Jet simulations and Gamma-ray burst afterglow jet breaks

TL;DR

This paper shows that gamma-ray burst afterglow jet breaks are chromatic across the self-absorption break, affecting the timing and inferred physical parameters of the jets, based on high-accuracy simulations.

Contribution

It demonstrates that jet breaks are frequency-dependent and provides a detailed simulation-based analysis of this chromaticity in GRB afterglows.

Findings

Radio jet break can be significantly delayed.

Conventional analysis overestimates jet break time.

Inferred opening angles and energies are underestimated.

Abstract

The conventional derivation of the gamma-ray burst afterglow jet break time uses only the blast wave fluid Lorentz factor and therefore leads to an achromatic break. We show that in general gamma-ray burst afterglow jet breaks are chromatic across the self-absorption break. Depending on circumstances, the radio jet break may be postponed significantly. Using high-accuracy adaptive mesh fluid simulations in one dimension, coupled to a detailed synchrotron radiation code, we demonstrate that this is true even for the standard fireball model and hard-edged jets. We confirm these effects with a simulation in two dimensions. The frequency dependence of the jet break is a result of the angle dependence of the emission, the changing optical depth in the self-absorbed regime and the shape of the synchrotron spectrum in general. In the optically thin case the conventional analysis systematically overestimates the jet break time, leading to inferred opening angles that are underestimated by a factor 1.32 and explosion energies that are underestimated by a factor 1.73, for explosions in a homogeneous environment.