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Abstract

For the first $\sim3$ years 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 $< 0.01\,\rm{cm^{-3}}$. We report on observational evidence for an excess of
X-ray emission at $\delta t>900$ days after the merger. With $L_x\approx5\times
10^{38}\,\rm{erg\,s^{-1}}$ at 1234 days, the recently detected X-ray emission
represents a $\ge 3.2\,\sigma$ (Gaussian equivalent) deviation from the
universal post jet-break model that best fits the multi-wavelength afterglow at
earlier times. In the context of JetFit afterglow models, current data
represent a departure with statistical significance $\ge 3.1\,\sigma$,
depending on the fireball collimation, with the most realistic models showing
excesses at the level of $\ge 3.7\,\sigma$. A lack of detectable 3 GHz radio
emission suggests a harder broad-band 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 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.