The Dynamics of Truncated Black Hole Accretion Disks. I. Viscous Hydrodynamic Case
Abstract
Truncated accretion disks are commonly invoked to explain the spectro-temporal variability in accreting black holes in both small systems, I.e., state transitions in galactic black hole binaries (GBHBs), and large systems, I.e., low-luminosity active galactic nuclei (LLAGNs). In the canonical truncated disk model of moderately low accretion rate systems, gas in the inner region of the accretion disk occupies a hot, radiatively inefficient phase, which leads to a geometrically thick disk, while the gas in the outer region occupies a cooler, radiatively efficient phase that resides in the standard geometrically thin disk. Observationally, there is strong empirical evidence to support this phenomenological model, but a detailed understanding of the dynamics of truncated disks is lacking. We present a well-resolved viscous, hydrodynamic simulation that uses an ad hoc cooling prescription to drive a thermal instability and, hence, produce the first sustained truncated accretion disk. With this simulation, we perform a study of the dynamics, angular momentum transport, and energetics of a truncated disk. We find that the time variability introduced by the quasi-periodic transition of gas from efficient cooling to inefficient cooling impacts the evolution of the simulated disk. A consequence of the thermal instability is that an outflow is launched from the hot/cold gas interface, which drives large, sub-Keplerian convective cells into the disk atmosphere. The convective cells introduce a viscous θ - ϕ stress that is less than the generic r - ϕ viscous stress component, but greatly influences the evolution of the disk. In the truncated disk, we find that the bulk of the accreted gas is in the hot phase.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2017
- DOI:
- 10.3847/1538-4357/aa774b
- arXiv:
- arXiv:1706.01489
- Bibcode:
- 2017ApJ...843...80H
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- hydrodynamics: HD;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 16 pgs, 14 figures, accepted for publication in ApJ