Observation of a Correlated X-Ray Transition in Cygnus X-1

Analysis of 16 months of Uhuru data on Cyg X-1 has shown a remarkable transition in the source which occurred during 1971 March and April. The average X-ray intensity in the 2-6-keV energy range decreased by about a factor of 4, the average X-ray intensity in the -keV band increased by a factor of 2, and a weak radio source suddenly appeared. This simultaneous X-ray and radio behavior provides strong evidence for the identification of the radio source with Cyg X-1. Uhuru also monitored Cyg X-1 for 35 consecutive days during 1971 December and 1972 January. The data were analyzed for an effect due to a binary system. Although large-scale fluctuations were present, no periodicity was found.