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A.M. Beloborodov (Stockholm Observatory), A.F. Illarionov (Astro-Space Center, Moscow)
Gas falling quasi-spherically onto a Schwarzschild black hole can form an inner thin accretion disk if its specific angular momentum, l, exceeds lmin=0.75rgc, where rg is Schwarzschild radius. The standard disk model assumes l>>lmin. We argue that, in many black-hole sources, accretion flows can have angular momenta just above the threshold for disk formation, lmin, and assess the mechanism of accretion in this regime. In a range lmin < l < lcr, a small-scale disk forms in which gas spirals fast into the black hole without any help of horizontal viscous stresses. Such an ``inviscid'' disk, however, interacts inelastically with the infall feeding the disk. The disk-infall interaction determines the observed luminosity and the radiative efficiency of accretion, which turns out to be comparable with the efficiency of the standard disk. The maximum radius of the inviscid disk is about 2lcr2/rgc2~14 rg, and the energy release peaks at 2rg. The disk emits a Comptonized X-ray spectrum, a power-law 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.