Outflow energy and black-hole spin evolution in collapsar scenarios

We explore the collapsar scenario for long gamma-ray bursts by performing axisymmetric neutrino-radiation 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 $>{10}^{53}\mathrm{erg}$ 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 ${10}^{52}\mathrm{erg}$ by magnetohydrodynamical effects. We discuss an issue of the overproduced energy for the early-jet-launch models.