X‐Ray Spectral Variability in Cygnus X‐1

Abstract

Spectral variability in different energy bands of X-rays from Cyg X-1 in different states is studied with Rossi X-Ray Timing Explorer (RXTE) observations and time-domain approaches. In the hard tail of the energy spectrum above ~10 keV, average peak-aligned shots are softer than the average steady emission and the hardness ratio decreases when the flux increases during a shot for all states. With regard to a soft band below ~10 keV, the hardness in the soft state varies in an opposite manner: it peaks when the flux of the shot peaks. For the hard and transition states, the hardness ratio with respect to a soft band during a shot is in general lower than that of the steady component, and a sharp rise is observed at about the shot peak. For the soft state, the correlation coefficient between the intensity and hardness ratio in the hard tail is negative and decreases monotonically as the timescale increases from 0.01 to 50 s, which is opposite to the behavior in a soft band. For the hard and transition states, the correlation coefficients are in general negative and have a trend of decreasing with increasing timescale.