Outflow energy and black-hole spin evolution in collapsar scenarios

We explore the collapsar scenario for long gamma -ray bursts by performing axisymmetric neutrinoradiation magnetohydrodynamics simulations in full general relativity for the first time. In this paper, we pay particular attention to the outflow energy and the evolution of the black -hole spin. We show that for a strong magnetic field with an aligned field configuration initially given, a jet is launched by magnetohydrodynamical effects before the formation of a disk and a torus, and after the jet launch, the matter accretion onto the black hole is halted by the strong magnetic pressure, leading to the spin -down of the black hole due to the Blandford-Znajek mechanism. The spin -down timescale depends strongly on the magnetic -field strength initially given because the magnetic -field strength on the black -hole horizon, which is determined by the mass infall rate at the jet launch, depends strongly on the initial condition, although the total jet -outflow energy appears to be huge > 10(53 )erg depending only weakly on the initial field strength and configuration. For the models in which the magnetic -field configuration is not suitable for quick jet launch, a torus is formed, and after a long-term magnetic -field amplification, a jet can be launched. For this case, the matter accretion onto the black hole continues even after the jet launch, and black -hole spin -down is not found. We also find that the jet launch is often accompanied by the powerful explosion of the entire star with the explosion energy of order 1052 erg by magnetohydrodynamical effects. We discuss an issue of the overproduced energy for the early -jet -launch models.