Gamma‐Ray Spectra and Variability of Cygnus X‐1 Observed by BATSE

Abstract

We present new BATSE Earth occultation observations of the 25 keV1.8 MeV spectrum and variability of Cyg X-1 made between 1993 August and 1994 May. We observed that the normal soft c-ray spectrum of Cyg X-1 has two components : a Comptonized part seen below 300 keV and a high-(c 2 ) energy tail in the 0.32 MeV range that was only hinted at in previous spectrum observed by HEAO c 2 3. The source went through an extended sequence of changes between 1993 August and 1994 May ; the 45140 keV ux rst decreased steadily from to below the ux level seen previously by HEAO 3 Dc 2 c 1 in 1979 to a new level, roughly one-quarter of its intensity over a period of D140 days. The ux c 0 , remained at this low level for about 40 days before returning swiftly (D20 days) to approximately the initial level. The spectrum may be interpreted in terms of an interacting two-region model, consist-c 2 c 2 ing of a high-temperature (D210250 keV) core embedded in an D50 keV corona. In this scenario, the observed 25300 keV photons were produced by Compton scattering of soft photons (D0.5 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 45140 keV ux from the to the level, the spectrum evolved to a form consistent with either a c 2 c 0 power law with a photon index of D2.6 or a single-temperature Compton model with an electron temperature, kT , of 108 ^11 keV and an optical depth, q, of 0.40 ^0.06 and then returned essentially to the original spectrum at the end of the active period. The overall cooling of the system during the low-c 2 ux period may be due to an increase in the soft photon population that e ectively quenched the hot electrons in these regions through Compton scattering.