OSSE has observed seven transient black hole candidates : GRO J0422+32 , GX339–4 , GRS 1716–249 , GRS 1009–45 , 4U 1543–47 , GRO J1655–40 , and GRS 1915+105 .
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 \Gamma < 2 ) that are exponentially cut off with folding energy \sim 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 \sim 100 keV .
The latter four objects exhibit a “ power-law gamma-ray state ” with a relatively soft spectral index ( \Gamma \sim 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 \sim 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 .