Collapsar Uncorking and Jet Eruption in GRB
  We show that the collapsar model of gamma-ray bursts results in a series of successive shocks and rarefaction waves propagating in the "cork" of stellar material being pushed ahead of the jet, as it emerges from the massive stellar progenitor. These shocks result in a series of characteristic, increasingly shorter and harder thermal X-ray pulses, as well as a non-thermal gamma-ray pulse, which precede the usual non-thermal MeV gamma-rays. We consider jets escaping from both compact (CO or He dwarf) and blue supergiant stellar progenitors. The period, fluence and hardness of the pulses serves as a diagnostic for the size and density of the outer envelope of the progenitor star. 
E. Waxman and P. Meszaros