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

(Abridged) We consider the interaction of a relativistically-moving shell,composed of thermal photons, a reversing magnetic field and a small admixtureof charged particles, with a dense Wolf-Rayet wind. A thin outer layer ofWolf-Rayet material is entrained by the jet head; it cools and becomesRayleigh-Taylor unstable, thereby providing an additional source of inertia andvariability. Pair creation in the wind material, and the associatedpre-acceleration, defines a characteristic radiative compactness at the pointwhere the reverse shock has completed its passage back through the shell. Weargue that the prompt gamma-ray emission is triggered by this external braking,at an optical depth ~1 to electron scattering. Torsional waves, excited by theforced reconnection of the reversing magnetic field, carry a fluctuatingcurrent, and are damped at high frequencies by the electrostatic accelerationof electrons and positrons. We show that inverse Compton radiation by theaccelerated charges is stronger than their synchrotron emission, and is beamedalong the magnetic field. Thermal radiation that is advected out from the baseof the jet cools the particles. The observed relation between peak energy andisotropic luminosity is reproduced if the blackbody seeds are generated in arelativistic jet core that is subject to Kelvin-Helmholtz instabilities withthe Wolf-Rayet envelope. This relation is predicted to soften to E_peak ~L_iso^{1/4} below an isotropic luminosity L_iso ~ 3x10^{50} ergs/s. Theduration of spikes in the inverse-Compton emission is narrower at higherfrequencies, in agreement with the observed relation. The transition fromprompt gamma-ray emission to afterglow can be explained by the termination ofthe thermal X-ray seed and the onset of synchrotron-self-Compton emission.Comment: 32 pages, 5 figures, in press in the Astrophysical Journal, vol. 64