Collapsar Jets , Bubbles and Fe Lines

According to collapsar models, gamma ray bursts are caused by relativistic jets that expel energy along the rotation axis of a collapsing stellar core. We discuss how the structure and time-dependence of such jets depends on the properties of the stellar envelope and the central engine. It takes a few seconds for the jet to bore its way through the core of the star; most of the energy output during that period goes into a cocoon of relativistic plasma surrounding the jet. This material subsequently forms a bubble of highly-magnetised plasma that would take several hours to expand, subrelativistically, through the extended envelope of a high-mass supergiant. Shock waves and magnetic dissipation in this plasma (where the internal fields could still be 10 5 G) can contribute a non thermal UV/X-ray afterglow, and also excite Fe line emission from thermal gas, in addition to the standard power-law afterglow from the jet deceleration shock.