X-RAY FLUCTUATIONS FROM BLACK-HOLE OBJECTS AND SELF-ORGANIZATION OF ACCRETION DISKS

Aperiodic fluctuations with 1/f-like power spectral density (PSD, wher e f is a frequency) are characteristics of X-ray radiation from accret ing compact objects, such as active galactic nuclei and Cyg X-1. To ac count for such intriguing features, we previously proposed a cellular- automaton model for accretion disks based on the concept of the self-o rganized criticality (SOC). In this model, mass accretion takes place in a form of avalanches only when the local mass density exceeds some critical value. Although the original model basically produced 1/f-lik e PSD, it has become clear that the X-ray observations yield an expone ntial distribution of the peak intensities of X-ray shots, instead of a power-law distribution that the SOC predicts. We have thus modified the original model by introducing a gradual diffusion which occurs reg ardless of the critical condition, finding a good agreement between th e new model and the observations. There is a strong correlation (but n o one-to-one correspondence) between the energy (epsilon) and lifetime (tau) of individual shots, except at very large epsilon, where tau is rather insensitive to epsilon. We further find that the shape of the PSD in low-frequency regimes may discriminate wheather the accretion t akes place two-dimensionally (via disk accretion) or three-dimensional ly (via spherical accretion). The structure of accretion disks produci ng 1/f-like X-ray fluctuations is discussed in relation to the model o f advection-dominated accretion disks.