X-ray shots of Cyg X-1 in di †erent energy bands and spectral states have been studied with proportional counter array (PCA)/Rossi X-Ray T iming Explorer (RXT E) observations. The detailed shot structure is obtained by superposing many shots with 1 ms time bin through aligning their peaks with an improved algorithm. In general, the shots are composed of a slow rise and fast decay. The shot structures in the di †erent states are di †erent. The duration of a shot in the high state is shorter than that in the low and transition states. The shot proÐle in the high-energy band is more asymmetric and narrower than that in the low-energy band. The average hardness of a shot is lower than that of steady emission in the transition and low states but higher than that in the high state. The time lags between the shots in higher and lower energy bands have been found in the di †erent states. In transition states, the time lag is largest among the di †erent states of Cyg X-1, and it is smallest in the low state. The implications of the observed shot features for shot models are discussed.

No abstract

Temporal and spectral properties of X-ray rapid variability of Cyg X-1 are studied through correlation analysis in the time domain on di †erent timescales. The correlation coefficients between the total intensity in the 2È60 keV band and the hardness ratio of the 13È60 keV to the 2È6 keV band on the timescale of D1 ms are always negative in all states. For soft states, the correlation coefficients are positive on all timescales from D0.01 to D100 s, which is signiÐcantly di †erent from those for transition and low states. The temporal structures in the high-energy band are narrower than those 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 implications of the observed temporal and spectral characteristics for the production region and mechanism of Cyg X-1 X-ray variations are discussed.