The merger of two neutron stars has been predicted to produce an optical–infrared transient (lasting a few days) known as a ‘kilonova’, powered by the radioactive decay of neutron-rich species synthesized in the merger1,2,3,4,5. Evidence that short γ-ray bursts also arise from neutron-star mergers has been accumulating6,7,8. In models2,9 of such mergers, a small amount of mass (10−4–10−2 solar masses) with a low electron fraction is ejected at high velocities (0.1–0.3 times light speed) or carried out by winds from an accretion disk formed around the newly merged object10,11. This mass is expected to undergo rapid neutron capture (r-process) nucleosynthesis, leading to the formation of radioactive elements that release energy as they decay, powering an electromagnetic transient1,2,3,9,10,11,12,13,14. A large uncertainty in the composition of the newly synthesized material leads to various expected colours, durations and luminosities for such transients11,12,13,14. Observational evidence for kilonovae has so far been inconclusive because it was based on cases15,16,17,18,19 of moderate excess emission detected in the afterglows of γ-ray bursts. Here we report optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and with a low-luminosity short-duration γ-ray burst20. Our observations, taken roughly every eight hours over a few days following the gravitational-wave trigger, reveal an initial blue excess, with fast optical fading and reddening. Using numerical models21, we conclude that our data are broadly consistent with a light curve powered by a few hundredths of a solar mass of low-opacity material corresponding to lanthanide-poor (a fraction of 10−4.5 by mass) ejecta.
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We are indebted to W. Rosing and the LCO staff for making these observations possible, and to the LIGO and Virgo science collaborations. We thank L. Singer, T. Piran and W. Fong for assistance with planning the LCO observing program. We appreciate assistance and guidance from the LIGO–Virgo Collaboration—Electromagnetic follow-up liaisons. We thank B. Tafreshi and G. M. Árnason for helping to secure Internet connections in Iceland while this paper was being reviewed. Support for I.A. and J.B. was provided by the National Aeronautics and Space Administration (NASA) through the Einstein Fellowship Program (via grant numbers PF6-170148 and PF7-180162, respectively). G.H., D.A.H. and C.M. are supported by US National Science Foundation (NSF) grant AST-1313484. D.P. and D.M. acknowledge support by Israel Science Foundation grant number 541/17. D.K. is supported in part by a Department of Energy (DOE) Early Career award DE-SC0008067, a DOE Office of Nuclear Physics award DE-SC0017616, and a DOE SciDAC award DE-SC0018297, and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the US Department of Energy under contract number DE-AC02-05CH11231. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US Department of Energy under contract number DE AC02-05CH11231. This research has made use of the NASA/IPAC Extragalactic Database, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The Digitized Sky Surveys were produced at the Space Telescope Science Institute (STScI) under US Government grant number NAG W-2166. The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council (later the UK Particle Physics and Astronomy Research Council), until June 1988, and thereafter by the Anglo-Australian Observatory. Supplementary funding for sky-survey work at the STScI is provided by the European Southern Observatory.
The authors declare no competing financial interests.
Reviewer Information Nature thanks R. Chevalier, C. Miller and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Figure 1 Timeline of the discovery and the observability of AT 2017gfo in the first 24 h following the merger.
The curved lines denote the airmass and altitude (in degrees above the horizon) of the position of AT 2017gfo on the sky at each LCO Southern Hemisphere site from the start of the night until the hour-angle limit of the LCO 1-m telescopes. The vertical thick lines denote the times when LCO images were obtained (colours correspond to the different filters as denoted in the legend of Fig. 3). AT 2017gfo was observable for approximately 1.5 h at the beginning of the night. Having three Southern Hemisphere sites allowed us to detect the kilonova approximately 6.5 h after the LIGO-Virgo localization, follow it approximately 10 h later, and continue to observe it three times per 24-h period for the following days (Fig. 3). Counterpart announcement is from ref. 31.
MCMC parameter distributions (a–f) and spectral energy distributions (luminosity density Lλ as a function of wavelength) with the blackbody fits (g–l) are shown for the six epochs (noted by their modified Julian dates, MJD) with observations in more than two bands after excluding w-band data. In the parameter distributions, contour lines denote 50% and 90% bounds, the red and blue solid lines overplotted on each histogram denote the mean and median of each parameter distribution (respectively), and the dashed lines denote 68% confidence bounds. Error bars on the luminosity densities denote 1σ uncertainties.
Extended Data Figure 3 Bolometric luminosity, photospheric radius and temperature deduced from blackbody fits.
Error bars denote 1σ uncertainties (n = 200). The large uncertainties in the later epochs might be due to a blackbody that peaks redward of our available data, so these data points should be considered to be temperature upper limits. Our MCMC fits of an analytical model32 to the bolometric luminosity are shown in blue, and the numerical models21 from Fig. 3 are shown in red in the top panel. The numerical models were tailored to fit Vriw bands, but not the g band, which is driving the high bolometric luminosity at early times.
Extended Data Figure 4 AT 2017gfo evolves faster than any known supernova, contributing to its classification as a kilonova.
We compare our w-band data of AT 2017gfo (red; arrows denote 5σ non-detection upper limits reported by others55,56) to r-band templates of common supernova types (types Ia and Ib/c normalized to peaks of −19 mag and −18 mag, respectively)50,51, to r-band data of two rapidly evolving supernovae52,53 (SN 2002bj and SN 2010X) and to R-band data of the drop from the plateau of the prototypical type IIP supernova54 SN 1999em (dashed line; shifted by 1 mag for clarity).
Extended Data Figure 5 Peak luminosity and time of AT 2017gfo compared to simple analytical predictions.
The parameters11 from equations (1) and (2) are shown for different values of the ejecta mass Mej (solid lines), the opacity κ (dashed lines), and for two different ejecta velocities vej (red and blue lines). The rise time and peak luminosity of AT 2017gfo (black arrow) can be reproduced by an ejecta velocity vej ≈ 0.3c and a low opacity of κ ≲ 1 cm2 g−1. Matching the data with higher opacities would require higher ejecta velocities.
Extended Data Figure 6 Parameter distribution for MCMC fits of analytical kilonova models32 to our bolometric light curve.
The contour lines denote 50% and 90% bounds. The red and blue solid lines overplotted on each histogram denote the mean and median of each parameter distribution (respectively). The dashed lines denote 68% confidence bounds. The fits converge on an ejecta mass of (4.02 ± 0.05) × 10−2M⊙ but they do not constrain the velocity (converging on the largest possible range) or the geometrical parameters (θej and Φej), nor do they reproduce the colour evolution of our event (not shown). This indicates that these models may not be entirely valid for AT 2017gfo (although in ref. 59 it is shown that the geometrical parameters cannot be constrained either way). Our numerical models21, on the other hand, which include detailed radiation transport calculations, do provide a good fit to the data (Fig. 3) with Mej = (2–2.5) × 10−2M⊙, vej = 0.3c, and a lanthanide mass fraction of Xlan = 10−4.5, corresponding to an effective opacity of κ ≲ 1 cm2 g−1.
The number of AT 2017gfo-like events per year detectable by r-band transient surveys in two (solid lines), three (dashed lines) and five (dotted lines) epochs before fading from view. The numbers of events refer to the entire sky, and should be multiplied by the fraction of sky covered by the survey. We assume that the intrinsic rate of events is one per year out to 40 Mpc (scaling accordingly to larger distances).
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Arcavi, I., Hosseinzadeh, G., Howell, D. et al. Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger. Nature 551, 64–66 (2017). https://doi.org/10.1038/nature24291
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