Variability and Instabilities of Accretion Disks

Abstract

In the Seyfert galaxy NGC 1566 and quasars 3C 345, 3C 446 long term (∼ 103 days) variations have been observed. We propose to explain them as a limit cycle behaviour, similar to that well studied in the case of dwarf novae outbursts. The key element of our analysis is the relationship between the accretion rate · and the surface density Σ.