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

Using RXTE/PCA data, we study the fast variability of the reflected emissio n in the soft spectral state of Cyg X-1 by means of Fourier frequency-resol ved spectroscopy. We find that the rms amplitude of variations of the refle cted emission has the same frequency dependence as the primary radiation do wn to time-scales of less than or similar to 30-50 ms. This might indicate that the reflected flux reproduces, with nearly flat response, variations o f the primary emission. Such behaviour differs notably from that of the har d spectral state, in which variations of the reflected flux are significant ly suppressed in comparison with the primary emission, on time-scales short er than similar to 0.5-1 s. If related to the finite light-crossing time of the reflector, these result s suggest that the characteristic size of the reflector, presumably an opti cally thick accretion disc, in the hard spectral state is larger by a facto r of greater than or similar to 5-10 than in the soft spectral state. Model ling the transfer function of the disc, we estimate the inner radius of the accretion disc to be R(in)similar to 100R(g) in the hard state and R(in)le ss than or similar to 10R(g) in the soft state for a 10-M. black hole.