TY - JOUR
T1 - How important is non-ideal physics in simulations of sub-Eddington accretion on to spinning black holes?
AU - Foucart, Francois
AU - Chandra, Mani
AU - Gammie, Charles F.
AU - Quataert, Eliot
AU - Tchekhovskoy, Alexander
N1 - Funding Information:
We thank Matt Kunz and Jono Squire for useful discussions over the course of this project. Support for FF was provided by NASA through Einstein Postdoctoral Fellowship grant numbered PF4- 150122 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. EQ was supported in part by NSF grant AST 13-33612, a Simons Investigator Award from the Simons Foundation, and the David and Lucile Packard Foundation. CFG was supported by NSF grant AST-1333612, a Simons Fellowship and a visiting fellowship at All Souls College, Oxford. CFG is also grateful to Oxford Astrophysics for a Visiting Professorship appointment. MC was supported by an Illinois Distinguished Fellowship from the University of Illinois and by NSF grant AST-1333612. MC also thanks EQ for a Visiting Scholar appointment at the University of California, Berkeley, where part of this work was done. We thank Pavan Yalamanichili and the team at arrayfire.com for help with performance optimizations in GRIM. This work was made possible by computing time granted by the Extreme Science and Engineering Discovery Environment (XSEDE) through allocation no. TG-PHY160040, supported by NSF Grant No. ACI-1053575. AT was supported by the Theoretical Astrophysics Center (TAC) Fellowship and by NSF through XSEDE allocation TG-AST100040.
Publisher Copyright:
© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2017/9/11
Y1 - 2017/9/11
N2 - Black holes with accretion rates well below the Eddington rate are expected to be surrounded by low-density, hot, geometrically thick accretion discs. This includes the two black holes being imaged at subhorizon resolution by the Event Horizon Telescope. In these discs, the mean free path for Coulomb interactions between charged particles is large, and the accreting matter is a nearly collisionless plasma. Despite this, numerical simulations have so farmodelled these accretion flows using ideal magnetohydrodynamics. Here, we present the first global, general relativistic, 3D simulations of accretion flows on to a Kerr black hole including the non-ideal effects most likely to affect the dynamics of the disc: the anisotropy between the pressure parallel and perpendicular to the magnetic field, and the heat flux along magnetic field lines. We show that for both standard and magnetically arrested discs, the pressure anisotropy is comparable to the magnetic pressure, while the heat flux remains dynamically unimportant. Despite this large pressure anisotropy, however, the time-averaged structure of the accretion flow is strikingly similar to that found in simulations treating the plasma as an ideal fluid. We argue that these similarities are largely due to the interchangeability of the viscous and magnetic shear stresses as long as the magnetic pressure is small compared to the gas pressure, and to the subdominant role of pressure/viscous effects in magnetically arrested discs. We conclude by highlighting outstanding questions in modelling the dynamics of low-collisionality accretion flows.
AB - Black holes with accretion rates well below the Eddington rate are expected to be surrounded by low-density, hot, geometrically thick accretion discs. This includes the two black holes being imaged at subhorizon resolution by the Event Horizon Telescope. In these discs, the mean free path for Coulomb interactions between charged particles is large, and the accreting matter is a nearly collisionless plasma. Despite this, numerical simulations have so farmodelled these accretion flows using ideal magnetohydrodynamics. Here, we present the first global, general relativistic, 3D simulations of accretion flows on to a Kerr black hole including the non-ideal effects most likely to affect the dynamics of the disc: the anisotropy between the pressure parallel and perpendicular to the magnetic field, and the heat flux along magnetic field lines. We show that for both standard and magnetically arrested discs, the pressure anisotropy is comparable to the magnetic pressure, while the heat flux remains dynamically unimportant. Despite this large pressure anisotropy, however, the time-averaged structure of the accretion flow is strikingly similar to that found in simulations treating the plasma as an ideal fluid. We argue that these similarities are largely due to the interchangeability of the viscous and magnetic shear stresses as long as the magnetic pressure is small compared to the gas pressure, and to the subdominant role of pressure/viscous effects in magnetically arrested discs. We conclude by highlighting outstanding questions in modelling the dynamics of low-collisionality accretion flows.
KW - Black hole physics
KW - Galaxies: nuclei
KW - Galaxy: centre
KW - MHD
KW - Methods: numerical
KW - Stars: black holes
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U2 - 10.1093/mnras/stx1368
DO - 10.1093/mnras/stx1368
M3 - Article
AN - SCOPUS:85023768697
SN - 0035-8711
VL - 470
SP - 2240
EP - 2252
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 2
ER -