Gamma-Ray Spectral States of Galactic Black Hole Candidates
Journal article preprint
NAVAL RESEARCH LAB WASHINGTON DC E O HULBURT CENTER FOR SPACE RESEARCH
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OSSE has observed seven transient black hole candidates GRO J042232, GX339-4, GRS 1716-249, GRS 1009-45, 4U 1543-47, GRO J1655-40, and GRS 1915105. 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, 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 , 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.