A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet

TL;DR

This study reports the continued decline in X-ray and radio emissions from GW170817, supporting the structured jet model over cocoon models, with observations spanning over 200 days post-merger.

Contribution

First evidence of a turnover in X-ray light curve from GW170817, supporting structured jet models with multi-wavelength data analysis.

Findings

X-ray light curve shows a significant decline at >5σ

Radio-to-X-ray spectral energy distribution remains stable during decline

Data consistent with a successful structured jet expanding into low-density medium

Abstract

We present new observations of the binary neutron star merger GW170817 at $\Delta t\approx 220-290$ days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at $\gtrsim5\sigma$ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until $\sim 1000$ days post-merger.