Magnetohydrodynamics of gamma-ray burst outflows

Using relativistic, axisymmetric, ideal MHD, we examine the outflow from a disk around a compact object, taking into account the baryonic matter, the electron-positron/photon fluid, and the large-scale electromagnetic field. Focusing on the parameter regime appropriate to gamma -ray burst outflows, we demonstrate, through exact selfsimilar solutions, that the thermal force (which dominates the initial acceleration) and the Lorentz force (which dominates farther out and contributes most of the acceleration) can convert up to similar to 50% of the initial total energy into asymptotic baryon kinetic energy. We examine how baryon loading and magnetic collimation affect the structure of the flow, including the regime where emission due to internal shocks could take place.