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D. Kazanas (NASA/GSFC), X. M. Hua (Raytheon Systems)
Timing observations of accreting Black Hole Candidate (BHC) sources are modeled and tested against the predictions of Compton scattering, the process of choice for the production of their observed high energy spectra. It is found that the time lags, \Delta t, between the soft (5-13 keV) and hard (13-30 keV) energy bands, observed to increase over a wide range in the Fourier period, P, are not consistent with the assumption that the high energy emission takes place in a region restricted to the black hole vicinity. It is proposed that these observations can be accounted for if the Comptonization region extends over several decades in radius. It is furthermore found that the dependence of the lags \Delta t on the Fourier period P has an one-to-one relation to the density profile of the scattering medium, which can thus be mapped through the lag measurements. The observations of galactic BHC sources seem to favor a profile of denisty n(r) \propto r-1, incommensurate with those of the simplest accretion models, though on occasions the density profile is consistent with the form n(r) \propto r-3/2 favored by ADAF.
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