Could the recent rebrightening of the GW170817A afterglow be caused by a counter jet?

TL;DR

This paper investigates whether the late-time rebrightening of GW170817A's afterglow could be caused by a counter jet, using new observations and a boosted fireball jet model, but finds the evidence inconclusive.

Contribution

The study introduces a new afterglow modeling code, Firefly, and analyzes the counter-jet's potential role in the observed rebrightening of GW170817A.

Findings

Counter-jet emission could explain the rebrightening around 800 days.

Apparent motion measurements do not match counter-jet predictions.

The excess flux is unlikely caused by the counter jet based on current data.

Abstract

GRB170817A (also GW170817) became the first binary neutron star (BNS) merger event detected via gravitational waves and electromagnetic signals. Over the next 4 years, various multiband observations have led to re-imagine the various short Gamma Ray Burts (sGRB) and interstellar medium interaction models. While these models successfully explain the observed afterglow until ~900 days, a re-brightening or excess flux was observed in the 1keV X-Ray band after ~1000 days. In this study, we re-evaluate the jet parameters using the new observations (until ~1234 days) with a boosted fireball jet model. We study the observable effects of the counter-jet for GRB170817A, using our new afterglow code, Firefly. Our results show that it is indeed possible for the observed excess to coincide with the emissions from the counter-jet (~800 days). We also computed an empirical scaling law between the jet and counter-jet peak emission timescales and the observer angle. The Firefly code can also track the simulated object through the observers' sky and numerically model the apparent motion. The calculated apparent motion (~2.6c) does not match the observed apparent motion (7.5c to 5.2c). Hence we conclude, the excess flux of GRB170817A may not be associated with the counter jet; however, it is not enough to reject this hypothesis from the traditional counter jet visibility time scales, which predicts > 5000 days. The apparent motion, combined with the multi-band lightcurves, is needed to break degeneracy between geometrical parameters and the microphysical parameters of the afterglow.