Broad-band X-Ray Spectra of the Black Hole Candidate GRO J1655{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 nd that an adequate t is obtained when a broad iron line and a re ection component are added to a model consisting of a power-law plus a soft excess component. The 95% con dence 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 cuto . After this observation, a signi cant 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 nd that the power-law index is harder ( = 2:415 0:011), the ux 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 ux previously reported for GRO J1655{40. Subject headings: accretion, accretion disks | BHXNe: general | stars: individual (GRO J1655{40) | stars: black holes | X-rays: stars 1 Department of Physics and Columbia Astrophysics Laboratory, Columbia University, 550 W. 120th Street, New York, NY 10027 2 Naval Research Laboratory, Code 7650, Washington DC 3 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])