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 \u3e 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 J 1719-24, 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 \u3e 250 keV will help answer questions regarding issues such as the nonthermal electron distribution, state transitions, and the connection to jets. © 2008 American Institute of Physics