1999A&A...347L..23R

We studied the frequency resolved energy spectra of Cyg X-1 during the standard low (hard) spectral state using the data of the Rossi X-Ray Timing Explorer. We found that the relative amplitude of the reflection features - the iron fluorescent line at ∼6.5keV and the smeared edge above ∼7keV - decreases with the increasing frequency. In particular we found that the equivalent width of the iron line decreases above ∼1Hz and drops twice in value at a frequency of ∼10Hz. The assumption that such behavior is solely due to a finite light crossing time of the reflecting media, would imply the characteristic size of the reflector ∼ 5x10⁸cm, corresponding to ∼150R_g for a 10M_☉ black hole. Alternatively the lack of high frequency oscillations of the reflected component may indicate that the short time scale, ∼50-100msec, variations of the primary continuum appear in the geometrically different, likely inner, part of the accretion flow and give a rise to a significantly weaker, if any, reflected emission than the longer time scale events.