Spectra of Poynting-flux powered GRB outflows

We investigate the production of the gamma-ray spectrum of a Poynting-flux dominated GRB ouflow. The very high magnetic field strengths (super-equipartition) in such a flow lead to very efficient synchrotron emission. In contrast with internal shocks, dissipation of magnetic energy by reconnection is gradual and does not produce the spectrum of cooling electrons associated with shock acceleration.We show that a spectrum with a break in the BATSE energy range is produced, instead, if the magnetic dissipation heats a small (\sim 10^{-4}) population of electrons.