Copernicus observations of variations in the x-ray flux from Cygnus X-1

The Mullard Space Science Laboratory X-ray instrumentation on board Copernicus has observed Cyg X-1 for a continuous period of 6.5 days from 1973 November 5 to November 11. A variation in flux level was observed on a time scale of 5.6 days, the period of the optical candidate. The peak of the X-ray flux corresponds to the tirne of closest approach to the observer of the secondary component of the optical candidate, HDE 226868, as calculated from optical radial-velocity measurements. This observation greatly strengthens the identification of HDE 226868 with the X-ray source. Subject headings: binaries - X-ray sources