We have analyzed energy spectra of low-frequency QPO in a number of transient X-ray binaries, around the time periods of state transitions. The QPO spectra show an interesting anti-correlation with the time averaged X-ray spectra: the QPO spectra are harder than the time averaged spectra when the latter are soft, while they are softer than the time averaged spectra when the latter are hard. We then model and interpret these results with models of spectral variability of Comptonized emission. We point out that the QPO in the soft state appear to be driven by modulations in the hot plasma, while most of the QPO models assumes that the modulations are driven by instabilities in the accretion disks. We have also considered the model for high-frequency QPO in accreting black holes, involving epicyclic motions of the accretion disk. Assuming that X-ray modulation comes solely from relativistic effects, we have constructed theoretical energy spectra of the QPO. These could be compared with good quality data.
Black Holes from Stars to Galaxies -- Across the Range of Masses
- Pub Date:
- April 2007
- 4 pages, Proc. of IAU Symposium 238, "Black Holes from Stars to Galaxies - across the range of masses", Prague, Aug 2006