We consider the effects of rapid pair creation by an intense pulse ofgamma-rays propagating ahead of a relativistic shock. Side-scattered photonscolliding with the main gamma-ray beam amplify the density of scatteringcharges. The acceleration rate of the pair-loaded medium is calculated, and itslimiting bulk Lorentz factor related to the spectrum and compactness of thephoton source. One obtains, as a result, a definite prediction for the relativeinertia in baryons and pairs. The deceleration of a relativistic shock in themoving medium, and the resulting synchrotron emissivity, are compared withexisting calculations for a static medium. The radiative efficiency isincreased dramatically by pair loading. When the initial ambient densityexceeds a critical value, the scattering depth traversed by the main gamma-raypulse rises above unity, and the pulse is broadened. These considerations placesignificant constraints on burst progenitors: a pre-burst mass loss rateexceeding 10^{-5} M_\odot per year is difficult to reconcile with individualpulses narrower than 10 s, unless the radiative efficiency is low. Ananisotropic gamma-ray flux (on an angular scale \Gamma^{-1} or larger) drives alarge velocity shear that greatly increases the energy in the seed magneticfield forward of the propagating shock.
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