Small-scale inviscid accretion discs around black holes |
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| Authors: | Andrei M Beloborodov Andrei F Illarionov |
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| Institution: | Stockholm Observatory, SE-13336 Saltsjöbaden, Sweden;Astro-Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117810 Moscow, Russia |
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| Abstract: | Gas falling quasi-spherically on to a black hole forms an inner accretion disc if its specific angular momentum l exceeds l ?～ r g c , where r g is the Schwarzschild radius. The standard disc model assumes l ? l ?. We argue that, in many black hole sources, accretion flows have angular momenta just above the threshold for disc formation, l ? l ?, and assess the accretion mechanism in this regime. In a range l ?< l < l cr, a small-scale disc forms in which gas spirals fast into the black hole without any help from horizontal viscous stresses. Such an 'inviscid' disc, however, interacts inelastically with the feeding infall. The disc–infall interaction determines the dynamics and luminosity of the accretion flow. The inviscid disc radius can be as large as 14 r g, and the energy release peaks at 2 r g. The disc emits a Comptonized X-ray spectrum with a break at ～100 keV. This accretion regime is likely to take place in wind-fed X-ray binaries and is also possible in active galactic nuclei. |
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| Keywords: | accretion accretion discs black hole physics radiation mechanisms: thermal binaries: general X-rays: galaxies X-rays: stars |
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