The binary neutron star merger GW170817 was the first multi-messenger event observed in both gravitational and electromagnetic waves1,2. The electromagnetic signal began approximately two seconds post-merger with a weak, short burst of gamma rays3, which was followed over the next hours and days by the ultraviolet, optical and near-infrared emission from a radioactively powered kilonova4,5,6,7,8,9,10,11. Later, non-thermal rising X-ray and radio emission was observed12,13. 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 ejecta13,14,15. 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 blue16,17. 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 gamma-ray burst 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.
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Based on observations made with the NASA/European Space Agency Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. These observations are associated with programmes GO 14771 (N.R.T.) and GO 14270 (A.J.L.). We thank the staff at STScI for their excellent support of these observations. A.J.L. acknowledges that this project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no 725246). J.D.L., A.J.L., D.S. and K.W. acknowledge support from the Science and Technology Facilities Council (STFC) via grant ST/P000495/1. N.R.T., P.O., J.P.O., G.P.L., I.M. and S.K. acknowledge support from STFC. G.P.L. acknowledges partial support from the Royal Astronomical Society and International Astronomical Union grants. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). The Cosmic Dawn Center is funded by the Danish National Research Foundation. A.d.U.P., C.C.T. and Z.C. acknowledge support from the Spanish project AYA 2014-58381-P. Z.C. also acknowledges support from the Juan de la Cierva Incorporación fellowship IJCI-2014-21669. M.I. acknowledges the support from the National Research Foundation of Korea grant, No. 2017R1A3A3001362. S.R. has been supported by the Swedish Research Council (VR) under grant number 2016- 03657_3, by the Swedish National Space Board under grant number Dnr. 107/16 and by the research environment grant ‘Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT)’ funded by the Swedish Research council (VR) under Dnr 2016-06012. P.A.E. acknowledges United Kingdom Space Agency support. D.J.W. is supported by the the Danish Agency for Science, Technology and Innovation under grant number DFF – 7014-00017. G.P.L. thanks A. Higgins and L. Raynard for useful conversations regarding Markov chain Monte Carlo, and G.P.L. and S.K. thank E. Nakar for helpful comments. I.M. thanks J. Granot for useful discussions.