OSSE has observed seven transient black hole candidates: GRO J0422+32, GX339-4, GRS 1716-249, GRS 1009-45, 4U 1543. Two gamma-ray spectral states are evident and, based on a limited number of contemporaneous X-ray and gamma-ray observations, these states appear to be correlated with X-ray states. The former three objects show hard spectra below 100 keV (photon number indices Γ < 2) that are exponentially cut off with folding energy ∼ 100 keV, a spectral form that is consistent with thermal Comptonization. This "breaking gamma-ray state" is the high-energy extension of the X-ray low, hard state. In this state, the majority of the luminosity is above the X-ray band, carried by photons of energy ∼100 keV. The latter four objects exhibit a "power-law gamma-ray state" with a relatively soft spectral index (Γ ∼ 2.5 − 3) and no evidence for a spectral break. For GRO J1655-40, the lower limit on the break energy is 690 keV. GRS 1716-249 exhibits both spectral states, with the power-law state having significantly lower gamma-ray luminosity. The power-law gamma-ray state is associated with the presence of a strong ultrasoft X-ray excess (kT ∼ 1 keV), the signature of the X-ray high, soft (or perhaps very high) state. The physical process responsible for the unbroken power law is not well understood, although the spectra are consistent with bulk-motion Comptonization in the convergent accretion flow.Here we summarize the low-energy gamma-ray spectra of seven transient galactic BHCs, all low-mass X-ray binaries (LMXBs), observed with OSSE on the Compton Gamma Ray Observatory (in order of increasing right ascension: GRO J0422+32, GRS 1009-45, 4U 1543-47, GRO J1655-40, GX339-4, GRS 1716 and demonstrate the existence of two spectral states in low-energy gamma rays. We compare these spectra with those of the archetypal galactic BHC, the high-mass X-ray binary Cyg X-1. Introductory remarks covering the historical record of each of these objects are given along with the details of the OSSE observations in the subsections below.

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We present broad-band (2 keV to 2 MeV) x-ray spectra of GRO J1655-40, a luminous X-ray transient and occasional source of relativistic radio jets, obtained with the Rossi X-Ray Timing Explorer (RXTE) and the Oriented Scintillation Spectrometer Experiment (OSSE). In one observation, the luminosity is found to be 18% of the Eddington limit, which is one of the highest luminosities ever observed from GRO J1655-40. For this observation, we find that an adequate fit is obtained when a broad iron line and a reflection component are added to a model consisting of a power-law plus a soft excess component. The 95% confidence lower limit on the rms line width is 0.86 keV. The power-law component has a photon index of 2.72 +0.03 −0.08 and extends to at least 800 keV without a cutoff.After this observation, a significant drop in the (5-12 keV)/(1.5-5 keV) hardness ratio occured on a timescale less than 2 hours. From an RXTE observation of GRO J1655-40 made after the hardness transition, we find that the power-law index is harder (α = 2.415 ± 0.011), the flux of the power-law component is lower, and the total luminosity is 10% of the Eddington limit. The change in the power-law component is consistent with the correlation between the spectral index and power-law flux previously reported for GRO J1655-40.