2001MNRAS.323..167B

Gas falling quasi-spherically on to a black hole forms an inner accretion disc if its specific angular momentum l exceeds l∗∼r_gc, where r_g is the Schwarzschild radius. The standard disc model assumes l{Gt}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 14r_g, and the energy release peaks at 2r_g. The disc emits a Comptonized X-ray spectrum with a break at ∼100keV. This accretion regime is likely to take place in wind-fed X-ray binaries and is also possible in active galactic nuclei.