Detection of an Anticorrelated Hard X-Ray Time Lag in Cygnus X-3

The wide-band X-ray spectra of the high mass X-ray binary Cygnus X-3 exhibitsa pivoting behavior in the `low' (as well as `hard') state, correlated to theradio emission. The time scale of the soft and hard X-rays' anti-correlation,which gave rise to the pivoting feature, was found to be less than a day fromthe monitoring observations by RXTE--ASM and CGRO--BATSE. In this Letter wereport the detection of a lag of $\lesssim$ 1000s in the anti-correlation ofthe hard X-ray emission (20--50 keV) to that of the soft X-ray emission (2--7keV), which may be attributed to the viscous time scale of flow of matter inthe accretion disk. This suggests the geometrical picture of a truncatedaccretion disc with a Compton cloud inside the disc, the relative sizes ofwhich determine the spectral shape. Any change in the disc structure will takeplace in a viscous time scale, with corresponding anti-correlated change in theCompton cloud. We also report the pivoting in the spectra in one span of apointed observation when an episode of the rearranging of the accretion systemis serendipitously observed. This is the first such observation of hard X-raydelay seen in the persistent Galactic microquasars, within the precincts of thehard state.Comment: Accepted in The Astrophysical Journal (Letters): in pres