The Variability of Cygnus X-1 at MeV Energies
M.L. McConnell, J.M. Ryan
W. Collmar, V. Schönfelder, H. Steinle, A.W. Strong
H. Bloemen, W. Hermsen, L. Kuiper
In previous work, we have used data from the first three years of the
CGRO mission to
assemble a broad-band g-ray spectrum of the
galactic black hole candidate Cygnus X-1. Contemporaneous data
from the COMPTEL, OSSE and BATSE experiments on CGRO were
further selected on the basis of the hard X-ray flux (45-140 keV)
as measured by BATSE.
The resulting spectrum provided a consistent picture of the
emission in the low energy g-ray range above 100 keV for
Cygnus X-1 in its canonical low soft X-ray state.
This spectrum can be generally described as an exponential-type spectrum
with clear evidence for emission out to at least 2 MeV and
perhaps as high as 5 MeV. Here we report on a comparison of this
spectrum to a COMPTEL-OSSE spectrum
collected during a high soft X-ray state of
Cygnus X-1 (May, 1996). This comparison results in evidence for significant
variability at energies above 1 MeV.
In particular, whereas the hard X-ray flux decreases during
the high soft X-ray state, the flux at energies above 1 MeV increases,
resulting in a significantly harder high energy spectrum.
Furthermore, the combined COMPTEL-OSSE spectrum shows much less
curvature and can be well-described by a single power-law form.
This behavior is consistent with the general picture that has emerged
over the last several years of galactic black hole
candidates having two distinct spectral forms at energies above 50 keV.
These data extend this picture, for the first time, to energies well
above 1 MeV.
File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:19.