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We demonstrate that the steep decay and long plateau in the early phases ofgamma ray burst (GRB) X-ray afterglows are naturally produced in the collapsarmodel, by a means ultimately related to the dynamics of relativistic jetpropagation through a massive star. We present two-dimensional axisymmetrichydrodynamical simulations which start from a collapsar engine and evolve allthe way through the late afterglow phase. The resultant outflow includes a jetcore which is highly relativistic after breaking out of the star, but becomesbaryon-loaded after colliding with a massive outer shell, corresponding to massfrom the stellar atmosphere of the progenitor star which became trapped infront of the jet core at breakout. The prompt emission produced before orduring this collision would then have the signature of a high Lorentz factorjet, but the afterglow is produced by the amalgamated post-collision ejectawhich has more inertia than the original highly relativistic jet core and thushas a delayed deceleration. This naturally explains the early light curvebehavior discovered by Swift, including a steep decay and a long plateau,without invoking late-time energy injection from the central engine. Thenumerical simulation is performed continuously from engine to afterglow,covering a dynamic range of over ten orders of magnitude in radius. Lightcurves calculated from the numerical output demonstrate that this mechanismreproduces basic features seen in early afterglow data. Initial steep decaysare produced by internal shocks, and the plateau corresponds to the coastingphase of the outflow.

From Engine to Afterglow: Collapsars Naturally Produce Top-Heavy Jets and Early-Time Plateaus in Gamma Ray Burst Afterglows