Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks
arxiv(2023)
摘要
The conventional accretion disk lore is that magnetized turbulence is the
principal angular momentum transport process that drives accretion. However,
when dynamically important magnetic fields thread an accretion disk, they can
produce mass and angular momentum outflows that also drive accretion. Yet, the
relative importance of turbulent and wind-driven angular momentum transport is
still poorly understood. To probe this question, we analyze a long-duration
(1.2 × 10^5 r_ g/c) simulation of a rapidly rotating (a=0.9)
black hole (BH) feeding from a thick (H/r∼0.3), adiabatic, magnetically
arrested disk (MAD), whose dynamically-important magnetic field regulates mass
inflow and drives both uncollimated and collimated outflows (e.g., "winds" and
"jets", respectively). By carefully disentangling the various angular momentum
transport processes occurring within the system, we demonstrate the novel
result that both disk winds and disk turbulence extract roughly equal amounts
of angular momentum from the disk. We find cumulative angular momentum and mass
accretion outflow rates of L̇∝ r^0.9 and Ṁ∝
r^0.4, respectively. This result suggests that understanding both turbulent
and laminar stresses is key to understanding the evolution of systems where
geometrically thick MADs can occur, such as the hard state of X-ray binaries,
low-luminosity active galactic nuclei, some tidal disruption events, and
possibly gamma ray bursts.
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