GW170817 4.5 years after merger: Dynamical ejecta afterglow constraints

TL;DR

This study uses deep radio observations 4.5 years after GW170817 to search for late-time emission from dynamical ejecta, providing new constraints on ejecta properties and neutron star characteristics.

Contribution

It offers the first late-time radio constraints on dynamical ejecta after GW170817, refining models of ejecta energy and speed distributions and implications for neutron star mass.

Findings

No excess radio emission detected at 3 GHz, confirming previous results.

Constraints favor single-speed ejecta with energy less than 10^50 erg.

Results imply larger maximum neutron star masses in equal mass scenarios.

Abstract

GW170817 is the first binary neutron star (NS) merger detected in gravitational waves (GWs) and photons, and so far remains the only GW event of its class with a definitive electromagnetic (EM) counterpart. Radio emission from the structured jet associated with GW170817 has faded below the sensitivity achievable via deep radio observations with the most sensitive radio arrays currently in operation. Hence, we now have the opportunity to probe the radio re-brightening that some models predict, should emerge at late times from the interaction of the dynamically-stripped merger ejecta with the interstellar medium. Here we present the latest results from our deep radio observations of the GW170817 field with the Karl G. Jansky Very Large Array (VLA), 4.5 years after the merger. Our new data at $3\,$GHz do not show any compelling evidence for emission in excess to the tail of the jet afterglow ($<3.3\,\mu$Jy), confirming our previous results. We thus set new constraints on the dynamical ejecta afterglow models. These constraints favor single-speed ejecta with energy $\lesssim 10^{50}\,$erg (for an ejecta speed of $\beta_0=0.5$), or steeper energy-speed distributions of the kilonova ejecta. Our results also suggest larger values of the cold, non-rotating maximum NS mass in equal mass scenarios. However, without a detection of the dynamical ejecta afterglow, obtaining precise constraints on the NS equation of state remains challenging.