On the afterglow from the receding jet of gamma-ray burst

TL;DR

This study provides a detailed numerical analysis of the receding jet's afterglow in gamma-ray bursts, revealing it is typically weak and difficult to detect, but can be enhanced under certain conditions.

Contribution

It offers a comprehensive numerical model including equal arrival time surface effects, showing the receding jet's afterglow is usually faint and rarely observable.

Findings

Receding jet emission peaks after 1000 days in radio bands.

Receding jet flux is about 4 orders of magnitude lower than forward jet in radio.

Optical receding jet emission is about 8 orders of magnitude weaker.

Abstract

According to popular progenitor models of gamma-ray bursts, twin jets should be launched by the central engine, with a forward jet moving toward the observer and a receding jet (or the counter jet) moving backwardly. However, in calculating the afterglows, usually only the emission from the forward jet is considered. Here we present a detailed numerical study on the afterglow from the receding jet. Our calculation is based on a generic dynamical description, and includes some delicate ingredients such as the effect of the equal arrival time surface. It is found that the emission from the receding jet is generally rather weak. In radio bands, it usually peaks at a time of $t \geq 1000$ d, with the peak flux nearly 4 orders of magnitude lower than the peak flux of the forward jet. Also, it usually manifests as a short plateau in the total afterglow light curve, but not as an obvious rebrightening as once expected. In optical bands, the contribution from the receding jet is even weaker, with the peak flux being $\sim 8$ orders of magnitude lower than the peak flux of the forward jet. We thus argue that the emission from the receding jet is very difficult to detect. However, in some special cases, i.e., when the circum-burst medium density is very high, or if the parameters of the receding jet is quite different from those of the forward jet, the emission from the receding jet can be significantly enhanced and may still emerge as a marked rebrightening. We suggest that the search for receding jet emission should mostly concentrate on nearby gamma-ray bursts, and the observation campaign should last for at least several hundred days for each event.