Volume 614, June 2018
|Number of page(s)||12|
|Section||Stellar structure and evolution|
|Published online||18 June 2018|
Doughnut strikes sandwich: the geometry of hot medium in accreting black hole X-ray binaries
Tuorla Observatory, Department of Physics and Astronomy, 20014 University of Turku,
2 Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
3 Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
4 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
Accepted: 26 February 2018
We study the effects of the mutual interaction of hot plasma and cold medium in black hole binaries in their hard spectral state. We consider a number of different geometries. In contrast to previous theoretical studies, we use a modern energy-conserving code for reflection and reprocessing from cold media. We show that a static corona above an accretion disc extending to the innermost stable circular orbit produces spectra not compatible with those observed. They are either too soft or require a much higher disc ionization than that observed. This conclusion confirms a number of previous findings, but disproves a recent study claiming an agreement of that model with observations. We show that the cold disc has to be truncated in order to agree with the observed spectral hardness. However, a cold disc truncated at a large radius and replaced by a hot flow produces spectra which are too hard if the only source of seed photons for Comptonization is the accretion disc. Our favourable geometry is a truncated disc coexisting with a hot plasma either overlapping with the disc or containing some cold matter within it, also including seed photons arising from cyclo-synchrotron emission of hybrid electrons, i.e. containing both thermal and non-thermal parts.
Key words: accretion, accretion discs / black hole physics / stars: individual: GX 339–4 / X-rays: binaries / X-rays: stars
© ESO, 2018
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