Detection of an anticorrelated hard X-Ray time lag in Cygnus X-3

Abstract

The wide-band X-ray spectra of the high-mass X-ray binary Cyg X-3 exhibit a pivoting behavior in the "low" (as well as "hard") state, correlated to the radio emission. The timescale of the soft and hard X-rays' anticorrelation, which gave rise to the pivoting feature, was found to be less than a day from the monitoring observations by the Rossi X-Ray Timing Explorer all-sky monitor and the Compton Gamma Ray Observatory BATSE. In this Letter we report the detection of a lag of l≲1000 s in the anticorrelation of the hard X-ray emission (20-50 keV) to that of the soft X-ray emission (2-7 keV), which may be attributed to the viscous timescale of the flow of matter in the accretion disk. This suggests the geometrical picture of a truncated accretion disk with a Compton cloud inside the disk, the relative sizes of which determine the spectral shape. Any change in the disk structure will take place in a viscous timescale, with corresponding anticorrelated change in the Compton cloud. We also report the pivoting in the spectra in one span of a pointed observation when an episode of the rearranging of the accretion system is serendipitously observed. This is the first such observation of hard X-ray delay seen in the persistent Galactic microquasars, within the precincts of the hard state.