What is the optimum stellar rotation rate for a collapsar?

Abstract

We consider low angular momentum, neutrino cooled accretion flows onto newborn black holes in the context of the collapsar model for long Gamma Ray Bursts, and find a considerable energy release for rotation rates lower than those usually considered. The efficiency for the transformation of gravitational binding energy into radiation is maximized when the equatorial angular momentum l0 ~ 2 Rg c, where Rg is the Schwarzschild radius.

To appear in the proceedings of the conference "SWIFT and GRBs: Unveiling the relativistic universe", Il Nuovo Cimento"