Axisymmetric Magnetohydrodynamic Simulations of the Collapsar Model for Gamma-Ray Bursts

We present results from axisymmetric, time-dependent magnetohydrodynamic (MHD) simulations of the collapsar model for gamma-ray bursts. We begin the simulations after the 1.7 M_solar iron core of a 25 M_solar presupernova star has collapsed and study the ensuing accretion of the 7 M_solar helium envelope onto the central black hole formed by the collapsed iron core. We consider a spherically symmetric progenitor model but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. Our MHD simulations include a realistic equation of state, neutrino cooling, photodisintegration of helium, and resistive heating. Our main conclusion is that, within the collapsar model, MHD effects alone are able to launch, accelerate, and sustain a strong polar outflow. We also find that the outflow is Poynting flux-dominated and note that this provides favorable initial conditions for the subsequent production of a baryon-poor fireball.