Efficient acceleration and radiation in Poynting flux powered GRB outflows

We investigate the eects of magnetic energy release by local magnetic dissipation processes in Poynting flux- powered GRBs. For typical GRB parameters (energy and baryon loading) the dissipation takes place mainly outside the pho- tosphere, producing non-thermal radiation. This process converts the total burst energy into prompt radiation at an eciency of 10-50%. At the same time the dissipation has the eect of accelerating the flow to a large Lorentz factor. For higher baryon loading, the dissipation takes place mostly inside the photosphere, the eciency of conversion of magnetic energy into radia- tion is lower, and an X-ray flash results instead of a GRB. We demonstrate these eects with numerical one-dimensional steady relativistic MHD calculations.

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