The nature of the high-energy emission (X-rays and Gamma rays) from black hole binaries remains elusive and so continues to be an area of active research.

Thermal Comptonization of accretion disk photons in a hot inner "corona" is expected to produce high energy emission. A spectral break related to the electron temperature of the "corona" is the fingerprint of this process. A break at energies of 100 keV or below is typically seen in the "low/hard" (low flux, spectrally hard) state during outbursts from stellar-mass black holes.

INTEGRAL provides excellent sensitivity and spectral coverage in the 3 keV to 1 MeV band via the JEM-X, IBIS/ISGRI and SPI detectors. The black hole transient GRO J1655-40 was observed with INTEGRAL in the "low/hard" state in 2005. The source was detected up to photon energies of 500 keV, and a strong and unbroken power-law spectrum was revealed (Caballero Garcia et al. 2007, ApJ in press, http://arxiv.org/abs/0706.1302v1). This remarkable spectrum without any break is shown in the Figure above. For the first time it is clear, that non-thermal emission processes can dominate the high energy emission of black holes in the "low/hard" state. Observations with INTEGRAL are changing our view of accretion flows and hard X-ray emission in black holes.