Models for accretion-disk fluctuations through self-organized criticality including relativistic effects

The possibility that some of the observed X-ray and optical variability in active galactic nuclei and galactic black hole candidates are produced in a ccretion disks through the development of a self-organized critical state i s reconsidered. New simulations, including more complete calculations of re lativistic effects, do show that this model can produce light-curves and po wer-spectra for the variability which agree with the range observed in opti cal and X-ray studies of AGN and X-ray binaries. However, the universality of complete self-organized criticality has not quite been achieved. This is mainly because the character of the variations depend quite substantially on the extent of the unstable disk region. If it extends close to the inner most stable orbit, a physical scale is introduced and the scale-free charac ter of self-organized criticality is vitiated. A significant dependence of the power spectrum density slope on the type of diffusion within the disk a nd a weaker dependence on the amount of differential rotation are noted. Wh en general-relativistic effects are incorporated in the models, additional substantial differences are produced if the disk is viewed from directions far from the accretion disk axis.