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

Abstract

We study the long-term evolution of relativistic jets in collapsars and\nexamine the effects of viewing angle on the subsequent gamma ray bursts. We\ncarry out a series of high-resolution simulations of a jet propagating through\na stellar envelope in 2D cylindrical coordinates using the FLASH relativistic\nhydrodynamics module. For the first time, simulations are carried out using an\nadaptive mesh that allows for a large dynamic range inside the star while still\nbeing efficient enough to follow the evolution of the jet long after it breaks\nout from the star. Our simulations allow us to single out three phases in the\njet evolution: a precursor phase in which relativistic material turbulently\nshed from the head of the jet first emerges from the star, a shocked jet phase\nwhere a fully shocked jet of material is emerging, and an unshocked jet phase\nwhere the jet consists of a free-streaming, unshocked core surrounded by a thin\nboundary layer of shocked jet material. The appearance of these phases will be\ndifferent to observers at different angles. The precursor has a wide opening\nangle and would be visible far off axis. The shocked phase has a relatively\nnarrow opening angle that is constant in time. During the unshocked jet phase\nthe opening angle increases logarithmically with time. As a consequence, some\nobservers see prolonged dead times of emission even for constant properties of\nthe jet injected in the stellar core. We also present an analytic model that is\nable to reproduce the overall properties of the jet and its evolution. We\nfinally discuss the observational implications of our results, emphasizing the\npossible ways to test progenitor models through the effects of jet propagation\nin the star. In an appendix, we present 1D and 2D tests of the FLASH\nrelativistic hydrodynamics module.\n