Early Evolution of Long Gamma-Ray Burst Jets: Ultra-Relativistic Hydrodynamic Simulations

Abstract

Early Evolution of Long Gamma-Ray Burst Jets: Ultra-Relativistic Hydrodynamic Simulations

Dongsu Ryu^1, Seonho Kim^1, Kyujin Kwak^1

1Depart. of Physics, College of Natural Sciences, UNIST, Rep. of Korea

Gamma-ray bursts (GRBs), one of the most energetic phenomena in the universe, involve ultra-relativistic jets with Lorentz factors reaching several hundred. With such large Lorentz factors, simulation studies of these jets require robust codes that can properly handle ultra-relativistic flows. Using a newly developed relativistic hydrodynamic code of high accuracy, we perform three-dimensional simulations of ultra-relativistic jets with Lorentz factors up to ~ 100, to explore the early evolution and structure of long GRB (LGRB). Our simulations incorporate realistic LGRB environments and investigate models with varying jet Lorentz factor, opening angle, density, and pressure, as well as varying ambient medium properties. In the simulations, we identify four distinct regions: the spine, the shocked spine, the shocked ambient medium, and the ambient medium from the interior of the jet outwards. We find that the structure of GRB jets differs from that of the relativistic jets in radio galaxies. Notably, the larger Lorentz factors and non-zero opening angles of GRB jets lead to wider spines. Additionally, instead of the back-flow cocoon, the shocked spine with forward moving flow develops. We analyze shocks, shear, and vorticity in the simulations, quantifying physical properties such as the probability distribution functions of shocks and relativistic shear and the energy dissipation due to shocks and turbulence. Our analysis indicates that shocks and shear predominantly develop within the shocked spine, suggesting that particle acceleration within LGRBs would occur primarily in this region.