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For the first $\sim3$ years after the binary neutron star merger event GW170817 the radio and X-ray radiation has been dominated by emission from astructured relativistic off-axis jet propagating into a low-density medium withn $< 0.01\,\rm{cm^{-3}}$. We report on observational evidence for an excess ofX-ray emission at $\delta t>900$ days after the merger. With $L_x\approx5\times10^{38}\,\rm{erg\,s^{-1}}$ at 1234 days, the recently detected X-ray emissionrepresents a $\ge 3.2\,\sigma$ (Gaussian equivalent) deviation from theuniversal post jet-break model that best fits the multi-wavelength afterglow atearlier times. In the context of JetFit afterglow models, current datarepresent a departure with statistical significance $\ge 3.1\,\sigma$,depending on the fireball collimation, with the most realistic models showingexcesses at the level of $\ge 3.7\,\sigma$. A lack of detectable 3 GHz radioemission suggests a harder broad-band spectrum than the jet afterglow. Theseproperties are consistent with the emergence of a new emission component suchas synchrotron radiation from a mildly relativistic shock generated by theexpanding merger ejecta, i.e. a kilonova afterglow. In this context, we presenta set of ab-initio numerical-relativity BNS merger simulations that show thatan X-ray excess supports the presence of a high-velocity tail in the mergerejecta, and argues against the prompt collapse of the merger remnant into ablack hole. Radiation from accretion processes on the compact-object remnantrepresents a viable alternative. Neither a kilonova afterglow noraccretion-powered emission have been observed before, as detections of BNSmergers at this phase of evolution are unprecedented.

Evidence for X-ray Emission in Excess to the Jet Afterglow Decay 3.5 yrs After the Binary Neutron Star Merger GW 170817: A New Emission Component