A turnover in the radio light curve of GW170817

TL;DR

This study reports radio observations of GW170817 showing the afterglow peaked around 149 days post-merger and is now declining, indicating mildly- or non-relativistic ejecta without evidence for a highly energetic off-axis jet.

Contribution

First detailed radio monitoring of GW170817's afterglow over 125-200 days revealing peak timing and decay, constraining the energy and geometry of the ejecta.

Findings

Radio afterglow peaked at 149 days post-merger.

No evidence for a highly energetic off-axis jet.

Ejecta likely mildly- or non-relativistic.

Abstract

We present 2-9 GHz radio observations of GW170817 covering the period 125-200 days post-merger, taken with the Australia Telescope Compact Array and the Karl G. Jansky Very Large Array. Our observations demonstrate that the radio afterglow peaked at $149\pm2$ days post-merger and is now declining in flux density. We see no evidence for evolution in the radio-only spectral index, which remains consistent with optically-thin synchrotron emission connecting the radio, optical, and X-ray regimes. The peak implies a total energy in the synchrotron-emitting component of a ${\rm few}\times 10^{50}\,$erg. The temporal decay rate is most consistent with mildly- or non-relativistic material and we do not see evidence for a very energetic off-axis jet, but we cannot distinguish between a lower-energy jet and more isotropic emission.