SAS 3 observations of Cygnus X-1 - The intensity dips

In general, the dips are observed to occur near superior conjunctions of the X-ray source, but one pair of 2-minute dips occurs when the X-ray source is closer to the observer than is the supergiant companion. The dips are analyzed spectrally with the aid of seven energy channels in the range 1.2-50 keV. Essentially, there is no change in the spectral index during the dips. Reductions in the count rates are observed at energies exceeding 6 keV for some of the dips, but the dip amplitude is always significantly greater in the 1.2-3 keV band. It is believed that absorption by partially ionized gas may best explain these results, since the observations of Pravdo et al. (1980) rule out absorption by unionized material. Estimates for the intervening gas density, extent, and distance from the X-ray source are presented. Attention is also given to the problems confronting the models for the injection of gas through the line of sight, believed to be inclined by approximately 30 deg from the binary pole.