Precursor emission has been observed in a non-negligible fraction of gamma-ray bursts.The time gap between the precursor and the main burst extends in some case up to hundreds of seconds , such as in GRB041219A , GRB050820A and GRB060124 .
Both the origin 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 .
Due to 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 re-accelerates the jet to a relativistic velocity and therefore limits the gap period to within about ten seconds .
This scenario therefore can not 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 have examined the propagation times of the weak precursor jet through the stellar progenitor .
We find that the initial weak jet can break out of the progenitor in a time less than ten seconds ( a typical precursor duration ) provided that it has a moderately high relativistic Lorentz factor \Gamma \gtrsim 10 .
The longer ( \sim 100 s ) time gap following this is determined , in this scenario , by the fall-back timescale , which is at the same time long enough for the exit channel to close after the precursor activity ceases , thus allowing for the collimation by the cocoon pressure of the subsequent main jet , as required .