1996A&AS..120C.155P

We report on ∼120-days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50keV to 1MeV, and we find evidence for a continuum of hard X-ray spectra over a broad range of fluxes 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 1MeV feature or a narrow 511keV 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 60-140keV energy band.