Galactic black hole binaries: High-energy radiation

Observations of galactic black hole candidates made by the instruments aboard the Compton GRO in the hard X-ray and γ-ray bands have significantly enhanced our knowledge of the phenomenology of the emission from 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 black holecandidates above 20 keV. The all-sky monitoring capability of BATSE allows, for the first time, nearly continuous studies of the high-energy emission from more than a dosen black holecandidates. These long-term datasets are particularly well-suited to multi-wavelength comparison studies, from the radio upward in frequency (Zhang et al. 1997a, these proceedings). Energy spectral evolution and/or spectral state transitions have been observed from many of the black hole candidates. Moderately deep searches of the galactic plane suggest a deficit of weak γ-ray transients. Such population studies have implications for the origin of black hole binaries and the nature of accretion events. Observations above 50 keV from OSSE demonstrate that in the γ-ray band there exist two 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 cuts off with e-folding energy ∼100 keV and has its peak luminosity near this energy; thus substantial corrections need to be made to historical estimates of the bolometric luminosity ofblack holes in the “low” state. In contrast, in the X-ray high (soft) state, the luminosity peaks in thesoft X-rays and the spectrum extends with an unbroken power law, even up to energies above 500 keV in some cases. COMPTEL has detected emission above 750 keV from Cyg X-1 and the transient GRO J0422+32. In both cases the data suggest that an additional weak, hardspectral component is required beyond that observed by OSSE at lower energies, although the precise spectral form is yet to be determined