The physics of GRB jets and their interaction with the progenitor star

It is now generally accepted that long gamma-ray bursts are associated with the final evolutionary stages of massive stars. As a consequence, their Jets must propagate through the stellar progenitor and break out on their surface, before they can reach the photospheric radius and produce the gamma-ray photons. We investigate the role of the progenitor star in shaping the jet propel-ties. We show that even a jet powered by a steady engine call develop a rich phenomenology at the stellar surface. We present special-relativistic simulations and compare the results to analytic considerations. We show that the jet is complex in the time as well as in the angular domain, so that observers located along different lines of sight detect significantly different bursts.