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,
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.
