  The binary neutron star merger GW170817 was the first multi-messenger event
observed in both gravitational and electromagnetic waves. The electromagnetic
signal began approximately 2 seconds post-merger with a weak, short burst of
gamma-rays, which was followed over the next hours and days by the ultraviolet,
optical and near-infrared emission from a radioactively- powered kilonova.
Later, non-thermal rising X-ray and radio emission was observed. The low
luminosity of the gamma-rays and the rising non-thermal flux from the source at
late times could indicate that we are outside the opening angle of the beamed
relativistic jet. Alternatively, the emission could be arising from a cocoon of
material formed from the interaction between a jet and the merger ejecta. Here
we present late-time optical detections and deep near-infrared limits on the
emission from GW170817 at 110 days post-merger. Our new observations are at
odds with expectations of late-time emission from kilonova models, being too
bright and blue. Instead, the emission arises from the interaction between the
relativistic ejecta of GW170817 and the interstellar medium. We show that this
emission matches the expectations of a Gaussian structured relativistic jet,
which would have launched a high luminosity short GRB to an aligned observer.
However, other jet structure or cocoon models can also match current data - the
future evolution of the afterglow will directly distinguish the origin of the
emission.
