A High Sensitivity Measurement of the MeV Gamma-Ray Spectrum of Cygnus X-1

The Compton Gamma-Ray Observatory (CGRO) has observed the Cygnus region on several ocassions since its launch in 1991. The data collected by the COMPTEL experiment on CGRO represent the most sensitive observations to date of Cygnus X-1 in the 0.75-30 MeV range. A spectrum accumulated by COMPTEL over 10 weeks of observation time shows significant evidence for emission extending out to several MeV. We have combined these data with contemporaneous data from both BATSE and OSSE to produce a broad-band $\gamma$-ray spectrum, corresponding to the low X-ray state of Cygnus X-1, extending from 50 keV up to $\sim$5 MeV. Although there is no evidence for any broad line-like emissions in the MeV region, these data further confirm the presence of a hard tail at energies above several hundred keV. In particular, the spectrum at energies above 650 keV can be described as a power-law with a photon spectral index of $\alpha$ = -3.2, with no evidence for a cutoff at high energies. The spectral data presented here should provide a useful basis for further theoretical modeling.