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L. Titarchuk (GMU/NRL/GSFC), C. Shrader (GSFC/USRA), K.S. Wood (NRL), M. Kafatos (GMU)
We present theoretical predictions for the converging inflow model, which is supported by realistic hydrodynamic modeling, and compare them with X-ray observations of the high- soft state of the black hole candidate transients. It is well known that the high-soft state spectra of black hole candidates consist of two distinct components: a blackbody like component and an extended power law. We argue that an extended hard tail of the high-soft state spectra with the specific photon index 2.5--2.7 and with a high energy cutoff around 300--400 keV is a real signature of the converging flow into a black hole. The position of the high energy cutoff and the photon index value are closely related to the curved geometry of the black hole background (photon bending and the gravitational redshift effect). We show that a ~200 Hz QPO phenomenon is detected in the very high state when the bolometric luminosity surges and the hard tail contribution in the spectrum increases significantly with respect to the black body like component. We interpret this QPO phenomenon, which is seen only in the power law hard tail, as oscillations of the innermost part of the accretion disk. The inner disk supplies the seed photons for the converging part of the accretion flow where the hard tail is formed because of Bulk Motion Comptonization. We predict that the same QPO effect, related to the power law hard tail, would be detected in AGN as well but with frequency at least seven orders of magnitude lower, namely of order of 10-5 Hz.
Support at GMU for this research is provided by the Center for Earth Observation and Space Research. X-ray Astronomy basic research at the Naval Research Laboratory is supported by NRL/ONR.
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