# Late-Time Afterglow from Double-Sided Structured Jets: Application to GRB 170817A

## TL;DR

This paper models the late-time radio afterglow of GW170817/GRB 170817A considering a double-sided structured jet, predicting a detectable counter-jet emission that can reveal jet and environmental properties through high-precision radio observations.

## Contribution

It introduces a model for double-sided structured jets and predicts observable late-time radio features, enhancing understanding of jet structure and surrounding medium in neutron star mergers.

## Key findings

- Counter-jet emission emerges around 2500 days post-merger in radio bands.
- Enhanced counter-jet emission can produce a plateau or rebrightening in radio light curves.
- Counter-jet is generally too faint in X-ray bands to detect.

## Abstract

The broadband afterglow of GW170817/GRB 170817A is believed to be from an off-axis structured jet. The central engine of a gamma-ray burst usually launches a pair of outflows that move oppositely, it is reasonable to consider the emission from a double-sided structured jets, with a near-jet moving toward us and a counter-jet moving away from us. Assuming that the two branches of the jet have the same physical parameters, we have calculated their radio emission. It is found that the counter-jet component will emerge in the radio light curves $\sim$ 2500 days post-merger.It typically leads to a plateau in the light curve, thus could be hopefully revealed by high accuracy radio observations. We have also considered the possibilities that both branches have different parameters, and found that if some of the physical parameters of the counter-jet are evaluated more favorably than those of the near-jet, then the counter-jet emission will be enhanced and will clearly show up as an obvious plateau or even a rebrightening. For example, when the circum-burst medium encountered by the counter-jet is assumed to be 100 times denser than that of the near-jet, a remarkable radio plateau will appear at $\sim$ 600 days. However, in X-ray bands, the counter-jet component is generally too faint to be discerned. It is argued that the late radio observations of GW170817/GRB 170817A can help determine the key parameters and diagnose the environment of the event.