Outflow energy and black-hole spin evolution in collapsar scenarios

TL;DR

This study uses advanced simulations to investigate how magnetic fields influence jet formation, black-hole spin evolution, and explosion energies in collapsar scenarios related to long gamma-ray bursts.

Contribution

First full general relativistic axisymmetric neutrino-radiation MHD simulations of collapsars reveal jet formation, black-hole spin-down, and explosion energies, highlighting the role of magnetic fields and initial conditions.

Findings

Jets can be launched before disk formation with strong magnetic fields.

Black-hole spin decreases due to the Blandford-Znajek mechanism after jet launch.

Explosion energies can reach around 10^{52} erg, often associated with jet activity.

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.