Frequency resolved spectroscopy of Cyg X-1: fast variability of the Fe K_alpha line

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 ~6.5 keV and the smeared edge above ~7 keV -- decreases with the increasing frequency. In particular we found that the equivalent width of the iron line decreases above ~1 Hz and drops twice at frequency of ~10 Hz. An 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 ~5E8 cm, corresponding to ~150 Rg for a 10Msun black hole. Alternatively lack of high frequency oscillations of the reflected component may indicate that the short time scale, ~50-100 msec, variations of the primary continuum appear in 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.