The Spectral Variability of Cygnus X-1 at MeV Energies

Abstract

In previous work, we have used data from the first three years of the CGRO mission to assemble a broad-band $\gamma$-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 COMPTEL-OSSE 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 {\it decreases} during the high X-ray state, the flux at energies above 1 MeV {\it 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 $\gamma$-ray energies. These data extend this picture, for the first time, to energies above 1 MeV.