Instabilities of thin, viscous and magnetized accretion disks

Abstract

We derive the dispersion equation by the perturbation method and discuss in detail the instabilities of thin accretion disks in four cases. The results show: pulsational instability exists in both pure viscous and pure magnetized disks; for disks that are both viscous and magnetized, there are two kinds of instability, pulsational and monotonic. Numerical calculations show that the pulsational instability most likely exists in the inner region of the disk, while the monotonic instability is confined to the outer region and is meaningful only for short wave perturbations. These results may be helpful in explaining the light variations observed in the nuclei of BL Lac objects, Seyfert galaxies and quasars.