Relativistic Jets from Collapsars: Gamma-Ray Bursts
  Growing observational evidence supports the proposition that gamma-ray bursts (GRBs) are powered by relativistic jets from massive helium stars whose cores have collapsed to black holes and an accretion disk (collapsars). We model the propagation of relativistic jets through the stellar progenitor and its wind using a two-dimensional special relativistic hydrodynamics code based on the PPM formalism. The jet emerges from the star with a plug in front and a cocoon surrounding it. During its propagation outside the star, the jet gains high Lorentz factor as its internal energy is converted into kinetic energy while the cocoon expands both outwards and sideways. External shocks between the cocoon and the stellar wind can produce $\gamma$-ray and hard x-ray transients. The interaction of the jet beam and the plug will also affect both of them substantially, and may lead to short-hard GRBs. Internal shocks in the jet itself may make long-soft GRBs. 
Weiqun Zhang, S. E. Woosley (UC Santa Cruz)