On different types of instabilities in black hole accretion discs: implications for X-ray binaries and active galactic nuclei

Abstract

We discuss two important instability mechanisms that may lead to the limit‐cycle oscillations of the luminosity of the accretion discs around compact objects: ionization instability and radiation pressure instability. Ionization instability is well established as a mechanism of X‐ray novae eruptions in black hole binary systems, but its applicability to active galactic nuclei (AGN) is still problematic. Radiation pressure theory has still a very weak observational background in any of these sources. In this paper, we attempt to confront the parameter space of these instabilities with the observational data. At the basis of this simple survey of sources properties, we argue that the radiation pressure instability is likely to be present in several Galactic sources with the Eddington ratios being above 0.15 and in AGN with the Eddington ratio above 0.025. Our results favour the parametrization of the viscosity through the geometrical mean of the radiation and gas pressure in both Galactic sources and AGN. More examples of the quasi‐regular outbursts in the time‐scales of 100 s in Galactic sources and hundreds of years in AGN are needed to formulate firm conclusions. We also show that the disc sizes in the X‐ray novae are consistent with the ionization instability. This instability may also considerably influence the lifetime cycle and overall complexity in the supermassive black hole environment.

Spectroscopic Indication of a Centi-Parsec Supermassive Black Hole Binary in the Galactic Center of Ngc 5548

“…It is possible the thermal instability of the accretion disk leads to repeat outbursts of accretion onto the black hole and gives rise to periodic continuum variations, but this generally occurs on a much long timescale of the order of 10 3 −10 5 yr (e.g., Mineshige & Shields 1990;Hameury et al 2009;Janiuk & Czerny 2011). Even if the instability were to develop only in a part of the disk and the continuum variations are modulated by the disk rotation, the amount of the modulation (i.e., the Doppler boosting) in the regions that produce the 5100 Å continuum (at ∼10 3 R g with a period of ∼14 yr) is insufficient for the observed large amplitude (see also Section 4.3).…”

Stability of black hole accretion disks

“…These are the radiation pressure instability and the partial hydrogen ionization instability, both known for over 40 years in theoretical astrophysics. a e-mail: agnes@cft.edu.pl For a detailed discussion of these instabilities the reader is referred to the recent article [5] as well as to the literature quoted therein. Below, we only briefly describe the fundamental physics behind these processes.…”

Stabilization of radiation pressure dominated accretion disks through viscous fluctuations

“…Specifically, we parametrized the jet (cf. Janiuk & Czerny 2011) by including an additional term in the energy equation…”

“…However, even for this prescription the disk is not stable, but undergoes luminosity oscillations with diminished amplitude. A combination of this viscous prescription and the assumption that a fraction of the power is transferred to a corona may explain the variability of the micro-quasar GRS 1915+105, in certain spectral states (Janiuk & Czerny 2011). GRS 1915+105 is not typical, and the apparent stability of other bright X-ray binaries and even for GRS 1915+105 in its stable states remains a puzzle (see e.g., Neilsen et al 2011).…”

“…The extension of the radiation pressure dominated part of the disk depends on the accretion rate, the black hole mass, and the viscosity. The parameter regions of the radiation pressure dominated disks in Galactic black holes and AGN were shown recently in Janiuk & Czerny (2011); the approximate fitting formula for this dependence is log R out R S = logṁ + 2.5 + 0.1 log α 0.01…”