THE ROTATION OF ACCRETION DISKS AND THE POWER SPECTRA OF X-RAY FLICKERING

The X-ray producing, inner region of the accretion disk in Active Galactic Nuclei (AGN) is likely to be non-stationary and non-axisymmetric. This non-stationarity and non-axisymmetry in disk surface brightness may be modeled by considering the presence of many "hot spots" on a steady, axisymmetric disk. As long as a "spot" can survive for a few orbital periods, its orbital frequency can be introduced into the light curve either by relativistic oribital motion or by eclipsing of the spot by the disk. These rotational effects vary with the local properties of the spot population. Depending on the radial variation of spot brightness, lifetime and number density, the observed variability power spectrum may differ from that due to the intrinsic variability of spots alone, within the orbital frequency range in which these spots occur. In this paper, we explore the relation between properties assumed for the spot population and the resulting predictions for the observed variability. The implications of our results for the "flickering" of X-ray sources powered by accretion disks (both AGN and galactic sources) are also discussed.