GAMMA-RAY OBSERVATIONS OF CYGNUS X-1 WITH THE ORIENTED SCINTILLATION SPECTROMETER EXPERIMENT

We report on similar to 120 days of observations of Cygnus X-1 with OS SE on board the Compton Gamma Ray Observatory. Emission is detected in the range 50 keV-1 MeV, and we find evidence for a continuum of hard X-ray flux levels rather than the existence of distinct flux states. C omparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average s pectrum in the OSSE energy band. Although we have measured a new minim um in the hard X-ray flux from the source, no evidence was found for e ither a broad 1 MeV feature or a narrow 511 keV line, previously repor ted in association with a low flux state. Upper limits on such emissio n features are an order of magnitude lower than earlier reported detec tions. The 5.6 day periodicity of the source measured at optical wavel engths was not detected with a sensitivity to the rms modulation fract ion of 5% in the 60-140 keV energy band.