  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 MSUN iron core of a 25 MSUN presupernova star has
collapsed and study the ensuing accretion of the 7 MSUN 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.
