Is a black hole accretion disk in a self-organized critical state?

In Mineshige et al. we have proposed that the inner portions of black hole accretion disks may be in a self-organized critical (SOC) state, thereby producing 1/f-like X-ray fluctuations (with f being a frequency) in spite of random mass input. The original model is modified so as to incorporate viscous diffusion process. Mass accretion, therefore, occurs either by an avalanche, which is triggered when the mass density of the disk exceeds some critical value, or by gradual gas diffusion. The model then produces 1/fbeta (with beta approximately 1.6) decline in the power spectrum, which is in good agreement with the observations that show a break to a steeper decline. We then discuss the observational features of the SOC state: (1) power spectra consisting of flat and power-law decline parts, (2) power-law-like peak-count distributions for individual shots and (3) non-random temporal distributions of very big shots. Detailed observational data analysis is urgently needed for confirmation.