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 occasions since its launch in 1991. The data collected by the COMPT EL 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 exten ding out to several MeV. We have combined these data with contemporaneous d ata from both BATSE and OSSE to produce a broadband gamma -ray spectrum, co rresponding to the low X-ray state of Cygnus X-1, extending from 50 keV up to similar to5 MeV. Although there is no evidence for any broad-line-like e missions in the MeV region, these data further confirm the presence of a ha rd tail at energies above several hundred keV. In particular, the spectrum at MeV energies can be described as a power law with a photon spectral inde x of alpha = -3.2, with no evidence for a cutoff at high energies. For the 200 keV-5 MeV spectrum, we provide a quantitative description of the underl ying electron spectrum, in the context of a hybrid thermal/nonthermal model for the emission. The electron spectrum can be described by a thermal Maxw ellian with a temperature of kT(e) = 86 keV and a nonthermal power-law comp onent with a spectral index of p(e) = 4.5. The spectral data presented here should provide a useful basis for further theoretical modeling.