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On the nature of X-ray emission in accreting black holes

Juri Poutanen, University of Oulu

We have developed a numerical code to study radiative processes in relativistic magnetized plasmas. We account for Compton scattering, synchrotron radiation (and self-absorption), and pair-production. For the first time, we solve coupled kinetic equations for electrons and photons without any approximations on the relevant cross-sections and compute self-consistently the resulting electron and photon distributions. We apply the developed code to study the X-ray emission from accreting galactic black holes (GBH). A source of luminosity of about at L=10^{37} erg/s, where only synchrotron and Compton processes operate, produces an X-ray spectrum with photon index 1.7, which strongly resembles the hard state of GBH. The feedback from the disc is not needed to produce such spectra. This argues in favour of hot inner disc model as a source of X-rays in GBH. At high luminosities of 10^{38} erg/s (in the absence of disc radiation), the spectrum is close to saturated Comptonization, peaking at 5 keV. This Wien-type spectrum might be associated with the thermally-looking emission in the very high states of e.g. GRS 1915+105. A behaviour similar to what is observed in the spectral state transitions in GBH can be reproduced by varying the ratio of soft disc luminosity and the power dissipated in the hot flow, consistent with interpretation of the disc moving towards the black hole in the soft state.