INTEGRAL Picture Of the Month
July 2007

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GRO J1655-40: A black hole transient without a break

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, ). 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.

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