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P. J. Wiita, G. Bao (Georgia St. U.)
Our models assume that the innermost part of an accretion disk around a black hole is unstable and clumpy and that it is illuminated by X-ray sources originating above it in a jet or disk corona. We study the X-ray spectral and temporal variability induced both by a clump of matter moving relativistically around the black hole and by the non-thermal source moving above the disk. One interesting new result is that even for a face-on geometry, rotation induced variability can still be observed as long as the non-thermal source is not exactly at the symmetry axis; this is true even if the accretion disk is smooth. We also show that the reflected X-ray component, peaked around ten or several tens of keV, can vary more than other spectral components, such as the thermal emission from the disk and the external power-law flux, and that it carries richer information on the non-thermal sources. This is because the reflected component experiences a double path through the gravitational field of the central mass and is twice Doppler shifted and gravitationally lensed. Thus our study is most relevant to X-ray variability of active galaxies and galactic black hole candidates. These results may provide a partial explanation of why some Seyfert 1 galaxies exhibit rapid X-ray variability with timescales comparable to the fastest orbital periods, and why soft X-rays and UV emission generally show slower variability than do the hard X-rays.
This work was supported in part by NASA grant NAG 5-3098 and RPE funds at GSU.
The author(s) of this abstract have provided an email address for comments about the abstract: wiita@chara.gsu.edu