Numerical Study of Gamma‐Ray Burst Jet Formation in Collapsars

Abstract

Two-dimensional magnetohydrodynamic simulations are performed using the ZEUS-2D code to investigate the dynamics of a collapsar that generates a GRB jet, taking account of realistic equation of state, neutrino cooling and heating processes, magnetic fields, and gravitational force from the central black hole and self gravity. It is found that neutrino heating processes are not so efficient to launch a jet in this study. It is also found that a jet is launched mainly by B φ fields that are amplified by the winding-up effect. However, since the ratio of total energy relative to the rest mass energy in the jet is not so high as several hundred, we conclude that the jets seen in this study are not be a GRB jet. This result suggests that general relativistic effects, which are not included in this study, will be important to generate a GRB jet. Also, the accretion disk with magnetic fields may still play an important role to launch a GRB jet, although a simulation for much longer physical time (∼ 10 − 100 s) is required to confirm this effect. It is shown that considerable amount of 56 Ni is synthesized in the accretion disk. Thus there will be a possibility for the accretion disk to supply sufficient amount of 56 Ni required to explain the luminosity of a hypernova. Also, it is shown that neutron-rich matter due to electron captures with high entropy per baryon is ejected along the polar axis. Moreover, it is found that the electron fraction becomes larger than 0.5 around the polar axis near the black hole by ν e capture at the region. Thus there will be a possibility that r-process and r/p−process nucleosynthesis occur at these regions. Finally, much neutrons will -2be ejected from the jet, which suggests that signals from the neutron decays may be observed as the delayed bump of the light curve of the afterglow or gammarays.

Nucleosynthesis in Magnetically Driven Jets from Collapsars

“…3). We note that winds have not appeared in MHD simulations by other groups (Proga et al 2003;Nagataki et al 2007) or in the hydrodynamic simulation with a low $ 0.001 by MacFadyen & Woosley (1999). The winds are therefore generated by an '' -viscosity'' and may be artificial.…”

“…The energy liberated in the disk can be greater than 9 ; 10 51 ergs s À1 even for the low efficiency (=0.1) of energy liberation. In addition, the energy of neutrinos, which emanates from the disk at rates higher than 5 ; 10 51 ergs s À1 , is partly transferred to the polar region from the disk and acts to heat the polar region (Nagataki et al 2007).…”

“…In addition to the changes in electron fraction due to neutrino capture, neutrino interactions are important for the dynamics of collapsar jets through neutrino heating ( Nagataki et al 2007). The heating from neutrino captures and annihilations between neutrinos and antineutrinos may increase the energy of the jets.…”

Searching for differences inSwift’s intermediate GRBs

“…The intrinsic shorter duration of the former events suggests that either the central engine is active for a shorter time than in long bursts or that the efficiency of the energy extraction is lower than in long events. A short-lived central engine may be produced by a thermally driven relativistic outflow (Aloy & Obergaulinger 2007;Nagataki et al 2007), or perhaps the magnetic flux in the vicinity of the outflow being only slightly above the critical value to activate the Blandford-Znajek mechanism (e.g., Komissarov & Barkov 2009). If the type-defining property of intermediate bursts were the magnetic field strength, an additional means of distinguishing the properties of their central engines would be provided by the early optical afterglow observations.…”

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…Many authors have contributed to the discovery and laying out first ideas for theoretical explanations (see the reviews by Piran [58] and Nagataki [48]). The collapsar model was proposed by Woosley, MacFadyen and others (see also [35,36,46,47,62]), based on neutrino heating from the accretion disk and/or the winding of strong magnetic fields and MHD jets [41,54]. Hydrodynamic simulations (injecting explosion energies artificially) were performed, either by introducing high explosion energies (up to 10 52 erg) in a spherically symmetric way or aspherically in order to understand jet-like explosions [53].…”