Small-scale inviscid accretion discs around black holes

Gas falling quasi-spherically onto a black hole forms an inner accretion disc if its specific angular momentum $l$ exceeds $\text{lmin}\sim r_{g}c$ where $r_g$ is the Schwarzschild radius. The standard disc model assumes $l\gg \text{lmin}$. We argue that, in many black-hole sources, accretion flows have angular momenta just above the threshold for disc formation, $l\text{simgt}\text{lmin}$, and assess the accretion mechanism in this regime. In a range $\text{lmin}<l<\text{lcr}$, a small-scale disc forms in which gas spirals fast into the black hole without any help of 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 $\sim 100$ keV. This accretion regime is likely to take place in wind-fed X-ray binaries and is also possible in active galactic nuclei.