A Formation Mechanism of Collapsar Black Hole -- early evolution phase

Abstract

The latest studies of massive star evolution indicate that an initially rapidly rotating star with sufficiently low metallicity can produce a rapidly rotating, massive stellar core that could be a progenitor of long-soft gamma-ray bursts (LGRBs). Motivated by these studies, we follow the collapse of a rapidly rotating massive stellar core to a 'collapsar' black hole (BH) surrounded by a massive, hot accretion disk performing fully general relativistic simulations. We focus on the general relativistic dynamics of the collapse, and the relevant microphysics is treated in a qualitative manner. The simulations are performed until the system consisting of the BH and the disk has relaxed to a quasi-stationary state. A novel mechanism found in this study is that strong shock waves are formed at the inner part of the disk after the formation of the BH. These shock waves propagate mainly along the rotation axis, heating the disk and sweeping materials around the rotational axis, and thereby forming a low density region. The temperature of the disk is high enough for copious neutrino emission. All these features indicate that the direct formation of a rapidly rotating BH is a promising source of LGRBs even in the absence of strong magnetic fields.