  Temporal and spectral properties of X-ray rapid variability of Cyg X-1 are
studied by an approach of correlation analysis in the time domain on different
time scales. The correlation coefficients between the total intensity in 2-60
keV and the hardness ratio of 13-60 keV to 2-6 keV band on the time scale of
about 1 ms are always negative in all states. For soft states, the correlation
coefficients are positive on all the time scales from about 0.01 s to 100 s,
which is significantly different with that for transition and low states.
Temporal structures in high energy band are narrower than that in low energy
band in quite a few cases. The delay of high energy photons relative to low
energy ones in the X-ray variations has also been revealed by the correlation
analysis. The implication of observed temporal and spectral characteristics to
the production region and mechanism of Cyg X-1 X-ray variations is discussed.
