Gamma-Ray Burst Afterglows In The Multi-Messenger Era: Numerical Models and Closure Relations

TL;DR

This paper develops a comprehensive framework for modeling gamma-ray burst afterglows from structured jets, especially relevant for off-axis observations like GW170817, introducing new closure relations and providing an open-source simulation tool.

Contribution

It extends traditional GRB afterglow models to include structured jets and off-axis viewing angles, with new closure relations and a publicly available numerical modeling package.

Findings

Derived new closure relations for structured jets.

Found the temporal slope depends on viewing and jet opening angles.

Provided an open-source Python package for modeling afterglows.

Abstract

Gamma-ray bursts (GRBs) associated with gravitational wave events are, and will likely continue to be, viewed at a larger inclination than GRBs without gravitational wave detections. As demonstrated by the afterglow of GW170817, this requires an extension of the common GRB afterglow models which typically assume emission from an on-axis top hat jet. We present a characterization of the afterglows arising from structured jets, providing a framework covering both successful and choked jets. We compute new closure relations for structured jets and compare with the established relations for energy injection and refreshed shock models. The temporal slope before the jet break is found to be a simple function of the ratio between the viewing angle and effective opening angle of the jet. A numerical model to calculate synthetic light curves and spectra is publicly available as the open source Python package afterglowpy.