Non-Stationary Discs and Instabilities

TL;DR

This paper reviews the current understanding of thermal and viscous instabilities in accretion discs around compact objects, covering classical models, ionization instabilities, MHD effects, and stability issues in radiation pressure-dominated regions.

Contribution

It provides a comprehensive review of classical and modern theories of disc instabilities, including new insights from MHD simulations and their implications for observed phenomena.

Findings

Ionization instability explains dwarf nova and X-ray binary outbursts.

MHD simulations offer new perspectives on angular momentum transport.

Discrepancies exist between predicted and observed instabilities in radiation pressure-dominated regions.

Abstract

We review our current knowledge of thermal and viscous instabilities in accretion discs around compact objects. We begin with classical disc models based on analytic viscosity prescriptions, discussing physical uncertainties and exploring time-dependent solutions of disc evolution. We also review the ionization instability responsible for outbursting dwarf nova and X-ray binary systems, including some detailed comparisons between alpha-based models and the observed characteristics of these systems. We then review modern theoretical work based on ideas around angular momentum transport mediated by magnetic fields, focusing in particular on knowledge gained through local and global computer simulations of MHD processes in discs. We discuss how magnetohydrodynamics (MHD) may alter our understanding of outbursts in white dwarf and X-ray binary systems. Finally, we turn to the putative thermal/viscous instabilities that were predicted to exist in the inner, radiation pressure-dominated regions of black hole and neutron star discs, in apparent contradiction to the observed stability of the high/soft state in black hole X-ray binaries.