  RXTE/PCA observations of several black hole X-ray binaries in the low
spectral state revealed a tight correlation between spectral parameters and
characteristic frequencies of variability of X-ray flux. In particular, the
amplitude of reflection increases and the slope of Comptonized radiation
steepens as the noise frequencies increase. The data also suggest that there is
a correlated increase of the width of the Fe fluorescent line, probably related
to the Doppler broadening of the reflection features. Moreover, the width of
the line seems to follow DeltaE/E ~ nu_qpo^{1/3} law. If confirmed with higher
energy resolution observations, this result will have significant impact on the
models of the accretion flow and on our understanding of the nature of
characteristic frequencies of variability in X-ray binaries. In particular, it
will lend support to the truncated disk picture, confirming that the spectral
variations are indeed related to changes of the position of the inner disk
boundary and that characteristic variability frequencies are proportional to
the Keplerian frequency at the inner boundary of the accretion disk.
