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

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

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...484..375l&link_type=abstract

Astrophysical Journal v.484, p.375

Astronomy and Astrophysics

Astronomy

38

Gamma Rays: Observations, Radiation Mechanisms: Nonthermal, Stars: Individual Constellation Name: Cygnus X-1

Scientific paper

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 ~ 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 ~140 days. The flux remained at this low level for about 40 days before returning swiftly (~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 (~210--250 keV) core embedded in an ~50 keV corona. In this scenario, the observed 25--300 keV photons were produced by Compton scattering of soft photons (~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 ~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.

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