Small-scale inviscid accretion discs around black holes

Gas falling quasi-spherically on to a black hole forms an inner accretion disc if its specific angular momentum l exceeds l∗∼rgc, where rg 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<lcr, 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 14rg, and the energy release peaks at 2rg. 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.