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

We present new observations of the binary neutron star merger GW170817 at $\Delta t\approx 220-260$ days post-merger, at radio (Karl G.~Jansky Very Large Array; VLA), X-ray ({\it Chandra X-ray Observatory}) and optical ({\it 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 structured jet expanding into a low density circumbinary medium. If future observations continue to track our predictions, we expect the radio and X-ray emission to remain detectable until $\sim 1000$ days post-merger.