Off-Axis Afterglow Light Curves from High-Resolution Hydrodynamical Jet Simulations

TL;DR

This paper uses high-resolution hydrodynamical simulations and synthetic light curves to analyze gamma-ray burst afterglows, revealing that jet breaks are often undetectable at larger observer angles, explaining observational discrepancies.

Contribution

It presents detailed 2D relativistic jet simulations combined with synthetic light curves, quantifying the likelihood of detecting jet breaks at various observer angles in Swift data.

Findings

Jet breaks become harder to detect at larger observer angles.

Only 12% chance of observing a jet break at 3 sigma level for typical Swift parameters.

Observer angle explains the lack of observed jet breaks in Swift sample.

Abstract

Numerical jet simulations serve a valuable role in calculating gamma-ray burst afterglow emission beyond analytical approximations. Here we present the results of high resolution 2D simulations of decelerating relativistic jets performed using the RAM adaptive mesh refinement relativistic hydrodynamics code. We have applied a separate synchrotron radiation code to the simulation results in order to calculate light curves at frequencies varying from radio to X-ray for observers at various angles from the jet axis. We provide a confirmation from radio light curves from simulations rather than from a simplified jet model for earlier results in the literature finding that only a very small number of local Ibc supernovae can possibly harbor an orphan afterglow. Also, recent studies have noted an unexpected lack of observed jet breaks in the Swift sample. Using a jet simulation with physical parameters representative for an average Swift sample burst, such as a jet half opening angle of 0.1 rad and a source redshift of z = 2.23, we have created synthetic light curves at 1.5 keV with artificial errors while accounting for Swift instrument biases as well. A large set of these light curves have been generated and analyzed using a Monte Carlo approach. Single and broken power law fits are compared. We find that for increasing observer angle, the jet break quickly becomes hard to detect. This holds true even when the observer remains well within the jet opening angle. We find that the odds that a Swift light curve from a randomly oriented 0.1 radians jet at z = 2.23 will exhibit a jet break at the 3 sigma level are only 12 percent. The observer angle therefore provides a natural explanation for the lack of perceived jet breaks in the Swift sample.