The frequency resolved spectroscopy of Cyg X-1: fast variability of the FeK alpha line

We studied the frequency resolved energy spectra of Cyg X-1 during the stan dard low (hard) spectral state using the data of the Rossi X-Ray Timing Exp lorer. We found that the relative amplitude of the reflection features - th e iron fluorescent line at similar to 6.5 keV and the smeared edge above si milar to 7 keV - decreases with the increasing frequency. In particular we found that the equivalent width of the iron line decreases above similar to 1 Hz and drops twice in value at a frequency of - 10 Hz. 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 re flector similar to 5 x 10(8) cm, corresponding to similar to 150R(g) for a 10M. black hole. Alternatively the lack of high frequency oscillations of t he reflected component may indicate that the short time scale, similar to 5 0-100 msec, variations of the primary continuum appear in the geometrically different, likely inner, part of the accretion flow and give a rise to a s ignificantly weaker, if any, reflected emission than the longer time scale events.