Observations of galactic BHCs made by the Compton GRO in the hard X-ray and
gamma-ray bands have significantly enhanced our knowledge of the emission
properties of these objects. Understanding these observations presents a
formidable challenge to theoretical models of the accretion flow onto the
compact object and of the physical mechanisms that generate high-energy
radiation. Here we summarize the current state of observations and theoretical
interpretation of the emission from BHCs above 20 keV.
The all-sky monitoring capability of BATSE allows nearly continuous studies
of the high-energy emission from more than a dozen BHCs. These long-term
datasets are particularly well-suited to multi-wavelength studies. Energy
spectral evolution and/or state transitions have been observed from many of the
BHCs.
Observations above 50 keV from OSSE demonstrate the existence of two gamma
ray spectral states that appear to be the extensions of the X-ray low/hard and
high/soft (or perhaps very high) states. The former state, the "breaking"
state, cuts off with e-folding energy ~100 keV and has its peak luminosity near
this energy. In contrast, the latter state has luminosity peaking in the soft
X-rays and an unbroken power law spectrum, even up to energies above 500 keV in
some cases.
The breaking gamma-ray spectrum can be well modeled by Comptonization of soft
photons in a hot thermal plasma. The power-law state creates more significant
theoretical challenges. It has been suggested that in this state the high-
energy emission arises from bulk-motion Comptonization in the convergent
accretion flow from the inner edge of the accretion disk.
