GW170817 was the first gravitational-wave detection of a binary neutron-star merger1. It was accompanied by radiation across the electromagnetic spectrum and localized2 to the galaxy NGC 4993 at a distance of 40 megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet being launched during the merger (and successfully breaking out of the surrounding material), directed away from our line of sight (off-axis)3,4,5,6. The presence of such a jet is predicted from models that posit neutron-star mergers as the drivers of short hard-γ-ray bursts7,8. Here we report that the radio light curve of GW170817 has no direct signature of the afterglow of an off-axis jet. Although we cannot completely rule out the existence of a jet directed away from the line of sight, the observed γ-ray emission could not have originated from such a jet. Instead, the radio data require the existence of a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material that was ejected dynamically during the merger, or a cocoon of material that breaks out when a jet launched during the merger transfers its energy to the dynamical ejecta. Because the cocoon model explains the radio light curve of GW170817, as well as the γ-ray and X-ray emission (and possibly also the ultraviolet and optical emission)9,10,11,12,13,14,15, it is the model that is most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a hitherto unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.
We acknowledge the support and dedication of the staff of the National Radio Astronomy Observatory and particularly thank the VLA Director, M. McKinnon, as well as A. Mioduszewski and H. Medlin, for making the VLA campaign possible. We thank B. Griswold (NASA/GSFC) for beautiful graphic arts (Fig. 2). S.R.K. thanks M. Shull for discussions. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank the GMRT staff for scheduling our observations. The GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. The Australia Telescope Compact Array is part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a National Facility managed by CSIRO. K.P.M. is a Hintze Fellow and so is supported by the Hintze Centre for Astrophysical Surveys, which is funded through the Hintze Family Charitable Foundation. E.N. acknowledges the support of an ERC starting grant (GRB/SN) and an ISF grant (1277/13). G.H. acknowledges the support of NSF award AST-1654815. A.C. acknowledges support from the National Science Foundation CAREER award number 1455090 titled ‘CAREER: Radio and gravitational-wave emission from the largest explosions since the Big Bang’. A.H. acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. T.M. acknowledges the support of the Australian Research Council through grant FT150100099. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. D.L.K. was supported by NSF grant AST-1412421. M.M.K.’s work was supported by the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the NSF under PIRE grant number 1545949. This work is part of the research programme Innovational Research Incentives Scheme (Vernieuwingsimpuls), which is financed by the Netherlands Organization for Scientific Research through NWO VIDI grant number 639.042.612-Nissanke and NWO TOP grant number 62002444-Nissanke. P.C. acknowledges support from the Department of Science and Technology via SwarnaJayanti Fellowship awards (DST/SJF/PSA-01/2014-15). T.P. acknowledges the support of the Advanced ERC grant TReX. V.B. acknowledges the support of the Science and Engineering Research Board, Department of Science and Technology, India, for the GROWTH-India project.