The Cessation of Flickering during Dips in Cygnus X-1

We report the discovery of the cessation of flickering in dips in the blackhole candidate Cygnus X-1, detected for the first time in the ASCA observationof May 9th., 1995. During this observation, particularly deep dipping tookplace resulting in strong changes in hardness ratio corresponding to absorptionof the power law spectral component. The deadtime corrected light curve withhigh time resolution clearly shows a dramatic decrease in the extent offlickering in the band 0.7 - 4.0 keV during dipping, but in the band 4.0 - 10.0keV, there is relatively little change. We show that the rms flickeringamplitude in the band 0.7 - 4.0 keV is proportional to the X-ray intensity inthis band which changes by a factor of almost three. This is direct evidencethat the strong Low State flickering is intrinsic to the power law emission; ietakes place as part of the emission process. The rms amplitude is proportionalto the intensity in the low energy band, except for a possible deviation fromlinearity at the lower intensities. If confirmed, this non-linearity couldimply a process such as electron scattering of radiation which will tend tosmear out the fluctuations, or a process of fluctuation generation whichdepends on radial position in the source. Thus timing observations duringabsorption dips can give information about the source region and may placeconstraints on its size.Comment: 6 pages including 4 figures, accepted for publication in Astrophysical Journal Letter