Precursors and e^± pair loading from erupting fireballs

Recent observations suggest that long-duration γ-ray bursts and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. As the jet makes its way out of the stellar mantle, a bow shock runs ahead and a strong thermal precursor is produced as the shock breaks out. Such erupting fireballs produce a very bright γ-ray precursor as they interact with the thermal break-out emission. The prompt γ-ray emission propagates ahead of the fireball before it becomes optically thin, leading to e^± pair loading and radiative acceleration of the external medium. The detection of such precursors would offer the possibility of diagnosing not only the radius of the stellar progenitor and the initial Lorentz factor of the collimated fireball, but also the density of the external environment.