Abstract
We consider the time-dependent evolution of a relativistic jet following its breakout through the surface of a massive compact star, as envisaged in the collapsar model of gamma-ray bursts. At breakout, the jet is tightly collimated by the pressure of its hot cocoon, which is created as the jet traverses the star. After breakout, the cocoon pressure drops and the jet expands toward its natural opening angle. We show that the evolving opening angle of the jet produces a stratification of the total energy with the off-axis angle, resulting in a universal morphology. The angular structure is largely independent of the initial beam pattern and depends only on the luminosity of the central engine. With the minimal assumption of a constant energy release we reproduce the theta^{-2} profile required to explain observations of afterglows.
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