  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 depends on the stellar envelope
and central engine properties, assuming a steady jet injection. It takes a few
seconds for the jet to bore its way through the stellar core; 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 magnetized
plasma that takes several hours to expand, subrelativistically, through the
envelope of a high-mass supergiant. Jet break-through and a conventional burst
would be expected not only in He stars but possibly also in blue supergiants.
Shock waves and magnetic dissipation in the escaping bubble can contribute a
non thermal UV/X-ray afterglow, and also excite Fe line emission from thermal
gas, in addition to the standard jet deceleration power-law afterglow.
