On different types of instabilities in black hole accretion discs. Implications for X-ray binaries and AGN

TL;DR

This paper examines ionization and radiation-pressure instabilities in black hole accretion disks, comparing theoretical models with observational data from X-ray binaries and AGN to understand their role in luminosity oscillations.

Contribution

It provides a comparative analysis of instability mechanisms in accretion disks and suggests conditions under which radiation pressure instability is likely present in Galactic sources and AGN.

Findings

Radiation pressure instability likely occurs in sources with high Eddington ratios.

Disk sizes in X-ray novae support ionization instability.

Viscosity parameterization through geometric mean of pressures is favored.

Abstract

We discuss two important instability mechanisms that may lead to the limit-cycle oscillations of the luminosity of the accretion disks 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 AGN is still problematic. Radiation pressure theory has still very weak observational background in any of these sources. In the present 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 above 0.15, and in AGN with the Eddington ratio above 0.025. Our results favor the parameterization of the viscosity through the geometrical mean of the radiation and gas pressure both in Galactic sources and AGN. More examples of the quasi-regular outbursts in the timescales of 100 seconds in Galactic sources, and hundreds of years in AGN are needed to formulate firm conclusions. We also show that the disk 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.