VARIABILITY SPECTRA OF ORBITING BLOBS AROUND A BLACK-HOLE

Variability power spectra of the distribution of orbiting blobs in an optically thick accretion disk around a black hole are investigated by a full relativistic approach. Both the well-known alpha disks and beta disks are considered. The calculations also include the cases when the emitting matter is optically thin or optically thick. The resulting variability spectra are characterized by a power-law shape with flattening frequency at its lower end and a drop-off frequency at its higher end, both corresponding to the borders of blobs distribution in the disk. The frequencies are intrinsic to all sources with an inhomogeneous accretion disk around a black hole, independent of either the specific property of the disk or the nature of the blobs. The drop-off frequency could be an important clue in determining the mass of the central black hole if the distribution of the blobs extends to the last stable circular orbit around a black hole. It may also be taken as evidence for the relativistic rotation of accretion disks around the hole. The average slopes of the observed X-ray power spectra are around -1.5 for both active galactic nuclei and Galactic black hole candidates, suggesting the number density distribution n(r) proportional to r(delta) with 0 less than or equal to delta less than or equal to 1. This may improve our understanding of fluctuations of accretion disks.