Gamma-ray spectra and variability of Cygnus X-1 observed by BATSE

We present new BATSE Earth occultation observations of the 25 keV-1.8 MeV spectrum and variability of Cyg X-1 made between 1993 August and 1994 May. We observed that the normal soft gamma-ray spectrum (gamma(2)) of Cyg X-1 has two components: a Comptonized part seen below 300 keV and a high-energy tail in the 0.3-2 MeV range that was only hinted at in previous gamma(2) spectrum observed by HEAO 3. The source went through an extended sequence of changes between 1993 August and 1994 May; the 45-140 keV flux first decreased steadily from similar to gamma(2) to below the gamma(1) flux level seen previously by HEAO 3 in 1979 to a new level, gamma(0), roughly one-quarter of its intensity over a period of similar to 140 days. The flux remained at this low level for about 40 days before returning swiftly (similar to 20 days) to approximately the initial gamma(2) level. The gamma(2) spectrum may be interpreted in terms of an interacting two-region model, consisting of a high-temperature (similar to 210-250 keV) core embedded in an similar to 50 keV corona. In this scenario, the observed 25-300 keV photons were produced by Compton scattering of soft photons (similar to 0.5 keV) by the hot electrons in the outer corona. These same hard X-rays were further upscattered by a population of energetic electrons in the inner core, producing the spectral tail above 300 keV. During the excursion of the 45-140 keV flux from the gamma(2) to the gamma(0) level, the spectrum evolved to a form consistent with either a power law with a photon index of similar to 2.6 or a single-temperature Compton model with an electron temperature, kT, of 108 +/- 11 keV and an optical depth, tau, of 0.40 +/- 0.06 and then returned essentially to the original gamma(2) spectrum at the end of the active period. The overall cooling of the system during the low-flux period may be due to an increase in the soft photon population that effectively quenched the hot electrons in these regions through Compton scattering.