Temporal and Angular Properties of Gamma‐Ray Burst Jets Emerging from Massive Stars

We study the long-term evolution of relativistic jets in collapsars andexamine the effects of viewing angle on the subsequent gamma ray bursts. Wecarry out a series of high-resolution simulations of a jet propagating througha stellar envelope in 2D cylindrical coordinates using the FLASH relativistichydrodynamics module. For the first time, simulations are carried out using anadaptive mesh that allows for a large dynamic range inside the star while stillbeing efficient enough to follow the evolution of the jet long after it breaksout from the star. Our simulations allow us to single out three phases in thejet evolution: a precursor phase in which relativistic material turbulentlyshed from the head of the jet first emerges from the star, a shocked jet phasewhere a fully shocked jet of material is emerging, and an unshocked jet phasewhere the jet consists of a free-streaming, unshocked core surrounded by a thinboundary layer of shocked jet material. The appearance of these phases will bedifferent to observers at different angles. The precursor has a wide openingangle and would be visible far off axis. The shocked phase has a relativelynarrow opening angle that is constant in time. During the unshocked jet phasethe opening angle increases logarithmically with time. As a consequence, someobservers see prolonged dead times of emission even for constant properties ofthe jet injected in the stellar core. We also present an analytic model that isable to reproduce the overall properties of the jet and its evolution. Wefinally discuss the observational implications of our results, emphasizing thepossible ways to test progenitor models through the effects of jet propagationin the star. In an appendix, we present 1D and 2D tests of the FLASHrelativistic hydrodynamics module.