Evolution of an Accretion Disk in an Active Galactic Nucleus

Details Evolution of an Accretion Disk in an Active Galactic Nucleus Evolution of an Accretion Disk in an Active Galactic Nucleus

Feb 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...458..491s&link_type=abstract

Astrophysical Journal v.458, p.491

Astronomy and Astrophysics

Astronomy

79

Accretion, Accretion Disks, Galaxies: Active, Galaxies: Quasars: General

Scientific paper

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.

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