Wang 2007 回落盘模型下的前兆时标
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Authors: Xiang-Yu Wang, Peter Meszaros
Comments: 6 pages, submitted to ApJ
Precursor emission has been observed in a non-negligible fraction of gamma-ray bursts, the time gap between the precursor and the main burst extending up to hundreds of seconds in some cases, such as in GRB041219A, GRB050820A and GRB060124. We have investigated 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 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 process associated with a short-lived proto-neutron star, while the main burst is produced by a stronger jet fed by the fallback disk accreting 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 provided that it has a moderately high relativistic Lorentz factor (Gamma>10). The longer time gap following this is dictated by the fall-back timescale, which is long enough for the exit channel to close after the precursor activity ceases, allowing for the collimation of the main jet by the cocoon pressure as it propagates outward.
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