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.
No abstract
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.