  Recent observations suggest that long-duration gamma-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 gamma-ray precursor as they
interact with the thermal break-out emission. The prompt gamma-ray emission
propagates ahead of the fireball before it becomes optically thin, leading to
electron-positron 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.

Astrophysics