Frequency resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state

Using the RXTE/PCA data we study the fast variability of the reflected emission in the soft spectral state of Cyg X-1 by means of Fourier frequency resolved spectroscopy. We find that the rms amplitude of variations of the reflected emission has the same frequency dependence as the primary radiation down to time scales of <30-50 msec. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behavior differs notably from the hard spectral state, in which variations of the reflected flux are significantly suppressed in comparison with the primary emission, on time scales shorter than ~0.5-1 sec. If related to the finite light crossing time of the reflector, these results suggest that the characteristic size of the reflector -- presumably an optically thick accretion disk, in the hard spectral state is larger by a factor of >5-10 than in the soft spectral state. Modeling the transfer function of the disk, we estimate the inner radius of the accretion disk R_in~100R_g in the hard and R_in<10R_g in the soft state for a 10M_sun black hole.