Variability in the noise properties of Cygnus X-1.

Abstract

The aperiodic variability in the X-ray emission of Cyg X-1 is examined from 13 Exosat observations covering the period July 1983 - October 1985, by means of broad band 0.001-100 H3 power spectra. The total rms fractional variations in the range 0.016-14 Hz is between 28 and 38 percent. The power distribution is flat below a characteristic break frequency nu b, follows a power law with slope about 1 up to about 1 Hz, and then steepens to a slope of roughly -2. Some wiggles in the power spectra are also observed. The break frequency is observed to vary in the range 0.04-0.4, showing no correlation with the source flux. The results can be interpreted with a shot noise model with a distribution in shot times, in which the presence of long living shots determines the position of the break frequency nu b.