ar X iv : a st ro - p h / 98 09 09 8 v 1 8 S ep 1 99 8 Broad - band X - Ray Spectra of the Black Hole Candidate GRO J 1655 – 40

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 −0.08 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. Subject headings: accretion, accretion disks — BHXNe: general — stars: individual (GRO J1655–40) — stars: black holes — X-rays: stars Department of Physics and Columbia Astrophysics Laboratory, Columbia University, 550 W. 120th Street, New York, NY 10027 Naval Research Laboratory, Code 7650, Washington DC Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 Current address: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02139 (e-mail: [email protected])