A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817

Abstract

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.

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Figure 1: The radio light curve of GW170817.
Figure 2: Schematic of the various possible jet and dynamical ejecta scenarios in GW170817.
Figure 3: Off-axis jet models.
Figure 4: Quasi-spherical ejecta models.

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Acknowledgements

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.

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K.P.M., E.N., K.H., G.H. and D.A.F. wrote the paper. A.C. compiled the references. A.C. and A.H. compiled Methods. D.D. and K.D. compiled the radio measurements table. K.P.M. managed the VLA observing programme and processed all of the VLA data. S.T.M., A.D. and S.B. helped to plan the VLA observations. E.N., K.H., D.L.K. and K.P.M. prepared the figures. T.M. planned and managed ATCA observations and data analysis and contributed to the manuscript text. D.L.K. helped to propose and plan the ATCA observations and contributed to the manuscript text. E.L., D.D., C.L. and K.W.B. helped with ATCA observations and data reduction. K.D. planned and managed GMRT observations and contributed to the manuscript text. K.P.M. and P.C. processed the GMRT data. V.B. helped with the GMRT observations. O.G. and E.N. provided the cocoon simulation. K.H. provided the spherical ejecta model. S.N. did the gravitational-wave and cocoon-rates analysis. S.R.K., T.P., M.M.K. and L.P.S. provided text for the paper. All co-authors discussed the results and provided comments on the manuscript.

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Correspondence to K. P. Mooley.

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Extended data figures and tables

Extended Data Figure 1 GW170817 radio image cut-outs.

Image cut-outs (30″?×?30″) from uGMRT (a, d), VLA (b, e) and ATCA (c, f) are shown, centred on NGC 4993. The position of GW170817 is marked by two black lines. ac, Images from August–September 2017, using the data reported in ref. 12. df, Data from October and December 2017. The flux density is indicated by the colour scale in each column. The synthesized beam is shown as an ellipse in the lower right corner of each image. Dec, declination; RA, right ascension.

Extended Data Figure 2 Confidence region for the radio spectral and temporal indices.

Joint confidence contours for α (the spectral power-law index) and δ (the temporal power-law index) are shown. The contours are 1σ, 2σ and 3σ confidence contours, and the location of the best-fit values, α?=?−0.61?±?0.05 and δ?=?0.78?±?0.05, is indicated by the red cross.

Extended Data Figure 3 Radio-only spectral indices of GW170817.

Radio spectral indices between 0.6 GHz and 15 GHz spanning multiple epochs (colour-coded) are shown. The corresponding days after the merger was detected and spectral indices are given in the legend. Error bars are 1σ. The joint analysis of all radio data (Methods) implies α?=?−0.61?±?0.05.

Extended Data Figure 4 Comparison between the radio and X-ray flux densities of GW170817.

The X-ray data are compared to the radio upper limits (arrows) and detections (filled symbols) at different epochs (colour-coded, see legend, and marked with different symbols). The difference epochs are 2017 August 19, August 26–28, September 2–3 and November 18 (2 days, around 10 days, around 15 days and 93 days after the merger was detected, respectively). Error bars are 1σ. The spectral index α?=?−0.60?±?0.03 and corresponding electron power-law index p?=?2.20?±?0.06 (assuming that the cooling frequency is beyond 1018?Hz, as expected for a mildly relativistic outflow) between 3 GHz and 1018?Hz, as derived from the September 2–3 data, are consistent with the radio-only spectral indices, and shown here as a dashed grey line. This indicates that the radio and X-ray emission originate from the same synchrotron source. The corresponding predicted soft-X-ray flux density on November 18 (0.3–2.2 nJy) is shown as an open magenta circle with an error bar. Note that the Chandra X-ray observations from December 3–6, reported after the submission of this paper, confirm this prediction. The flux densities in the ultraviolet (around 1015?Hz) and near-infrared (around 1014 Hz) bands, which are dominated by thermal emission at early times, are shown for reference.

Extended Data Table 1 Radio data for GW170817

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Mooley, K., Nakar, E., Hotokezaka, K. et al. A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817. Nature 554, 207–210 (2018). https://doi.org/10.1038/nature25452

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