On the Composition of GRBs' Collapsar Jets
  The duration distribution of long Gamma Ray Bursts reveals a plateau at durations shorter than ~20 s (in the observer frame) and a power-law decline at longer durations (Bromberg et al., 2012). Such a plateau arises naturally in the Collapsar model. In this model the engine has to operate long enough to push the jet out of the stellar envelope and the observed duration of the burst is the difference between the engine's operation time and the jet breakout time. We compare the jet breakout time inferred from the duration distribution (~10 s in the burst's frame) to the breakout time of a hydrodynamic jet (~10 s for typical parameters) and of a Poynting flux dominated jet with the same overall energy (<~1 s). As only the former is compatible with the duration of the plateau in the GRB duration distribution, we conclude that the jet is hydrodynamic during most of the time that its head is within the envelope of the progenitor star and around the time when it emerges from the star. This would naturally arise if the jet forms as a hydrodynamic jet in the first place or if it forms Poynting flux dominated but dissipates most of its magnetic energy early on within the progenitor star and emerges as a hydrodynamic jet. 
Omer Bromberg, Jonathan Granot, Tsvi Piran
astro-ph.HE