Astronomy and Astrophysics – Astrophysics
Scientific paper
2007-06-09
Astronomy and Astrophysics
Astrophysics
14 figures. Accepted for publication in ApJ. Revised text for typos and deleted last line of third paragraph in page 14 (last
Scientific paper
10.1086/520764
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.
Caballero-Garcia M. D.
Diaz Trigo Maria
Domingo Albert
Fabian Andrea C.
Fender Rob P.
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