Title (preliminary): Magnetohydrodynamics of Gamma-Ray Burst Outflow
Authors: N. Vlahakis, A. Königl
Contact: N.Vlahakis, Enrico Fermi Institute, University of Chicago

ABSTRACT
Using an exact solution of the relativistic steady axisymmetric ideal magnetohydrodynamic equations, we examine the outflow from a torus around a compact object, taking into account the baryonic matter, the electron-positron-photon fluid, and the large-scale electromagnetic field. We clarify the relationship between the thermal (fireball) and magnetic (Poynting flux) mechanisms, identify the parameter regimes where qualitatively different behaviours are expected, and demonstrate that the observationally inferred properties of these outflows can be attributed to magnetic driving.