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Abstract

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
$>10^{53}$ 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 with the powerful explosion of the entire star with the explosion
energy of order $10^{52}$ erg by magnetohydrodynamical effects. We discuss an
issue of the overproduced energy for the early-jet-launch models.