  The binary neutron-star (BNS) merger GW170817 is the first celestial object
from which both gravitational waves (GWs) and light have been detected enabling
critical insight on the pre-merger (GWs) and post-merger (light) physical
properties of these phenomena. For the first $\sim 3$ years after the merger
the detected radio and X-ray radiation has been dominated by emission from a
structured relativistic jet initially pointing $\sim 15-25$ degrees away from
our line of sight and propagating into a low-density medium. Here we report on
observational evidence for the emergence of a new X-ray emission component at
$\delta t>900$ days after the merger. The new component has luminosity $L_x
\approx 5\times 10^{38}\rm{erg s^{-1}}$ at 1234 days, and represents a $\sim
3.5\sigma$ - $4.3\sigma$ excess compared to the expectations from the off-axis
jet model that best fits the multi-wavelength afterglow of GW170817 at earlier
times. A lack of detectable radio emission at 3 GHz around the same time
suggests a harder broadband spectrum than the jet afterglow. These properties
are consistent with synchrotron emission from a mildly relativistic shock
generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this
context our simulations show that the 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. However, radiation from accretion
processes on the compact-object remnant represents a viable alternative to the
kilonova afterglow. Neither a kilonova afterglow nor accretion-powered emission
have been observed before.
