Gamma-Ray Spectral States of Galactic Black Hole Candidates

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 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. Subject headings: Black Holes — Gamma Rays: Observations — X-rays:Binaries E-mail: [email protected] CSI, George Mason University, Fairfax, VA 22030