The dynamics of a highly magnetized jet propagating inside a star

The collapsar model explains the association of long duration gamma-ray bursts (GRBs) with stellar collapse. It involves a relativistic jet that forms at the core of a collapsing massive star. The jet penetrates the stellar envelope and the prompt GRB emission is produced once the jet is well outside the star. Most current models for generation of relativistic jets involve Poynting-flux-dominated outflows. We explore here the propagation of such a jet through a stellar envelope. The jet forms a bow shock in front of it. Energy dissipation at the shock generates an energetic cocoon that surrounds the jet. This cocoon exerts pressure on the jet and collimates it. While this description resembles the propagation of a hydrodynamic jet there are significant qualitative differences. Two strong shocks, the reverse shock that slows down the hydrodynamic jet and the collimation shock that collimates it, cannot form within the Poynting-flux-dominated jet. As a result this jet moves much faster and dissipates much less energy while it crosses the stellar envelope. We construct here a simple analytic model that explores, self consistently, the jet-cocoon interaction and dynamics. Using this model we determine the properties of the jet, including its velocity, propagation time and shape.