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 OSSE 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. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we have measured a new minimum in the hard X-ray flux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line, previously reported in association with a low flux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6 day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5% in the 60-140 keV energy band.