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Deceleration of a Relativistic, Photon-rich Shell: End of Preacceleration, Damping of Magnetohydrodynamic Turbulence, and the Emission Mechanism of Gamma-Ray Bursts

We consider the interaction of a relativistically moving shell, composed of thermal photons, a reversing magnetic field, and a small admixture of charged particles, with a dense Wolf-Rayet wind. A thin layer of Wolf-Rayet material is entrained at the head of this outflow; it cools and becomes Rayleigh-Taylor unstable, thereby providing an additional source of inertia and variability. The gamma rays streaming across the forward shock load the wind material with electron-positron pairs and push it to relativistic speeds close to the engine. This defines a characteristic radiative compactness at the point where the reverse shock has completed its passage back through the shell. We argue that the prompt gamma-ray emission is triggered by this external braking, at an optical depth ~1 to electron scattering. Torsional MHD waves, excited by the forced reconnection of the reversing magnetic field, carry a fluctuating current and are Landau damped at high frequencies on the parallel motion of the light charges. We show that the heated charges cool primilarly by inverse Compton radiation, which is beamed along the magnetic field. Thermal radiation that is advected out from the base of the jet cools the particles. The observed relation between peak energy and isotropic luminosity—both its amplitude and scaling—is reproduced if the blackbody seeds are generated in a relativistic jet core that is subject to Kelvin-Helmholtz instabilities with the Wolf-Rayet envelope. This relation is predicted to soften below an isotropic luminosity Liso ~ 3 × 1050 ergs s-1. Spectrally harder bursts will arise in outflows which encounter no dense stellar envelope. The duration of spikes in the inverse-Compton emission is narrower at higher frequencies, as observed. The transition from prompt gamma-ray emission to afterglow can be explained by the termination of the thermal X-ray seed and the onset of synchrotron-self-Compton emission.