  We have performed a timing and spectral analysis of a set of black-hole
binaries to study the correlation between the photon index and the time lag of
the hard photons with respect to the soft ones. We provide further evidence
that the timing and spectral properties in black-hole X-ray binaries are
coupled. In particular, we find that the average time lag increases as the
X-ray emission becomes softer. Although a correlation between the hardness of
the X-ray spectrum and the time (or phase) lag has been reported in the past
for a handful of systems, our study confirms that this correlated behaviour is
a global property of black-hole X-ray binaries. We also demonstrate that the
photon-index - time-lag correlation can be explained as a result of inverse
Comptonization in a jet.
