The spectral variability of Cygnus X-1 at MeV energies

McConnell, M. L.; Bennett, K.; Bloemen, H.; Collmar, W.; Hermsen, W.; Kuiper, L.; Phlips, B.; Ryan, J. M.; Scho¨nfelder, V.; Steinle, H.; Strong, A. W.
April 2000
AIP Conference Proceedings;2000, Vol. 510 Issue 1, p114
Academic Journal
In previous work, we have used data from the first three years of the CGRO mission to assemble a broad-band γ-ray spectrum of the galactic black hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and BATSE experiments on CGRO were selected on the basis of the hard X-ray flux (45–140 keV) as measured by BATSE. This provided a spectrum of Cygnus X-1 in its canonical low X-ray state (as measured at energies below 10 keV), covering the energy range from 50 keV to 5 MeV. Here we report on a comparison of this spectrum to a COMPTELOSSE spectrum collected during a high X-ray state of Cygnus X-1 (May, 1996). These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the high X-ray state, the flux at energies above 1 MeV increases, resulting in a significantly harder high energy spectrum. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft γ-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. © 2000 American Institute of Physics.


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