  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 February 2005. We find that the
high energy emission of GRO J1655-40 can be modelled well with an unbroken
power-law (with photon indices of 1.72+-0.03,2.21+-0.04 for the first and the
second observations, respectively). 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 cut-off is not required for
the hard state, contrary to previous expectations for this state. We show for
the first time that Comptonization by non-thermal electrons is the dominant
process for the high energy emission in the hard state. We discuss our results
in terms of models for broad-band emission and accretion flows in stellar-mass
black holes.
