Temporal and Spectral Correlations of Cygnus X‐1

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Abstract
Temporal and spectral properties of X-ray rapid variability of Cyg X-1 are studied through correlation analysis in the time domain on different timescales. The correlation coefficients between the total intensity in the 2-60 keV band and the hardness ratio of the 13-60 keV to the 2-6 keV band on the timescale of ~1 ms are always negative in all states. For soft states, the correlation coefficients are positive on all timescales from ~0.01 to ~100 s, which is significantly different 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.