A Comment on the Paper by Mineshige et al. on the Cellular Automaton Model for the Variability of Accretion Disks around Black Holes

Abstract Stellar black-hole candidates and active galactic nuclei are believed to contain accretion disks around black holes. Their X-ray variability is characterized by power spectra with a power law dependence on the variability frequency. The power slope β is in the range between 1 and 2 and its most typical value is around 1.5. Mineshige et al. (1994, PASJ 46, 97) have recently simulated this type of accretion disk fluctuations by a cellular automaton “ant-lion” model and obtained β = 1.8. We point out that their model describes intrinsic fluctuations of accretion disks and that the observed value β = 1.8 is to be expected for disks oriented pole-on with respect to the detector. For disks with a non-zero inclination, the observed power spectrum slope should be flatter. We have calculated variability power spectra for accretion disk fluctuations described by the Mineshige et al. model, and obtained β = 1.7, 1.6, and 1.4 for disks with inclinations i = 20°, 60°, and 80° respectively, in an excellent agreement with observations.