X‐Ray Spectral Variability in Cygnus X‐1

Spectral variability in different energy bands of X-rays from Cyg X-1 indifferent states is studied with RXTE observations and time domain approaches.In the hard tail of energy spectrum above $\sim 10$ keV, average peak alignedshots are softer than the average steady emission and the hardness ratiodecreases when the flux increases during a shot for all states. In regard to asoft band lower $\sim 10$ keV, the hardness in the soft state varies in anopposite way: it peaks when the flux of the shot peaks. For the hard andtransition states, the hardness ratio in respect to a soft band during a shotis in general lower than that of the steady component and a sharp rise isobserved at about the shot peak. For the soft state, the correlationcoefficient between the intensity and hardness ratio in the hard tail isnegative and decreases monotonically as the timescale increases from 0.01 s to50 s, which is opposite to that in regard to a soft band. For the hard andtransition states, the correlation coefficients are in general negative andhave a trend of decrease with increasing timescale.Comment: 14 pages, 3 figures, accepted by Ap