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M.L. McConnell, J.M. Ryan (UNH), W. Collmar, V. Schönfelder, H. Steinle, A.W. Strong (MPE), H. Bloemen, W. Hermsen, L. Kuiper (SRON-Utrecht), K. Bennett (SSD-ESA), W. Paciesas (UAH), B. Phlips (NRL), J. Poutanen (Stockholm Observ.), A.A. Zdziarski (Copernicus Astronomical Ctr.)
The time-dependent behavior of Cygnus X-1 at energies below several hundred keV has been extensively observed. At X-ray energies, it is highly variable on time scales ranging from msecs to months. On long time scales, the emission varies between the low ('hard') X-ray state and the high ('soft') X-ray state. The low X-ray state spectrum can be described as a power-law at soft X-ray energies followed by an exponential cutoff at higher energies with an e-folding energy of ~100 keV. The high X-ray state spectrum can be described as a blackbody at soft X-ray energies followed by a power-law extending up to at least several hundred keV. At higher energies, near 1 MeV and above, relatively little is known about the time variations of the emission. We have used observations from CGRO to study the variation in the MeV emission between the low and high X-ray states. These data, in particular those from COMPTEL, provide a measurement of the spectrum above 1 MeV. The high state MeV spectrum is found to be much harder than that of the low state MeV spectrum. In particular, the power-law emission seen in the high state spectrum (with a photon spectral index of 2.5) is found to extend out to at least 5 MeV with no evidence for any cutoff. The extension of the powerlaw spectrum to such high energies is inconsistent with models based on bulk motion Comptonization. Here we shall present the data and describe the results from modeling both the low state and high state spectra using a hybrid thermal/nonthermal model in which the emission results from the Comptonization of an electron population that consists of both a thermal and nonthermal component.
The author(s) of this abstract have provided an email address for comments about the abstract: Mark.McConnell@unh.edu