The Cessation of Flickering during Dips in Cygnus X-1

We report the discovery of the cessation of flickering in dips in the black hole candidate Cygnus X-1, detected for the first time in the ASCA observation of 1995 May 9. During this observation, particularly deep dipping took place, which resulted in strong changes in hardness ratio corresponding to the absorption of the power-law spectral component. The dead time corrected light curve with high time resolution clearly shows a dramatic decrease in the extent of flickering in the 0.7-4.0 keV band during dipping, but in the 4.0-10.0 keV band, there is relatively little change. We show that the rms flickering amplitude in the 0.7-4.0 keV band is proportional to the X-ray intensity in this band, which changes by a factor of almost 3. This is direct evidence that the strong low-state flickering is intrinsic to the power-law emission; i.e., it takes place as part of the emission process. The rms amplitude is proportional to the intensity in the low-energy band, except for a possible deviation from linearity at the lower intensities. If confirmed, this nonlinearity could imply a process such as electron scattering of radiation, which will tend to smear out the fluctuations, or a process of fluctuation generation which depends on radial position in the source. Thus, timing observations during absorption dips can give information about the source region and may place constraints on its size.