21-cm Absorption Spectrum in Front of Cygnus X-3

Abstract

WE have previously presented observations1 made with the Nancay radio telescope of the 21-cm absorption spectrum in front of the highly variable radio source which is believed to be associated with Cygnus X-3 (ref. 2). This spectrum was derived from drift scans in different frequency channels. A baseline was fitted to the drift scans outside the source and subtracted from the record of the source; in a region exhibiting large variations in the 21-cm emission, like the surroundings of Cyg X-3, this procedure may involve large uncertainties due to an insufficient knowledge of the HI emission at the location of the source. Fortunately, in the case of strongly variable radio sources such as pulsars or Cyg X-3, an absorption spectrum free of these uncertainties can be derived from observations at two epochs when the fluxes of the source are widely different. For Cyg X-3, during September 1972 we obtained a few preliminary observations to compare with the initial ones and we suspected the existence of absorption around −68 km s−1 (ref. 1). We recently obtained good comparison observations after the source had become indistinguishable from the surrounding galactic background. Comparing these with the initial observations of September 4 and 5 (Fig. 1) we find three changes.