GAMMA-RAY OBSERVATIONS OF CYGNUS X-1 WITH OSSE

We report on similar to 120 days of observations of Cygnus X-1 with OS SE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV, and we find evidence for a continuum of hard X-ray sp ectra over a broad range of fluxes rather than the existence of distin ct flux states. Comparisons of the source spectra with various theoret ical models show that an exponentially truncated power law best descri bes the average spectrum in the OSSE energy band. Although we have mea sured a new minimum in the hard X-ray flux from the source, no evidenc e 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 earlie r 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.