Hard X-Ray Emission of the Microquasar GRO J1655-40 during the Rise of Its 2005 Outburst

We present an analysis of the high-energy emission of the Galactic black hole GRO J1655-40 at the beginning of its 2005 outburst. The data from 458 ks of INTEGRAL observations, spread over 4 weeks, are analyzed, along with the existing simultaneous RXTE and Swift data. The high-energy data allow us to detect the presence of a high-energy cutoff and to study its evolution during the outburst rise. This high-energy feature is generally related to thermal mechanisms in the framework of Comptonization models from which we can estimate the plasma parameters. We found an electron temperature of about 30-40 keV and an optical depth of around 1.8-2.1. The high-energy cutoff decreased along with the radio flux and disappeared as the jet turned off.