  We present a timing analysis of threeRossi X-ray Timing Explorer observations
of the black hole binary Cygnus X-1 with the propagating mass accretion rate
fluctuations model PROPFLUC. The model simultaneously predicts power spectra,
time lags, and coherence of the variability as a function of energy. The
observations cover the soft and the hard state of the source, and the
transition between the two. We find good agreement between model pre-dictions
and data in the hard and in the soft state. Our analysis suggests that in the
soft state the fluctuations propagate in an optically thin hot flow extending
up to large radii above and below a stable optically thick disc. In the hard
state, our results are consistent with a truncated disc geometry, where the hot
flow extends radially inside the inner radius of the disc. In the transition
from soft to hard state, the characteristics of the rapid variability are too
complex to be successfully described with PROPFLUC. The surface density profile
of the hot flow predicted by our model and the lack of QPOs in the soft and
hard state, suggest that the spin of the black hole is aligned with the inner
accretion disc and therefore probably with the rotational axis of the binary
system.
