Broadband 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, aluminous X-ray transient and occasional source of relativistic radio jets,obtained with RXTE and OSSE. In one observation, the luminosity is found to be18% of the Eddington limit, which is one of the highest luminosities everobserved from GRO J1655-40. For this observation, we find that an adequate fitis obtained when a broad iron line and a reflection component are added to amodel 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-lawcomponent has a photon index of 2.72 and extends to at least 800 keV without acutoff. After this observation, a significant drop in the (5-12 keV)/(1.5-5keV) hardness ratio occurred on a timescale less than 2 hours. From an RXTEobservation of GRO J1655-40 made after the hardness transition, we find thatthe power-law index is harder (2.415 +/- 0.011), the flux of the power-lawcomponent is lower, and the total luminosity is 10% of the Eddington limit. Thechange in the power-law component is consistent with the correlation betweenthe spectral index and power-law flux previously reported for GRO J1655-40.Comment: 20 pages, 5 figures, Accepted for publication in the Astrophysical Journa