Observational Constraints on Viscosity in the AGN Accretion Discs

Abstract

The optical/UV/soft X-ray big bump can be modelled as a thermal radiation from an accretion disc. Assuming that the observed UV variability in AGN spectra is caused by accretion disc instabilities we can set constraints on the viscosity. Simple bimodal disc approximation was used — the outer regions of the disc radiate locally as a black body whilst the inner regions locally become optically thin in a thermal time scale giving a negligible contribution to the UV radiation. Calculation were made for several models with L/L Edd = 1 and L/L Edd = 0.01 and various central masses. Comparison between thermal and observed variability time scales in 10 Seyfert galaxies and 16 QSOs indicate the parameter α value of the order of 0.01.