2007ApJ...670.1247W

Precursor emission has been observed in a nonnegligible fraction of gamma-ray bursts. The time gap between the precursor and the main burst extends in some cases up to hundreds of seconds. Both the origins of the precursor and the large value of the time gap are controversial. Here we investigate the maximum possible time gaps arising from the jet propagation inside the progenitor star in models which assume that the precursor is produced by the jet bow shock or the cocoon breaking out of the progenitor. Because of the pressure drop ahead of the jet head after it reaches the stellar surface, a rarefaction wave propagates back into the jet at the sound speed, which reaccelerates the jet to a relativistic velocity and therefore limits the gap period to within about 10 s. This scenario therefore cannot explain gaps which are hundreds of seconds long. Instead, we ascribe such long time gaps to the behavior of the central engine and suggest a fallback collapsar scenario for these bursts. In this scenario, the precursor is produced by a weak jet formed during the initial core collapse, possibly related to MHD processes associated with a short-lived proto-neutron star, while the main burst is produced by a stronger jet fed by fallback accretion onto the black hole resulting from the collapse of the neutron star. We find that the initial weak jet can break out of the progenitor in a time less than 10 s (a typical precursor duration) provided that it has a moderately high relativistic Lorentz factor Γ≳10.