  We report the results from our timing analysis of 15 RXTE observations of
Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be
divided into 3 distinct phases: (1) transition from the hard to soft state, (2)
soft state, and (3) transition from the soft state back to the hard state. The
observed X-ray properties in phases 1 and 3 are remarkably similar, suggesting
that the same physical processes are likely involved in triggering such
transitions. The power density spectrum (PDS) during the transition can be
characterized by a red noise component, followed by a white noise component
which extends to roughly 1-3 Hz where it is cut off, and a steeper power law at
higher frequencies. The X-ray flux also exhibits apparent quasi-periodic
oscillation (QPO) with the centroid frequency varying in the range of 4-12 Hz.
The QPO shows no correlation with the source flux, but becomes more prominent
at higher energies. This type of PDS bears resemblance to that of other black
hole candidates often observed in a so-called very high state, although the
origin of the observed QPO may be very different. The low-frequency red noise
has not been observed in the hard state, thus seems to be positively correlated
with the disk mass accretion rate which is presumably low in the hard state and
high in the soft state; in fact, it completely dominates the PDS in the soft
state. In the framework of thermalComptonization models, Cui et al. (see
astro-ph/9610071 and astro-ph/9610072) speculated that the difference in the
observed spectral and timing properties between the hard and soft states is due
to the presence of a ``fluctuating'' Comptonizing corona during the transition.
Here we present the measured hard X-ray time lags and coherence functions
between various energy bands, and show that the results strongly support such a
scenario.
