Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 470, Issue 2, Pages 2240-2252Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stx1368
Keywords
black hole physics; MHD; methods: numerical; stars: black holes; Galaxy: centre; galaxies: nuclei
Categories
Funding
- NASA [PF4-150122, NAS8-03060]
- Chandra X-ray Center
- NSF [AST 13-33612, AST-1333612, ACI-1053575, TG-AST100040]
- Simons Investigator Award from the Simons Foundation
- David and Lucile Packard Foundation
- All Souls College, Oxford
- Illinois Distinguished Fellowship from the University of Illinois
- Theoretical Astrophysics Center (TAC) Fellowship
- Direct For Mathematical & Physical Scien [1333682] Funding Source: National Science Foundation
- Division Of Astronomical Sciences [1333682] Funding Source: National Science Foundation
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1333091] Funding Source: National Science Foundation
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1333612] Funding Source: National Science Foundation
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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 far modelled 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.
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