Time Domain Studies of X‐Ray Shot Noise in Cygnus X‐1

We investigate the variability of Cygnus X-1 in the context of shot moisemodels, and employ a peak detection algorithm to select individual shots. For along observation of the low, hard state, the distribution of time intervalsbetween shots is found to be consistent with a purely random process, contraryto previous claims in the literature. The detected shots are fit to severalmodel templates and found to have a broad range of shapes. The fitted shotshave a distribution of timescales from below 10 milliseconds to above 1 second.The coherence of the cross spectrum of light curves of these data in differentenergy bands is also studied. The observed high coherence implies that thetransfer function between low and high energy variability is uniform. Theuniformity of the tranfer function implies that the observed distribution ofshot widths cannot have been acquired through Compton scattering. Our resultsin combination with other results in the literature suggest that shotluminosities are correlated with one another. We discuss how our experimentalmethodology relates to non-linear models of variability.Comment: Accepted for publication in Astrophysical Journal on July 16, 200