Two gamma-ray spectral classes of black hole transients

The observation of seven transient black hole candidates by the oriented scintillation spectrometer experiment (OSSE) is reviewed: GRO J0422+32; GX 339-4; GRS 1716-249; GRS 1009-45; 4U 1543-47; GRO J1655-40, and GRS 1915+105. Two gamma ray spectral classes are apparent. The former three objects show Comptonized spectra with exponential cutoff at approximately 100 keV, while the latter four have fairly soft power law spectra. The Comptonized spectra appear to be associated with the X-ray low state, while the power law spectra appear to be associated with the X-ray high state.