Accretion Disk Instabilities

Abstract

In this article we discuss two instabilities of stationary accretion disks which lead to an understanding of observed light variations in accretion disk systems, the dwarf novae and the rapid burster MXB 17030-335. The accretion disks in these systems avoid instability at the cost of stationarity and perform stable cycles in which sudden changes of the accretion flow lead to corresponding, often dramatic, variations of their accretion luminosity.Figure 1 shows a light curve of U Geminorum. It was discovered In 1855 by J.R. Hind and has become a prototype of the dwarf novae. In these systems an extended time of quiescence of up to several weeks Is followed by a short outburst of a few days during which the luminosity rises by a factor of 30 to 100. The dwarf novae belong to the cataclysmic variables. They are all close binaries In which a white dwarf primary is orbited by a Roche lobe-filling low mass secondary. Through the inner Lagrangian point mass flows over from the secondary and forms a luminious accretion disk around the white dwarf. In the case of the dwarf novae this disk has temperatures below about 10000K in Its outer region. It will be discussed how partial lonizatlon and convection then affect the vertical structure of the disk such that the stationary flow becomes unstable.Fig. 1. Light curve of the dwarf nova U Geminorum. Abszissa in days С [2])