GRB Precursors in the Fallback Collapsar Scenario

Precursor emission has been observed in a non-negligible fraction ofgamma-ray bursts.The time gap between the precursor and the main burst extendsin some case up to hundreds of seconds, such as in GRB041219A, GRB050820A andGRB060124. Both the origin of the precursor and the large value of the time gapare controversial. Here we investigate the maximum possible time gaps arisingfrom the jet propagation inside the progenitor star, in models which assumethat the precursor is produced by the jet bow shock or the cocoon breaking outof the progenitor. Due to the pressure drop ahead of the jet head after itreaches the stellar surface, a rarefaction wave propagates back into the jet atthe sound speed, which re-accelerates the jet to a relativistic velocity andtherefore limits the gap period to within about ten seconds. This scenariotherefore cannot explain gaps which are hundreds of seconds long. Instead, weascribe such long time gaps to the behavior of the central engine, and suggesta fallback collapsar scenario for these bursts. In this scenario, the precursoris produced by a weak jet formed during the initial core collapse, possiblyrelated to MHD processes associated with a short-lived proto-neutron star,while the main burst is produced by a stronger jet fed by fallback accretiononto the black hole resulting from the collapse of the neutron star. We haveexamined the propagation times of the weak precursor jet through the stellarprogenitor. We find that the initial weak jet can break out of the progenitorin a time less than ten seconds (a typical precursor duration) provided that ithas a moderately high relativistic Lorentz factor \Gamma>=10 (abridged).Comment: 8 pages, accepted by ApJ, this version contains significantly expanded discussion and an additional figure, conclusions unchange