Numerical simulations of relativistic jets in collapsars

Relativistic jets in collapsars are studied by numerical simulations. Such jets are believed to give rise to outbursts of high-energy emission known as gamma-ray bursts. The propagation and break out of the jets are examined in multi-dimensional numerical simulations using a special relativistic hydrodynamics code. During its propagation, the jet is collimated by the passage through the stellar mantle. As it erupts, the highly relativistic jet core is surrounded by a cocoon of less energetic, but still moderately relativistic ejecta that expands and becomes visible at larger polar angles. We predict a distribution of energy and Lorentz factor with viewing angle in the jet beam and its cocoon. These imply that what is seen may vary greatly with viewing angle. In particular, we predict the existence of a large number of low energy GRBs with mild Lorentz factors that may be related to GRB 980425/SN 1998bw and to the recently recognized cosmological X-ray flashes.