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L Titarchuk (George Mason University/ Naval Research Laboratory/GSFC), R. Fiorito (University of Maryland and GSFC)
Recent studies have shown that strong correlations are observed between the low frequencies (1-10 Hz) of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources, in low hard state, steep power-law (soft) state and in transition between these states. Strong QPOs (> 20% rms) are present in the power density spectrum in the spectral range where the power-law component is dominant ( i.e. 60-90 %). This evidence contradicts the dominant long standing interpretation of QPOs as a signature of the thermal accretion disk. We present the data from the literature and our own data to illustrate the dominance of power-law index-QPO frequency correlations. We provide a model, that identifies and explains the origin of the QPOs and how they are imprinted on the properties of power-law flux component. We argue the existence of a bounded compact coronal region which is a natural consequence of the adjustment of Keplerian disk flow to the innermost sub-Keplerian boundary conditions near the central object and that ultimately leads to the formation of a transition layer (TL) between the adjustment radius and the innermost boundary. The model predicts two phases or states dictated by the photon upscattering produced in the TL: (1) hard state, in which the TL is optically thin and very hot (kT~ 50 keV) producing photon upscattering via thermal Componization; the photon spectrum index \Gamma~ 1.7 for this state is dictated by gravitational energy release and Compton cooling in an optically thin shock near the adjustment radius; (2) a soft state which is optically thick and relatively cold ( kT ~ 5 keV); the index for this state, \Gamma~ 2.8 is determined by soft-photon upscattering and photon trapping in converging flow into BH. In the TL model for corona the QPO frequency \nuhigh is related to the gravitational (close to Keplerian) frequency \nu\rm{K} at the outer (adjustment) radius and \nulow is related to the TL's normal mode (magnetoacoustic) oscillation frequency \nuMA. The observed correlations between index and low and high QPO frequencies are readily explained in terms of this model. We also suggest a new method for evaluation of the BH mass using the index-frequency correlation.
The author(s) of this abstract have provided an email address for comments about the abstract: ltitarchuk@ssd5.nrl.navy.mil
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.