Stability of black hole accretion disks

TL;DR

This paper examines the stability of black hole accretion disks, focusing on thermal-viscous instabilities that can cause cyclic activity, and explores mechanisms, including stochastic viscosity fluctuations, that may stabilize these oscillations.

Contribution

It introduces the stabilizing effect of stochastic viscosity fluctuations as a new mechanism for preventing limit-cycle oscillations in high accretion rate disks.

Findings

Thermal-viscous instabilities can induce cyclic activity in accretion disks.

Observational evidence for these instabilities is limited and mostly indirect.

Stochastic viscosity fluctuations can stabilize the accretion disk oscillations.

Abstract

We discuss the issues of stability of accretion disks that may undergo the limit-cycle oscillations due to the two main types of thermal-viscous instabilities. These are induced either by the domination of radiation pressure in the innermost regions close to the central black hole, or by the partial ionization of hydrogen in the zone of appropriate temperatures. These physical processes may lead to the intermittent activity in AGN on timescales between hundreds and millions of years. We list a number of observational facts that support the idea of the cyclic activity in high accretion rate sources. We conclude however that the observed features of quasars may provide only indirect signatures of the underlying instabilities. Also, the support from the sources with stellar mass black holes, whose variability timescales are observationally feasible, is limited to a few cases of the microquasars. Therefore we consider a number of plausible mechanisms of stabilization of the limit cycle oscillations in high accretion rate accretion disks. The newly found is the stabilizing effect of the stochastic viscosity fluctuations.