Thermal and Viscous Instability of Accretion Disc in AGN
  The standard accretion discs are known to be thermally and viscously unstable over a certain range of temperatures. In the inner disc regions there may develop radiation pressure driven instability, which is possibly related to the rapid variability detected in AGNs in the UV range. In the outer disc develops the ionization instability, similar to that in the cataclysmic variables, but operating on much longer timescales. Due to this process the spectrum of the accretion disc differs from that of a stationary one. 
Agnieszka Janiuk, Bozena Czerny