  We analyzed RXTE/PCA observations of Cyg X-1 from 1996-1998. We found a tight
correlation between the characteristic noise frequencies (e.g. the break
frequency nu_br) and the spectral parameters in the low spectral state. The
amplitude of reflection increases and the spectrum of primary radiation
steepens as the noise frequency increases (nu_br changes by a factor of ~15 in
our sample). This can be understood assuming that increase of the noise
frequency is associated with the shift of the inner boundary of the optically
thick accretion disk towards the compact object. The related increase of the
solid angle, subtended by the disk, and of the influx of the soft photons to
the Comptonization region lead to an increase of the amount of reflection and
steepening of the Comptonized spectrum. The correlation between the slope of
primary radiation and the amplitude of reflection extends to the soft spectral
state. A similar correlation between reflection and slope was found for the
frequency resolved spectra in the 0.01-15 Hz frequency range. Such a
correlation could appear if the longer time scale variations are associated
with emission originating closer to the optically thick disk and, therefore,
having steeper Comptonized spectra with larger reflection.
