Constraining the Jet Launching Time of GRB 170817A by Utilizing the Baryon Loading

TL;DR

This study models the structured jet of GRB 170817A to constrain its launch time, suggesting the delay is mainly due to jet propagation, with the launch occurring within tens to hundreds of milliseconds after the merger.

Contribution

It introduces a structured jet model with angle-dependent baryon loading to fit multi-wavelength afterglow data and infer jet launch timing.

Findings

Jet launch time is less than hundreds of milliseconds after merger.

The delay is primarily due to jet propagation time.

The model successfully fits multi-wavelength afterglow observations.

Abstract

The observed delay of GRB 170817A relative to GW170817 carries significant information about gamma-ray burst (GRB) physics and is subject to intense debate. In this letter, we put forward an approach to discuss the major source of this time delay. First of all, we use the structured jet model to fit the X-ray/optical/radio afterglows of GRB 170817A together with superluminal motion measured by the Very Long Baseline Interferometry. Our structured jet is modelled with angle-dependent energy and baryon loading. It is found that our model can well fit the afterglows of GRB 170817A. After that, the baryon loading in the jet is inferred based on our fitting results. By comparing the baryon loading to the mass outflow in different stages, we infer that the time lag of the jet launch relative to the merger is less than hundreds or tens of milliseconds. It suggests that the time delay of GRB 170817A relative to GW170817 is defined mostly by the spreading time of the jet propagating to its dissipation radius.