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.

Automated summary

This paper reports on the excess of X-ray emission observed more than 900 days after the GW 170817 binary neutron star merger event, which deviates from the previous models and may be caused by the mildly relativistic shock generated by the merger ejecta.