  It is shown that the energy dependence of the time lags in Cygnus X-1
excludes any significant contribution of the standard reflected component to
the observed lags. The conclusion is valid in the 0.1--10 Hz frequency range
where time lags have been detected with high enough significance. In fact the
data hint that reflected component is working in opposite direction, reducing
the lags at energies where contribution of the reflected component is
significant.
  We argue that the observed logarithmic dependence of time lags on energy can
be understood as due to the small variations of the spectrum power law index in
a very simple phenomenological model. We assume that an optically thin
flow/corona, emitting a power law like spectrum, is present at a range of
distances from the compact object. The slope of the locally emitted spectrum is
a function of distance with the hardest spectrum emitted in the innermost
region. If perturbations with different time scales are introduced to the
accretion flow at different radii the observed X-ray lags naturally appear due
to the inward propagation of perturbations on the diffusion time scales.
