2007ApJ...665..569M

We study the long-term evolution of relativistic jets in collapsars and examine the effects of viewing angle on the subsequent gamma-ray bursts. We carry out a series of high-resolution simulations of a jet propagating through a stellar envelope in 2D cylindrical coordinates using the FLASH relativistic hydrodynamics module. For the first time, simulations are carried out using an adaptive mesh that allows for a large dynamic range inside the star while still being efficient enough to follow the evolution of the jet long after it breaks out from the star. We single out three phases in the jet evolution: a precursor phase in which relativistic material turbulently shed from the head of the jet first emerges from the star, a shocked-jet phase where a fully shocked jet of material is emerging, and an unshocked-jet phase where the jet consists of a free-streaming, unshocked core surrounded by a thin boundary layer of shocked-jet material. The appearance of these phases will be different to observers at different angles. The precursor has a wide opening angle, the shocked phase has a relatively narrow opening angle, and in the unshocked phase the opening angle increases logarithmically with time. As a consequence, some observers see prolonged dead times of emission even for constant properties of the jet injected in the stellar core. We also present an analytic model that is able to reproduce the overall properties of the jet and its evolution. In an appendix, we present 1D and 2D tests of the FLASH relativistic hydrodynamics module.