoretical standpoint. We perform GRMHD simulations that take into account a population of non-thermal electrons with energy distributions and injection rates that are motivated by PIC simulations of magnetic reconnection. We explore the effects of including these non-thermal electrons on the predicted broadband variability of Sgr A* and find that X-ray variability is a generic result of localizing non-thermal electrons to highly magnetized regions, where particles are likely to be accelerated via magnetic reconnection. The proximity of these high-field regions to the event horizon forms a natural connection between IR and X-ray variability and accounts for the rapid timescales associated with the X-ray flares. The qualitative nature of this variability is consistent with observations, producing X-ray flares that are always coincident with IR flares, but not vice versa, i.e., there are a number of IR flares without X-ray counterparts.
• Psaltis, D., Marrone, D. P., Chan, C., Psaltis, D., Ozel, F., Medeiros, L., Marrone, D. P., Kim, J., & Chan, C. (2016). BAYESIAN TECHNIQUES FOR COMPARING TIME-DEPENDENT GRMHD SIMULATIONS TO VARIABLE EVENT HORIZON TELESCOPE OBSERVATIONS. The Astrophysical Journal, 832(2), 156. doi:10.3847/0004-637x/832/2/156
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  NSF [AST-1207752, AST-1440254, AST 1108753, AST 1312034, 1228509]; NASA/NSF TCAN award [NNX14AB48G]; NFS GRFP [DGE 1144085]
• Stone, J. M., Ziurys, L. M., Strittmatter, P. A., Spilker, J., Psaltis, D., Marrone, D. P., Chan, C., Cappallo, R. J., Ziurys, L. M., Zensus, J. A., Young, K. H., Young, A., Yamaguchi, P., Wright, M., Woody, D., Weintroub, J., Wardle, J. F., Wagner, J., Vertatschitsch, L., , Titus, M., et al. (2016). Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales. The Astrophysical Journal, 820(2), 90. doi:10.3847/0004-637x/820/2/90
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  The Galactic Center black hole Sagittarius A^* (Sgr A^*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A^* obtained with the EHT on a total of 13 observing nights over four years. Closure phases, which are the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180° rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A^* that is not obscured by refraction due to interstellar electrons along the line of sight.
• Chan, C., Psaltis, D., Marrone, D. P., Sadowski, A., Psaltis, D., Ozel, F., Narayan, R., Medeiros, L., Marrone, D. P., & Chan, C. (2015). FAST VARIABILITY AND MILLIMETER/IR FLARES IN GRMHD MODELS OF Sgr A* FROM STRONG-FIELD GRAVITATIONAL LENSING. The Astrophysical Journal, 812(2), 103. doi:10.1088/0004-637x/812/2/103
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  We explore the variability properties of long, high cadence GRMHD simulations across the electromagnetic spectrum using an efficient, GPU-based radiative transfer algorithm. We focus on both disk- and jet-dominated simulations with parameters that successfully reproduce the time-averaged spectral properties of Sgr A ∗ and the size of its image at 1.3mm. We find that the disk-dominated models produce short timescale variability with amplitudes and power spectra that closely resemble those inferred observationally. In contrast, jet-dominated models generate only slow variability, at lower flux levels. Neither set of models show any X-ray flares, which most likely indicate that additional physics, such as particle acceleration mechanisms, need to be incorporated into the GRMHD simulations to account for them. The disk-dominated models show strong, short-lived mm/IR flares, with short (. 1hr) time lags between the mm and IR wavelengths, that arise from strong-field gravitational lensing of magnetic flux tubes near the horizon. Such events provide a natural explanation for the observed IR flares with no X-ray counterparts. Subject headings: accretion, accretion disks — black hole physics — Galaxy: center — radiative transfer
• Chan, C., Psaltis, D., Sadowski, A., Psaltis, D., Ozel, F., Narayan, R., & Chan, C. (2015). Erratum: “The Power of Imaging: Constraining the Plasma Properties of GRMHD Simulations using EHT Observations of Sgr A*” (2015, ApJ, 799, 1). The Astrophysical Journal, 807(1), 114. doi:10.1088/0004-637x/807/1/114
• Chan, C., Psaltis, D., Sa¸dowski, A., Psaltis, D., Ozel, F., Narayan, R., & Chan, C. (2015). The power of imaging: Constraining the plasma properties of grmhd simulations using eht observations of Sgr A∗. The Astrophysical Journal, 799(1), 1. doi:10.1088/0004-637x/799/1/1
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  Recent advances in general relativistic magnetohydrodynamic simulations have expanded and improved our understanding of the dynamics of black-hole accretion disks. However, current simulations do not capture the thermodynamics of electrons in the low density accreting plasma. This poses a significant challenge in predicting accretion flow images and spectra from first principles. Because of this, simplified emission models have often been used, with widely different configurations (e.g., disk- versus jet-dominated emission), and were able to account for the observed spectral properties of accreting black holes. Exploring the large parameter space introduced by such models, however, requires significant computational power that exceeds conventional computational facilities. In this paper, we use GRay, a fast graphics processing unit (GPU) based ray-tracing algorithm, on the GPU cluster El Gato, to compute images and spectra for a set of six general relativistic magnetohydrodynamic simulations with different magnetic field configurations and black-hole spins. We also employ two different parametric models for the plasma thermodynamics in each of the simulations. We show that, if only the spectral properties of Sgr A* are used, all 12 models tested here can fit the spectra equally well. However, when combined with the measurement of the image size of the emission using the Event Horizon Telescope, current observations rule out all models with strong funnel emission, because the funnels are typically very extended. Our study shows that images of accretion flows with horizon-scale resolution offer a powerful tool in understanding accretion flows around black holes and their thermodynamic properties.
• Psaltis, D., Chan, C., Marrone, D. P., Psaltis, D., Ozel, F., Marrone, D. P., & Chan, C. (2015). A General Relativistic Null Hypothesis Test with Event Horizon Telescope Observations of the black-hole shadow in Sgr A*. The Astrophysical Journal, 814(2), 115. doi:10.1088/0004-637x/814/2/115
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  In General Relativity, the shadow cast by a black hole on its surrounding emission has a quasicircular shape with a size that depends very weakly on the spin of the black hole or the orientation of the observer. The half opening angle of the shadow, as measured by different observers at infinity and for black holes with different spins, is always equal to (5 ± 0.2)GM/Dc 2 , where M is the mass of the black hole and D is its distance from the Earth. Therefore, measuring the size of the shadow of a black hole of known mass-to-distance ratio and verifying whether it is within the 4% range predicted for the Kerr metric constitutes a null hypothesis test of General Relativity. We show that the black hole in the center of the Milky Way, Sgr A ∗ , is the optimal target for performing this test with upcoming observations using the Event Horizon Telescope. We use the results of optical/IR monitoring of stellar orbits to show that the mass-to-distance ratio M/D for Sgr A ∗ is already known to an accuracy of ∼ 6%. We investigate our prior knowledge of the properties of the scattering screen between Sgr A ∗ and the Earth, the effects of which will need to be corrected for in order for the black-hole shadow to appear sharp against the background emission. We argue that, even though the properties of the scattering ellipse at longer wavelengths are known to within ∼ 3 − 20%, extrapolating them down to the 1.3 mm wavelength of the Event Horizon Telescope observations requires further theoretical work and a model for the wavelength dependence of the axis ratio and position angle in the presence of anisotropic turbulence. Finally, we employ recent GRMHD simulations of the accretion flow around Sgr A ∗ , which have been tuned to match the currently available spectroscopic and interferometric observations, to investigate the visibility and sharpness of the black-hole shadow, which will determine the accuracy at which the null hypothesis test can be performed. We explore an edge detection scheme for interferometric data and a pattern matching algorithm based on the Hough/Radon transform and demonstrate that the shadow of the black hole at 1.3 mm can be localized, in principle, to within ∼ 9%. All these results suggest that our prior knowledge of the properties of the black hole, of the scattering broadening in the interstellar medium, and of the accretion flow can only limit this General Relativistic null hypothesis test with Event Horizon Telescope observations of Sgr A ∗ to . 10%.
• Chan, C., Psaltis, D., Psaltis, D., Ozel, F., & Chan, C. (2013). GRay: a Massively Parallel GPU-Based Code for Ray Tracing in Relativistic Spacetimes. The Astrophysical Journal, 777(1), 13. doi:10.1088/0004-637x/777/1/13
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  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.
• Pessah, M. E., & Chan, C. (2013). Angular Momentum Transport in Accretion Disk Boundary Layers Around Weakly Magnetized Stars. EPJ Web of Conferences, 46, 04004. doi:10.1051/epjconf/20134604004
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  The standard model for turbulent shear viscosity in accretion disks is based on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. This implies that the turbulent stress must be negative and thus transport angular momentum inwards, in the boundary layer where the accretion disk meets the surface of a weakly magnetized star. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. Motivated by the need of a deeper understanding of the behavior of an MHD fluid in a differentially rotating background that deviates from a Keplerian profile, we study the dynamics of MHD waves in configurations that are stable to the standard MRI. Employing the shearing-sheet framework, we show that transient amplification of shearing MHD waves can generate magnetic energy without leading to a substantial generation of hydromagnetic stresses. While these results are in agreement with numerical simulations, they emphasize the need to better understand the mechanism for angular momentum transport in the inner disk regions on more solid grounds.
• Chan, C., & Pessah, M. E. (2012). On magnetohydrodynamic turbulence and angular momentum transport in accretion disk boundary layers. Proceedings of the International Astronomical Union, 8(S294), 349-352. doi:10.1017/s1743921313002731
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  AbstractThe physical modeling of the accretion disk boundary layer, the region where the disk meets the surface of the accreting star, usually relies on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity, widely adopted in astrophysics, satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability is inefficient in this inner disk region. I will discuss the results of a recent study on the generation of hydromagnetic stresses and energy density in the boundary layer around a weakly magnetized star. Our findings suggest that although magnetic energy density can be significantly amplified in this region, angular momentum transport is rather inefficient. This seems consistent with the results obtained in numerical simulations and suggests that the detailed structure of turbulent MHD boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity.
• Chan, C., Mitra, D., & Brandenburg, A. (2012). Dynamics of saturated energy condensation in two-dimensional turbulence.. Physical review. E, Statistical, nonlinear, and soft matter physics, 85(3 Pt 2), 036315. doi:10.1103/physreve.85.036315
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  In two-dimensional forced Navier-Stokes turbulence, energy cascades to the largest scales in the system to form a pair of coherent vortices known as the Bose condensate. We show, both numerically and analytically, that the energy condensation saturates and the system reaches a statistically stationary state. The time scale of saturation is inversely proportional to the viscosity and the saturation energy level is determined by both the viscosity and the force. We further show that, without sufficient resolution to resolve the small-scale enstrophy spectrum, numerical simulations can give a spurious result for the saturation energy level. We also find that the movement of the condensate is similar to the motion of an inertial particle with an effective drag force. Furthermore, we show that the profile of the saturated coherent vortices can be described by a Gaussian core with exponential wings.
• Pessah, M. E., & Chan, C. (2012). ON HYDROMAGNETIC STRESSES IN ACCRETION DISK BOUNDARY LAYERS. The Astrophysical Journal, 751(1), 48. doi:10.1088/0004-637x/751/1/48
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  Detailed calculations of the physical structure of accretion disk boundary layers, and thus their inferred observational properties, rely on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. In order to shed light on physically viable mechanisms for angular momentum transport in this inner disk region, we examine the generation of hydromagnetic stresses and energy density in differentially rotating backgrounds with angular frequencies that increase outward in the shearing-sheet framework. We isolate the modes that are unrelated to the standard MRI and provide analytic solutions for the long-term evolution of the resulting shearing MHD waves. We show that, although the energy density of these waves can be amplified significantly, their associated stresses oscillate around zero, rendering them an inefficient mechanism to transport significant angular momentum (inward). These findings are consistent with the results obtained in numerical simulations of MHD accretion disk boundary layers and challenge the standard assumption of efficient angular momentum transport in the inner disk regions. This suggests that the detailed structure of turbulent MHD accretion disk boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity
• Chan, C. (2011). A CLASS OF PHYSICALLY MOTIVATED CLOSURES FOR RADIATION HYDRODYNAMICS. The Astrophysical Journal, 727(2), 67. doi:10.1088/0004-637x/727/2/67
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  Radiative transfer and radiation hydrodynamics use the relativistic Boltzmann equation to describe the kinetics of photons. It is difficult to solve the six-dimensional time-dependent transfer equation unless the problem is highly symmetric or in equilibrium. When the radiation field is smooth, it is natural to take angular moments of the transfer equation to reduce the degrees of freedom. However, low order moment equations contain terms that depend on higher order moments. To close the system of moment equations, approximations are made to truncate this hierarchy. Popular closures used in astrophysics include flux-limited diffusion and the M 1 closure, which are rather ad hoc and do not necessarily capture the correct physics. In this paper, we propose a new class of closures for radiative transfer and radiation hydrodynamics. We start from a different perspective and highlight the consistency of a fully relativistic formalism. We present a generic framework to approximate radiative transfer based on relativistic Grad's moment method. We then derive a 14-field method that minimizes unphysical photon self-interaction.
• Chan, C. (2009). OSCILLATIONS OF THE INNER REGIONS OF VISCOUS ACCRETION DISKS. The Astrophysical Journal, 704(1), 68-79. doi:10.1088/0004-637x/704/1/68
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  Although quasi-periodic oscillations (QPOs) have been discovered in different X-ray sources, their origin is still a matter of debate. Analytical studies of hydrodynamic accretion disks have shown three types of trapped global modes with properties that appear to agree with the observations. However, these studies take only the linear effects into account. Moreover, observations suggest that resonances between modes play a crucial role. A systematic, numerical study of this problem is therefore needed. In this paper, we use a pseudo-spectral algorithm to perform a parameter study of the inner regions of hydrodynamic disks. By assuming α-viscosity, we show that steady state solutions rarely exist. The inner edges of the disks oscillate and excite axisymmetric waves, which provide a plausible explanation for the high-frequency QPOs observed from accreting black holes. In addition, the flows inside the inner edges are sometimes unstable to non-axisymmetric perturbations. One-armed, or even two-armed, spirals are developed. When the Reynolds numbers are above certain critical values, the inner disks go through some transient turbulent states characterized by strong trailing spirals; while large-scale leading spirals are developed in the outer disks. We compared our numerical results with standard thin disk oscillation models. Albeit the non-axisymmetric features have their analytical counterparts, more careful study is needed to explain the axisymmetric oscillations.
• Chan, C., Psaltis, D., Melia, F., Rockefeller, G., Psaltis, D., Ozel, F., Melia, F., Liu, S., Fryer, C. L., & Chan, C. (2009). MHD SIMULATIONS OF ACCRETION ONTO Sgr A*: QUIESCENT FLUCTUATIONS, OUTBURSTS, AND QUASIPERIODICITY. The Astrophysical Journal, 701(1), 521-534. doi:10.1088/0004-637x/701/1/521
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  High-resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations (QPOs), making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudospectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic (MHD) model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme and calibrating the parameters by observations, we show that the MHD turbulence in the environment of Sgr A* can by itself only produce factor two fluctuations in luminosity. These fluctuations cannot explain the magnitude of flares observed in this system. However, we also demonstrate that external forcing of the accretion disk, which may be generated by the 'clumpy material' raining down onto the disk from the large-scale flow, do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and by ground-based facilities in the near infrared. Strong, but short-term QPOs emerge naturally in the simulated light curves. We attribute these to nonaxisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state.
• Chan, C., Psaltis, D., Psaltis, D., Ozel, F., & Chan, C. (2009). SPECTRAL METHODS FOR TIME-DEPENDENT STUDIES OF ACCRETION FLOWS. III. THREE-DIMENSIONAL, SELF-GRAVITATING, MAGNETOHYDRODYNAMIC DISKS. The Astrophysical Journal, 700(1), 741-751. doi:10.1088/0004-637x/700/1/741
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  Accretion disks are three-dimensional, turbulent, often self-gravitating, magnetohydrodynamic (MHD) flows, which can be modeled with numerical simulations. In this paper, we present a new algorithm that is based on a spectral decomposition method to simulate such flows. Because of the high order of the method, we can solve the induction equation in terms of the magnetic vector potential and, therefore, ensure trivially that the magnetic fields in the numerical solution are divergence free. The spectral method also suffers minimally from numerical dissipation and allows for an easy implementation of models for subgrid physics. Both properties make our method ideal for studying MHD turbulent flows such as those found in accretion disks around compact objects. We verify our algorithm with a series of standard tests and use it to show the development of MHD turbulence in a simulation of an accretion disk. Finally, we study the evolution and saturation of the power spectrum of MHD turbulence driven by the magnetorotational instability.
• Pessah, M. E., & Chan, C. (2008). Viscous, Resistive Magnetorotational Modes. The Astrophysical Journal, 684(1), 498-514. doi:10.1086/589915
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  We carry out a comprehensive analysis of the behavior of the magnetorotational instability (MRI) in viscous, resistive plasmas. We find exact, nonlinear solutions of the nonideal magnetohydrodynamic (MHD) equations describing the local dynamics of an incompressible, differentially rotating background threaded by a vertical magnetic field when disturbances with wavenumbers perpendicular to the shear are considered. We provide a geometrical description of these viscous, resistive MRI modes and show how their physical structure is modified as a function of the Reynolds and magnetic Reynolds numbers. We demonstrate that when finite dissipative effects are considered, velocity and magnetic field disturbances are no longer orthogonal (as is the case in the ideal MHD limit) unless the magnetic Prandtl number is unity. We generalize previous results found in the ideal limit and show that a series of key properties of the mean Reynolds and Maxwell stresses also hold for the viscous, resistive MRI. In particular, we show that the Reynolds stress is always positive and the Maxwell stress is always negative. Therefore, even in the presence of viscosity and resistivity, the total mean angular momentum transport is always directed outward. We also find that, for any combination of the Reynolds and magnetic Reynolds numbers, magnetic disturbances dominate both the energetics and the transport of angular momentum and that the total mean energy density is an upper bound for the total mean stress responsible for angular momentum transport. The ratios between the Maxwell and Reynolds stresses and between magnetic and kinetic energy densities increase with decreasing Reynolds numbers for any magnetic Reynolds number; the lowest limit of both ratios is reached in the ideal MHD regime. The analytical results presented here provide new benchmarks for the various algorithms employed to solve the viscous, resistive MHD equations in the shearing box approximation.
• Chan, C., Psaltis, D., Psaltis, D., Pessah, M. E., & Chan, C. (2007). ANGULAR MOMENTUM TRANSPORT IN ACCRETION DISKS: SCALING LAWS IN MRI-DRIVEN TURBULENCE. The Astrophysical Journal, 668(1), L51-L54. doi:10.1086/522585
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  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.
• Chan, C., Psaltis, D., Psaltis, D., Pessah, M. E., & Chan, C. (2007). The fundamental difference between shear alpha viscosity and turbulent magnetorotational stresses. Monthly Notices of the Royal Astronomical Society, 383(2), 683-690. doi:10.1111/j.1365-2966.2007.12574.x
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  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.
• Chan, C., Psaltis, D., Psaltis, D., Pessah, M. E., & Chan, C. (2006). Local model for angular-momentum transport in accretion disks driven by the magnetorotational instability.. Physical review letters, 97(22), 221103. doi:10.1103/physrevlett.97.221103
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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.
• Psaltis, D., Chan, C. K., Psaltis, D., Pessah, M. E., & Chan, C. K. (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. doi:10.1111/j.1365-2966.2006.10824.x
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  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.
• Psaltis, D., Chan, C., Psaltis, D., Ozel, F., & Chan, C. (2006). Spectral Methods for Time-dependent Studies of Accretion Flows. II. Two-dimensional Hydrodynamic Disks with Self-Gravity. The Astrophysical Journal, 645(1), 506-518. doi:10.1086/500394
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  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.
• Psaltis, D., Chan, C., Psaltis, D., Ozel, F., & Chan, C. (2005). Spectral Methods for Time-dependent Studies of Accretion Flows. I. Two-dimensional, Viscous, Hydrodynamic Disks. The Astrophysical Journal, 628(1), 353-367. doi:10.1086/430511
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  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.

Proceedings Publications

• Norman, D., Cruz, K., Desai, V., Lundgren, B., Bellm, E., Economou, F., Smith, A., Bauer, A., Nord, B., Schafer, C., Narayan, G., Li, T., Tollerud, E., Sip{H{o}cz}, B., Stevance, H., Pickering, T., Sinha, M., Harrington, J., Kartaltepe, J., , Vohl, D., et al. (2019, sep). The Growing Importance of a Tech Savvy Astronomy and Astrophysics Workforce. In baas, 51.
• Smith, A., Norman, D., Cruz, K., Desai, V. a., Bellm, E., Lundgren, B., Economou, F., Nord, B. D., Schafer, C., Narayan, G., Harrington, J., Tollerud, E., Sip{H{o}cz}, B., Pickering, T., Peeples, M. S., Berriman, B., Teuben, P., Rodriguez, D., Gradvohl, A., , Shamir, L., et al. (2019, sep). Elevating the Role of Software as a Product of the Research Enterprise. In baas, 51.
• Tollerud, E., Smith, A., Price-Whelan, A., Cruz, K., Norman, D., Narayan, G., Mumford, S., Allen, A., Chan, C., Cherinka, B., Drlica-Wagner, A., Foreman-Mackey, D., Ginsburg, A., Gradvhol, A., Harrington, J., Hogg, D., Jartaltepe, J., Kinney, J., Merchant, N., , Momcheva, I., et al. (2019, sep). Sustaining Community-Driven Software for Astronomy in the 2020s. In baas, 51.

Others

• Chael, A. A., Bouman, K. L., Johnson, M. D., Narayan, R., Doeleman, S. S., Wardle, J. F., Blackburn, L. L., Akiyama, K., Wielgus, M., Chan, C., Farah, J. R., Palumbo, D., & Pesce, D. (2019). ehtim: Imaging, analysis, and simulation software for radio interferometry.
• Chael, A., Bouman, K., Johnson, M., Wielgus, M., Blackburn, L., Chan, C., Farah, J. R., Palumbo, D., & Pesce, D. (2019). eht-imaging: v1.1.0: Imaging interferometric data with regularized maximum likelihood.