The High-Energy Emission of GRO J1655-40 As Revealed with INTEGRAL Spectroscopy of the 2005 Outburst

We present broadband (3-500 keV) INTEGRAL X-ray spectra and X-ray/optical light curves of the luminous black hole X-ray transient and relativistic jet source GRO J1655-40. Our analysis covers four Target of Opportunity observations of the outburst that started in 2005 February. We find that the high-energy emission of GRO J1655-40 can be modeled well with an unbroken power law (with photon indices Gamma of 1.72+/-0.03 and 2.21+/-0.04 for the first and second observations). These correspond to hard and thermal-dominant states, respectively. In contrast to many other black hole spectra, high-energy complexity in the form of a break or cutoff is not required for the hard state, contrary to previous expectations. We show for the first time that Comptonization by nonthermal electrons is the dominant process behind the high-energy emission in the hard state. We discuss our results in terms of models for broadband emission and accretion flows in stellar-mass black holes.