A Simulation Study of Gamma-ray Bursts: Early Evolution and Structure of Ultra-Relativistic Jets

Abstract

A Simulation Study of Gamma-ray Bursts: Early Evolution and Structure of Ultra-Relativistic Jets

Sunho Kim1, Kyujin Kwak1, and Dongsu Ryu1

1Department of Physics, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

Gamma-ray bursts (GRBs) represent one of the most extreme astrophysical phenomena, in which ultra-relativistic jets with Lorentz factors () as large as a few hundred are believed to be involved. With such large ’s, simulation studies of GRBs demand a robust code that can properly handle the ultra-relativistic flow. By using a new relativistic hydrodynamics code developed at UNIST, we perform three-dimensional ultra-relativistic hydrodynamic simulations with  up to ~ 100, to probe the early evolution and structure of long GRB (LGRB) jets. In our simulations, jets are launched in an environment relevant to LGRBs. We consider a set of models to explore the effects of the density, pressure,  (or velocity), and opening angle of the jet. We also investigate the effects of the environment. In our simulations, we delineate four distinctive regions that develop as the jet proceeds through the background medium: the spine, the shocked spine, the shocked ambient medium, and the ambient medium from the inside out. Notably, we find that the GRB jets have wider spines when the jets are injected with larger ’s and opening angles. The morphology of GRB jet cocoons substantially differs from that of the relativistic jets in radio galaxies due to the difference in , opening angle, and ambient density profile. We analyze our simulations to quantify shocks, shear, and vorticity, and investigate physical properties such as energy dissipation rates and probability distribution functions of shocks and relativistic shear. We find that shock and shear develop predominantly within the shocked spine, suggesting that particle acceleration in GRBs would occur primarily in this region.