BATSE observations of Cygnus X-1

We report preliminary results of ∼65 days of BATSE Earth‐occultation observations of the low energy gamma‐ray (20 keV–1.8 MeV) emission of Cygnus X‐1 covering the period from 23 April to 27 June 1991. Cygnus X‐1 underwent two transitions in this period. The 45–140 keV flux was first observed between the γ1 and γ2 levels (Ling et al. 1987) from 23 April to 14 May. The source began a transition to γ1 on 14 May and stayed near this level from 19 to 29 May, and then moved toward γ2 and remained at approximately this level from 10 to 27 June. Both γ1 (19–29 May) and γ2 (10–27 June) spectra observed by BATSE can be best characterized by the Comptonized model with temperature of 67 keV and 62 keV, respectively, and optical depth of ∼2. There was no evidence for any broad spectral feature near 1 MeV seen previously by several satellite and balloon experiments. A composite Cygnus X‐1 spectrum observed by BATSE and COMPTEL in Viewing Period 2 (31 May–7 June) suggests that a power‐law with index of ∼3.5 may best characterize the spectrum from 200 keV to several MeV.