Collapsar jets, bubbles, and Fe lines

In the collapsar scenario, gamma-ray bursts are caused by relativistic jets expelled along the rotation axis of a collapsing stellar core. We discuss how the structure and time dependence of such jets depend on the stellar en velope and central engine properties, assuming a steady jet injection. It t akes a few seconds for the jet to bore its way through the stellar core; mo st of the energy output during that period goes into a cocoon of relativist ic plasma surrounding the jet. This material subsequently forms a bubble of magnetized plasma that takes several hours to expand, subrelativistically, through the envelope of a high-mass supergiant. Jet breakthrough and a con ventional burst would be expected not only in He stars but possibly also in blue supergiants. Shock waves and magnetic dissipation in the escaping bub ble can contribute a nonthermal UV/X-ray afterglow, and also excite Fe line emission from thermal gas, in addition to the standard jet deceleration po wer-law afterglow.