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Session 10 - Studies of Black Holes.
Display session, Monday, June 09
South Main Hall,

[10.06] The MeV \gamma-Ray Spectrum of Cygnus X-1

M. McConnell, J. Ryan (UNH), W. Collmar, V. Schönfelder, H. Steinle, A. Strong (MPE), H. Bloemen, W. Hermsen, L. Kuiper (SRON-Utrecht), K. Bennett, R. van Dijk, R. Much (SSD-ESA)

Measurements of the \gamma-ray spectrum of Cygnus X-1 can provide useful information on the extent to which non-thermal processes may be operating within the accretion flow. The COMPTEL experiment on the Compton Gamma-Ray Observatory (CGRO) has made several observations of Cygnus X-1 since its launch in April 1991. These data provide the most sensitive observations ever made of Cygnus X-1 in the 0.75--30 MeV energy band. Here, we shall summarize the latest findings from an analysis of these data. The time-averaged COMPTEL spectrum clearly indicates evidence for emission extending up to several MeV. There is no evidence for any Wien-type feature such as that previously reported from the HEAO-3 data. However, the presence of emission up to several MeV suggests, in itself, that standard thermal Comptonization models may be insufficient in explaining the broad-band spectrum. A comparison of these data has been made with contemporaneous OSSE and BATSE data. We review and discuss the implications of these comparisons.

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