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 { \Gamma } = 1.72 { \pm } 0.03 , 2.21 { \pm } 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 .