Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 Yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

For the first similar to 3 yrs after the binary neutron star merger event GW 170817, the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n < 0.01 cm(-3). We report on observational evidence for an excess of X-ray emission at delta t > 900 days after the merger. With L ( x ) approximate to 5 x 10(38) erg s(-1) at 1234 days, the recently detected X-ray emission represents a >= 3.2 sigma (Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance >= 3.1 sigma, depending on the fireball collimation, with the most realistic models showing excesses at the level of >= 3.7 sigma. A lack of detectable 3 GHz radio emission suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e., a kilonova afterglow. In this context, we present a set of ab initio numerical relativity binary neutron star (BNS) merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.