Small-scale inviscid accretion discs around black holes

Abstract

Gas falling quasi-spherically onto a black hole forms an inner accretion disc if its specific angular momentum $l$ exceeds $\lmin\sim r_gc$ where $r_g$ is the Schwarzschild radius. The standard disc model assumes $l\gg\lmin$. We argue that, in many black-hole sources, accretion flows have angular momenta just above the threshold for disc formation, $l\simgt\lmin$, and assess the accretion mechanism in this regime. In a range $\lmin<l<\lcr$, a small-scale disc forms in which gas spirals fast into the black hole without any help of horizontal viscous stresses. Such an `inviscid&#39; 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 $2r_g$. 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.