Long-term gamma-ray spectral variability of Cygnus X-1

The JPL HEAO 3 Gamma-Ray Spectroscopy experiment monitored the soft gamma-ray (0.05 - 10 MeV) emission of Cygnus X-1 for ≡90 days each in the fall of 1979 and spring of 1980. The source exhibited a steady increase of the 100 keV emission, by a factor of ≡3, during the first 70 days of observations. The spectrum obtained during the first 14 days of observations showed strong γ-ray emission in the 0.4 - 1.5 MeV band. As the 100 keV flux increased, the MeV flux disappeared, while the overall spectrum softened. The implications of these variations for the physical processes in the accretion disk are discussed.