Low Energy Gamma-Ray Emission from Galactic Black Holes

X-ray observations of Galactic black holes (GBHs) such as Cygnus X-1 have greatly advanced the understanding of these objects. However, the vast majority of the observations have been restricted to energies below ∼ 200 keV. The Compton Gamma-Ray Observatory (CGRO) allowed for the first time simultaneous observations at energies from ∼ 25 keV up to > 1 GeV. In particular, the BATSE experiment aboard CGRO was able to monitor low-energy gamma-ray emission from Cygnus X-1, as well as other GBHs, nearly continuously over a nine year period. Using the Enhanced BATSE Occultation Package (EBOP), light curves and spectra in the energy range 25−2000 keV have been obtained for six GBHs. Based on the spectra when the GBHs were in a high gamma-ray flux state, it is suggested that at least two different classes of GBHs exist. The first is characterized by a Comptonization spectrum below ∼ 200 keV followed by a soft power law excess as exhibited by Cygnus X-1, GRO J0422+32, GRO J1719–24, and GX 339–4. The second class is characterized by simple power law spectrum in the full 25− 2000 keV range, with no evidence for a Comptonization component, as exhibited by GRO J1655–40 and GRS 1915+105. Gamma-ray observations can serve as an important diagnostic in studying the physical processes around GBHs. More sensitive observations in the future at energies > 250 keV will help answer questions regarding issues such as the nonthermal electron distribution, state transitions, and the connection to jets.