Prompt emission from the counter jet of a short gamma-ray burst

TL;DR

This paper predicts the optical-infrared prompt emission from the counter jet of short gamma-ray bursts, suggesting it is detectable and can help constrain jet properties, especially with future gravitational wave detectors.

Contribution

It introduces calculations of counter jet emission brightness and proposes a new method to determine jet parameters using both forward and counter jet observations.

Findings

Counter jet emission is brighter than early kilonova and detectable by LSST.

Counter jet optical-infrared brightness peaks 10-10^3 seconds post-merger.

Future GW detectors can forecast merger details, aiding counter jet detection.

Abstract

The counter jet of a short gamma-ray burst (sGRB) has not yet been observed, while recent discoveries of gravitational waves (GWs) from a binary neutron star (NS) merger GW170817 and the associated sGRB 170817A have demonstrated that off-axis sGRB jets are detectable. We calculate the prompt emission from the counter jet of an sGRB and show that it is typically 23-26 mag in the optical-infrared band 10-10^3 sec after the GWs for an sGRB 170817A-like event, which is brighter than the early macronova (or kilonova) emission and detectable by LSST in the near future. We also propose a new method to constrain the unknown jet properties, such as the Lorentz factor, opening angle, emission radii, and jet launch time, by observing both the forward and counter jets. To scrutinize the counter jets, space GW detectors like DECIGO are powerful in forecasting the merger time (<~ 1 sec) and position (<~ 1 arcmin) (~ a week) before the merger.