Abstract
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical–near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process)1,2,3. Such transients, named ‘macronovae’ or ‘kilonovae’4,5,6,7, are believed to be centres of production of rare elements such as gold and platinum8. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst9,10 at redshift z = 0.356, although findings indicating bluer events have been reported11. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source12 GW170817 and γ-ray burst13,14 GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models15,16. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.
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Acknowledgements
This work is based on observations made with the ESO telescopes at the Paranal Observatory under programmes ID 099.D-0382 (Principal Investigator (PI): E. Pian), 099.D-0622 (PI: P.D’A.), 099.D-0191 (PI: A. Grado) and with the REM telescope at the ESO La Silla Observatory under programme ID 35020 (PI: S. Campana). Gemini observatory data were obtained under programme GS-2017B-DD-1 (PI: L. P. Singer). We thank the Gemini Observatory for performing these observations, the ESO Director General for allocating discretionary time and the ESO operation staff for support. We thank D. Fugazza for technical support with operating the REM telescope remotely and REM telescope director E. Molinari. We acknowledge INAF for supporting the project ‘Gravitational Wave Astronomy with the first detections of adLIGO and adVirgo experiments—GRAWITA’ (PI: E.B.) and support from ASI grant I/004/11/3. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). M.M.K. acknowledges support from the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE grant number 1545949.
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Contributions
E. Pian and P.D’A. coordinated the work. J.S. reduced all the X-shooter spectra presented in Fig. 2 and wrote the relevant sections. M.T. developed the kilonova spectral models. E.C. assisted with the spectral analysis. P.A.M. linked the spectral observations with kilonova theory, coordinated their theoretical interpretation, matched the synthetic with observed spectra (Fig. 3), and wrote the corresponding parts of the paper. S. Campana coordinated the REM observations. S. Covino, A. Grado and A.M. reduced and analysed the optical photometry data (Fig. 1). M.M.K. provided the Gemini spectrum. D.M. assisted with early observation planning. G. Ghirlanda, G. Ghisellini and O.S.S. wrote the section on the off-beam jet (with contributions from L.A., M.G.B., Y.Z.F., Z.-P.J., B.P., T.P. and A.S.). D.W. assisted with the analysis of spectra using thermal models and with writing the paper. E.B. was the PI of the GRAvitational Wave Inaf TeAm (GRAWITA), which works on gravitational-wave electromagnetic follow-up programmes at ESO and other telescopes in Italy and the Canary Islands. M.B. liaised between the GRAWITA and LIGO-Virgo collaborations. A. Grado coordinated the ESO-VST observations. L.L. and F.G. developed the pipeline to reduce the VST data. N.R.T. and A.L. assisted with near-infrared data calibration issues. J.P.U.F., J.H. and C.K. helped write the paper and provided short-GRB expertise. L. Nicastro supervised the data flow and handling. S.P. and V.D. contributed to the data reduction and analysis of the X-shooter spectra. E. Palazzi, A.R., G.S. and G. Greco participated in the organization of the observations and image analysis and provided specific input for photometry calibration. L.T., S.Y. and S.B. contributed to the data analysis, with particular reference to interstellar medium spectral features. P.M. assisted with issues related to ESO policies and observation planning. All GRAWITA members contributed to several phases of the work, from the preparation of proposals, coordination with the LIGO–Virgo collaborations and activation of approved programmes at many facilities to data acquisition, reduction, analysis, interpretation and presentation.
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Extended data figures and tables
Extended Data Figure 1 Image of the NGC 4993 galaxy.
The image was obtained with the X-shooter acquisition camera (z filter). The X-shooter slit is overlaid as a rectangle. The position of the optical transient is marked by a blue circle. The position of the line emission in the slit is marked by an ellipse. The dust lanes that are visible in the host intersect the slit at the position of the line emission.
Extended Data Figure 2 Blackbody fit to the AT 2017gfo spectra.
The two early X-shooter spectra of GW170817, obtained 1.5 and 3.5 days after discovery, are compared with the spectra of the type-Ib supernova SN 2008D59, obtained 2–5 days after the explosion (light grey, arbitrarily scaled in flux, ×10−16). The shaded areas represent wavelength intervals with low atmospheric transmission. The dotted green lines show the black-body fits of the optical continuum of GW170817 with temperature 5,000 K and 3,200 K.
Extended Data Figure 3 Two-dimensional image of the AT 2017gfo spectrum.
Top, the rectified, X-shooter two-dimensional image. The dark line visible across the entire spectral window is the bright continuum of the optical transient and the offset. The dark green blobs indicate the position of the line emission from N ii λ ≈ 6,549 Å, Hα and N ii λ ≈ 6,583 Å. Bottom, the line emission and the line fits. The integrated line fluxes are given, normalized by a factor of 10−17 for clarity. The error bars on the black points represent the individual 1σ spectral uncertainties. The blue shaded area represents 1σ uncertainty.
Extended Data Figure 4 Off-axis GRB afterglow model.
Synthetic X-ray (black curve), optical (dark grey curve) and radio (light grey curve) light curves of the GRB afterglow, as predicted by an off-axis jet model, derived using standard afterglow dynamics and radiation codes78. The filled circle shows the X-ray detection23 and the squares with arrows show two representative radio upper limits76,77.
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Pian, E., D’Avanzo, P., Benetti, S. et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature 551, 67–70 (2017). https://doi.org/10.1038/nature24298
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