M. Abramowicz et G. Bao, THE CELLULAR-AUTOMATON MODEL FOR THE VARIABILITY OF ACCRETION DISKS AROUND BLACK-HOLES - COMMENT, Publications of the Astronomical Society of Japan, 46(5), 1994, pp. 523-525
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 beta 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 flu
ctuations by a cellular automation ''ant-lion'' model and obtained bet
a = 1.8. We point out that their model describes intrinsic fluctuation
s of accretion disks and that the observed value beta = 1.8 is to be e
xpected for disks oriented pole-on with respect to the detector. For d
isks with a non-zero inclination, the observed power spectrum slope sh
ould be flatter. We have calculated variability power spectra for accr
etion disk fluctuations described by the Mineshige et al. model, and o
btained beta = 1.7, 1.6, and 1.4 for disks with inclinations i = 20-de
grees, 60-degrees, and 80-degrees respectively, in an excellent agreem
ent with observations.