Accretion modes in collapsars: Prospects for gamma-ray burst production

We explore low angular momentum accretion flows onto black holes formed after the collapse of massive stellar cores. In particular, we consider the state of the gas falling quasi-spherically onto stellar mass black holes in the hypercritical regime, where the accretion rates are in the range 10(-3) M-circle dot s(-1) less than or similar to M less than or similar to 0.5 M-circle dot s(-1) and neutrinos dominate the cooling. Previous studies have assumed that in order to have a black hole switch to a luminous state, the condition l >> r(g)c, where l is the specific orbital angular momentum of the infalling gas and r(g) is the Schwarszchild radius, needs to be fulfilled. We argue that flows in hyperaccreting, stellar mass disks around black holes are likely to transition to a highly radiative state when their angular momentum is just above the threshold for disk formation, l similar to 2r(g)c. In a range r(g)c < l < 2r(g)c, a dwarf disk forms in which gas spirals rapidly into the black hole due to general relativistic effects, without any help from horizontal viscous stresses. For high rotation rates l >= 2r(g)c, the luminosity is supplied by large, hot equatorial bubbles around the black hole. The highest neutrino luminosities are obtained for l approximate to 2rgc, and this value of angular momentum also produces the most energetic neutrinos and thus also the highest energy deposition rates. Given the range of l explored in this work, we argue that, as long as l similar to 2r(g)c, low angular momentum cores may in fact be better suited for producing neutrino-driven explosions following core collapse in supernovae and gamma-ray bursts.