The Compton Telescope (COMPTEL) experiment on the Compton Gamma-Ray Observatory (CGRO) has observed the Cygnus region on two occasions during the first (sky survey) phase of its mission. These data represent the most sensitive observations to date of Cygnus X-1 in the 0.75-30 MeV range. The observations in 1991 June and August both showed evidence for emission in the 0.75-2 MeV energy range. The flux level was larger by about a factor of 2 during the August observation. The spectral data were analyzed in the context of a Wien spectral model (the high-energy limit of the Sunyaev-Titarchuk Comptonization spectrum). Fits to this model gave electron plasma temperature (kT) values of 192 +/- 27 keV and 204 +/- 21 keV for the June and August data, respectively. These values are much higher than those typically derived from hard X-ray data, suggesting that an alternative to the single-temperature Comptonization model may be required to explain the entire spectrum. The present data may be explained by resolving the inadequacies of the standard Comptonization model which is used to fit the X-ray data and/or incorporating a spectral component which represents the reflection of hard X-rays from an optically thick accretion disk. Finally, there is no evidence in these data for any substantial hardening of the spectrum above 1 MeV (the so-called 'MeV excess'), such as that suggested in the past by several balloon and satellite observations.
American Astronomical Society
Digital Object Identifier (DOI)
M. McConnell, D. Forrest, J. Ryan, W. Collmar, V. Schoenfelder, H. Steinle, A. Strong, R. van Dijk, W. Hermsen, and K. Bennett, ‘Observations of Cygnus X-1 COMPTEL during 1991’, The Astrophysical Journal, vol. 424, p. 933, Mar. 1994.