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Published November 2, 2017 | Submitted + Supplemental Material
Journal Article Open

A kilonova as the electromagnetic counterpart to a gravitational-wave source


Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of −1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90–140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

Additional Information

© 2017 Macmillan Publishers Limited, part of Springer Nature. Received 14 September 2017; Accepted 25 September 2017; Published online 16 October 2017. Data availability: The reduced, calibrated spectral data presented in this paper are openly available on the Weizmann Interactive Supernova data REPository (https://wiserep.weizmann.ac.il) and at the ePESSTO project website http://www.pessto.org. The raw data from the VLT, NTT and GROND (for spectra and imaging) are available from the ESO Science Archive facility at http://archive. eso.org. The raw pixel data from Pan-STARRS1 and the 1.5m Boyden telescope are available from the authors on request. Code availability: The light curve fitting code described here is publicly available at the following website: https://star.pst.qub.ac.uk/wiki/doku.php/users/ajerkstrand/ lightcurvecodes. A code to produce the posteriors in this paper is available at: https://github.com/mcoughlin/gwemlightcurves. TARDIS is an open-source Monte Carlo radiative-transfer spectral synthesis code for one-dimensional models of supernova ejecta and is publicly available at https://tardis.readthedocs.io/en/latest/. Standard software within the IRAF environment was used to carry out the spectral and imaging reductions and photometry. This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, as part of ePESSTO (the extended Public ESO Spectroscopic Survey for Transient Objects Survey) ESO programme 199.D-0143 and 099.D-0376. We thank ESO staff for their support at La Silla and Paranal and for making the NACO and VISIR data public to LIGO–Virgo collaborating scientists. We thank J. Ward for permitting a time switch on the NTT. Part of the funding for GROND was generously granted from the Leibniz Prize to G. Hasinger (DFG grant HA 1850/28-1). Pan-STARRS1 and ATLAS are supported by NASA grants NNX08AR22G, NNX12AR65G, NNX14AM74G and NNX12AR55G issued through the SSO Near Earth Object Observations Program. We acknowledge help in obtaining GROND data from A. Hempel, M. Rabus and R. Lachaume on La Silla. The Pan-STARRS1 Surveys were made possible by the IfA, University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society, MPIA Heidelberg and MPE Garching, Johns Hopkins University, Durham University, the University of Edinburgh, Queen's University Belfast, Harvard-Smithsonian Center for Astrophysics, Las Cumbres Observatory Global Telescope Network Incorporated, National Central University of Taiwan, Space Telescope Science Institute, the National Science Foundation under grant number AST-1238877, the University of Maryland, and Eotvos Lorand University (ELTE) and the Los Alamos National Laboratory. We acknowledge EU/FP7-ERC grants 291222 and 615929 and STFC funding through grants ST/P000312/1 and ERF ST/M005348/1. A.J. acknowledges Marie Sklodowska-Curie grant number 702538. M.G., A.H., K.A.R. and Ł.W. acknowledge the Polish NCN grant OPUS 2015/17/B/ST9/03167, J.S. is funded by the Knut and Alice Wallenberg Foundation. C.B., M.D.V., N.E.-R., A.P. and G.T. are supported by the PRIN-INAF 2014. M.C. is supported by the David and Ellen Lee Prize Postdoctoral Fellowship at the California Institute of Technology. M.F. is supported by a Royal Society Science Foundation Ireland University Research Fellowship. M.S. and C.I. acknowledge support from EU/FP7-ERC grant number 615929. P.G.J. acknowledges the ERC consolidator grant number 647208. GREAT is funded by V.R. J.D.L. acknowledges STFC grant ST/P000495/1. T.W.C., P.S. and P.W. acknowledge support through the Alexander von Humboldt Sofja Kovalevskaja Award. J.H. acknowledges financial support from the Vilho, Yrjö and Kalle Väisälä Foundation. J.V. acknowledges FONDECYT grant number 3160504. L.G. was supported in part by the US National Science Foundation under grant AST-1311862. MB acknowledges support from the Swedish Research Council and the Swedish Space Board. A.G.-Y. is supported by the EU via ERC grant number 725161, the Quantum Universe I-Core programme, the ISF, the BSF and by a Kimmel award. L.S. acknowledges IRC grant GOIPG/2017/1525. A.J.R. is supported by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO) through project number CE110001020. I.R.S. was supported by the Australian Research Council grant FT160100028. We acknowledge Millennium Science Initiative grant IC120009. This paper uses observations obtained at the Boyden Observatory, University of the Free State, South Africa. Contributions: S.J.S. is Principal Investigator of ePESSTO and co-lead for the Pan-STARRS gravitational-wave follow-up. S.J.S. led the writing of the text and managed the project. A.J. wrote the light curve fitting code, led the modelling and co-wrote the text. M.C. provided code for modelling and Markov chain Monte Carlo analysis, provided analysis and text. K.W. provided input. S.A.S., L.J.S. and M.M. did the TARDIS modelling, assisted by A.G.-Y. in line identification. T.-W.C., J.G., S.K., A.R., P.S., T.S., T.K., P.W. and A.N.G. managed, executed, reduced and provided GROND data. E.K., M.F., C.I., K.M., T.K. and G.L. reduced and analysed photometry and spectra and contributed to analysis, text and figures. K.W.S. and D.R.Y. ran data management for ATLAS and Pan-STARRS analysis. J.T. is the ATLAS lead and provided data. K.C.C. is the Pan-STARRS director, the co-lead of the gravitational-wave follow-up and managed the observing sequences. Pan-STARRS and ATLAS data and products were provided by the team of M.E.H., J.B., L.D., H.F., T.B.L., E.A.M., A.R., A.S.B.S., B.S., R.J.W., C.W., H.W., M.W. and D.E.W. C.W.S. is the ATLAS co-lead for the gravitational-wave follow-up and contributed to the text. O.M.B. and P.C. checked the ATLAS data for candidates and O.M.B. provided manuscript editing support. The ePESSTO project was delivered by the following, who have contributed to data, analysis and text comments: R.K., J.D.L., D.S.H., C.A., J.P.A., C.R.A., C.A., C.B., F.E.B., M.B., M.B., Z.C., R.C., A.C., P.C., A.D.C., M.T.B., M.D.V., M.D., G.D., N.E.-R., R.E.F., A. Flörs, A. Franckowiak, C.F., L.B., S.G.-G., M.G., C.P.G., A.H., J.H., K.E.H., A.H., M.-S.H., S.T.H., I.M.H., L.I., P.A.J., P.G.J., Z.K.-R., M. Kowalski, M. Kromer, H.K., A.L., I.M., S.M., J.N., D.O'N., F.O., J.T.P., A.P., F.P., G.P., M.L.P., S.J.P., T.R., R.R., A.J.R., K.A.R., I.R.S., M.S., J.S., M.S., F.T., S.T., G.T., J.V., N.A.W., Ł.W., O.Y., G.C. and A.R. P.P. provided text and analysis comments. The 1.5B telescope data were provided, reduced and analysed by L.H., A.M.-C., L.S., H.S. and B.v.S. A.M. reduced and analysed the NACO and VISIR data. The authors declare no competing financial interests.

Attached Files

Submitted - 1710.05841.pdf

Supplemental Material - nature24303-sf1.jpg

Supplemental Material - nature24303-sf2.jpg

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Supplemental Material - nature24303-sf5.jpg

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Supplemental Material - nature24303-st2.jpg

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August 21, 2023
October 17, 2023