Evolution of an Accretion Disk in an Active Galactic Nucleus

We present results of the modeling of thermal-viscous ionization instabilities in accretion disks in active galactic nuclei (AGNs). This instability, analogous to the instability which drives outbursts in cataclysmic variables or X-ray transients, is expected also to occur in accretion disks around supermassive black holes unless self-gravity effects or irradiation prevents the existence of the partial ionization zone (which causes the instability). We assume that viscosity scales with the gas pressure (β-viscosity). We discuss in detail evolution of the accretion disk around an Mbh = 10⁸ Msun black hole mass, assuming a range of accretion rates corresponding to luminosities between 10⁻⁵ and 1, in Eddington units. We show that the ionization instability does operate in such disks. Depending on the viscosity, the instability can develop in a very narrow unstable zone, thus causing a flickering, or can propagate over the entire disk, resulting in the large-amplitude outbursts on timescales of order 10³-10⁵ yr. As a consequence: (1) AGNs are not always in their active state, and (2) the emitted optical-UV spectra are quite different from the spectrum of a thin, stationary accretion disk. External irradiation is shown to have a large effect on the instability and on the emitted optical-UV spectra. We briefly consider observational consequences.