Could the Recent Rebrightening of the GW170817A Afterglow Be Caused by a Counterjet?

GRB 170817A (also GW170817) became the first binary neutron star merger event detected via gravitational waves and electromagnetic signals. Over the next 4 yr, various multiband observations have led to reimaging of the various short gamma-ray burst and interstellar medium interaction models. While these models successfully explain the observed afterglow until ∼900 days, a rebrightening or excess flux was observed in the 1 keV X-ray band after ∼1000 days. In this study, we reevaluate the jet parameters using new observations (until ∼1234 days) with a boosted fireball jet model. We study the observable effects of the counterjet for GRB 170817A, using our new afterglow code, firefly . Our results show that it is indeed possible for the observed excess to coincide with the emissions from a counterjet (∼800 days). We also computed an empirical scaling law between the jet and counterjet peak emission timescales and the observer angle. The firefly code can also track the simulated object through the observer’s sky and numerically model the apparent motion. The calculated apparent motion (≈2.6 c ) does not match the observed apparent motion (5.2 c −7.5 c ). Hence we conclude that the excess flux of GRB 170817A may not be associated with a counterjet; however, it is not enough to reject this hypothesis from the traditional counterjet visibility timescale, which predicts ≥5000 days. The apparent motion, combined with the multiband lightcurves, is needed to break the degeneracy between the geometrical parameters and the microphysical parameters of the afterglow.