X-Ray Fluctuations from Black-Hole Objects and Self Organization of Accretion Disks

Details X-Ray Fluctuations from Black-Hole Objects and Self Organization of Accretion Disks X-Ray Fluctuations from Black-Hole Objects and Self Organization of Accretion Disks

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995pasj...47..617t&link_type=abstract

Publications of the Astronomical Society of Japan, v.47, p.617-627.

Physics

39

Accretion, Accretion Disks, Black Hole Physics, Galaxies: Active, Galaxies: Nuclei

Scientific paper

Aperiodic fluctuations with 1/f-like power spectral density (PSD, where f is a frequency) are characteristics of X-ray radiation from accreting compact objects, such as active galactic nuclei and Cyg X-1. To account for such intriguing features, we previously proposed a cellular-automaton model for accretion disks based on the concept of the self-organized 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-like PSD, it has become clear that the X-ray observations yield an exponential 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 regardless of the critical condition, finding a good agreement between the new model and the observations. There is a strong correlation (but no 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 takes place two-dimensionally (via disk accretion) or three-dimensionally (via spherical accretion). The structure of accretion disks producing 1/f-like X-ray fluctuations is discussed in relation to the model of advection-dominated accretion disks.

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