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

TLDR: In this paper, the authors 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.

Abstract: We discuss two important instability mechanisms that may lead to the limit-cycleoscillations of the luminosity of the accretion disks around compact objects: ionizationinstability and radiation-pressure instability. Ionization instability is well establishedas a mechanism of X-ray novae eruptions in black hole binary systems but its ap-plicability to AGN is still problematic. Radiation pressure theory has still very weakobservational background in any of these sources. In the present paper we attempt toconfront the parameter space of these instabilities with the observational data. At thebasis of this simple survey of sources properties we argue that the radiation pressureinstability is likely to be present in several Galactic sources with the Eddington ratiosabove 0.15, and in AGN with the Eddington ratio above 0.025. Our results favor theparameterization of the viscosity through the geometrical mean of the radiation andgas pressure both in Galactic sources and AGN. More examples of the quasi-regularoutbursts in the timescales of 100 seconds in Galactic sources, and hundreds of yearsin AGN are needed to formulate firm conclusions. We also show that the disk sizes inthe X-ray novae are consistent with the ionization instability. This instability may alsoconsiderably influence the lifetime cycle and overall complexity in the supermassiveblack hole environment.Keywords: physical processes:accretion; X-rays:binaries; galaxies: active – galaxies:evolution – galaxies