Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger

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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Figure 1: Multiband optical light curve of AT 2017gfo.
Figure 2: Time evolution of the AT 2017gfo spectra.
Figure 3: Kilonova models compared with the AT 2017gfo spectra.

References

  1. 1

    Lattimer, J. M., Mackie, F., Ravenhall, D. G. & Schramm, D. N. The decompression of cold neutron star matter. Astrophys. J. 213, 225–233 (1977)

    CAS  ADS  Google Scholar 

  2. 2

    Eichler, D., Livio, M., Piran, T. & Schramm, D. Nucleosynthesis, neutrino bursts and gamma-rays from coalescing neutron stars. Nature 340, 126–128 (1989)

    ADS  Google Scholar 

  3. 3

    Li, L.-X. & Paczyn´ski, B. Transient events from neutron star mergers. Astrophys. J. 507, L59–L62 (1998)

    ADS  Google Scholar 

  4. 4

    Kulkarni, S. R. Modelling supernova-like explosions associated with gamma-ray bursts with short durations. Preprint at https://arxiv.org/abs/astro-ph/0510256 (2005)

  5. 5

    Tanaka, M. & Hotokezaka, K. Radiative transfer simulations of neutron star merger ejecta. Astrophys. J. 775, 113 (2013)

    ADS  Google Scholar 

  6. 6

    Rosswog, S. et al. Detectability of compact binary merger macronovae. Class. Quantum Gravity 34, 104001 (2017)

    ADS  Google Scholar 

  7. 7

    Wollaeger, R. T. et al. Impact of ejecta morphology and composition on the electromagnetic signatures of neutron star mergers. Preprint at https://arxiv.org/abs/1705.07084 (2017)

  8. 8

    Metzger, B. D. Kilonovae. Living Rev. Relativ. 20, 3 (2017)

    ADS  PubMed  PubMed Central  Google Scholar 

  9. 9

    Tanvir, N. R. et al. A ‘kilonova’ associated with the short-duration γ-ray burst GRB 130603B. Nature 500, 547–549 (2013)

    CAS  ADS  PubMed  PubMed Central  Google Scholar 

  10. 10

    Berger, E., Fong, W. & Chornock, R. An r-process kilonova associated with the short-hard GRB 130603B. Astrophys. J. 774, L23 (2013)

    ADS  Google Scholar 

  11. 11

    Jin, Z.-P. et al. The macronova in GRB 050709 and the GRB-macronova connection. Nat. Commun. 7, 12898 (2016)

    CAS  ADS  PubMed  PubMed Central  Google Scholar 

  12. 12

    The LIGO Scientific Collaboration and the Virgo Collaboration. GW170817: observation of gravitational waves from a binary neutron star inspiral. Phys. Rev. Lett. 119, 161101 (2017)

  13. 13

    Goldstein, A. et al. An ordinary short gamma-ray burst with extraordinary implications: Fermi-GBM detection of GRB 170817A. Astrophys. J. 848, https://doi.org/10.3847/2041-8213/aa8f41 (2017)

    ADS  Google Scholar 

  14. 14

    Savchenko, V. et al. INTEGRAL detection of the first prompt gamma-ray signal coincident with the gravitational event GW170817. Astrophys. J. 848, https://doi.org/10.3847/2041-8213/aa8f94 (2017)

    ADS  Google Scholar 

  15. 15

    Kasen, D., Fernández, R. & Metzger, B. D. Kilonova light curves from the disc wind outflows of compact object mergers. Mon. Not. R. Astron. Soc. 450, 1777–1786 (2015)

    CAS  ADS  Google Scholar 

  16. 16

    Tanaka, M. et al. Properties of kilonovae from dynamical and post-merger ejecta of neutron star mergers. Preprint at https://arxiv.org/abs/1708.09101 (2017)

  17. 17

    Coulter, D. A. et al. Swope Supernova Survey 2017a (SSS17a), the optical counterpart to a gravitational wave source. Science http://doi.org/10.1126/science.aap9811 (2017)

    CAS  ADS  PubMed  PubMed Central  Google Scholar 

  18. 18

    Valenti, S. et al. The discovery of the electromagnetic counterpart of GW170817: kilonova AT 2017gfo/DLT17ck. Astrophys. J. 848, https://doi.org/10.3847/2041-8213/aa8edf (2017)

    ADS  Google Scholar 

  19. 19

    Jones, D. H. et al. The 6dF galaxy survey: final redshift release (DR3) and southern large-scale structures. Mon. Not. R. Astron. Soc. 399, 683–698 (2009)

    CAS  ADS  Google Scholar 

  20. 20

    Kasliwal, M. M. et al. Illuminating gravitational waves: a concordant picture of photons from a neutron star merger. Science http://doi.org/10.1126/science.aap9455 (2017)

    CAS  ADS  PubMed  PubMed Central  Google Scholar 

  21. 21

    Cardelli, J. A., Clayton, G. C. & Mathis, J. S. The relationship between infrared, optical, and ultraviolet extinction. Astrophys. J. 345, 245–256 (1989)

    CAS  ADS  Google Scholar 

  22. 22

    Malesani, D. et al. LIGO/Virgo G298048: optical spectral energy distribution of SSS17a. GCN Circ. 21577 (2017)

  23. 23

    Troja, E. et al. The X-ray counterpart to the gravitational-wave event GW170817. Nature http://doi.org/10.1038/nature24290 (2017)

    ADS  Google Scholar 

  24. 24

    Patricelli, B. et al. Prospects for joint observations of gravitational waves and gamma rays from merging neutron star binaries. J. Cosmol. Astropart. Phys. 11, 56 (2016)

    ADS  Google Scholar 

  25. 25

    Salafia, O. S., Ghisellini, G., Pescalli, A., Ghirlanda, G. & Nappo, F. Structure of gamma-ray burst jets: intrinsic versus apparent properties. Mon. Not. R. Astron. Soc. 450, 3549–3558 (2015)

    CAS  ADS  Google Scholar 

  26. 26

    Lazzati, D. et al. Off-axis prompt X-ray transients from the cocoon of short gamma-ray bursts. Preprint at https://arxiv.org/abs/1709.01468 (2017)

  27. 27

    Nakar, E. & Piran, T. The observable signatures of GRB cocoons. Astrophys. J. 834, 28 (2016)

    ADS  Google Scholar 

  28. 28

    Bannister, K. et al. LIGO/Virgo G298048: ATCA detection of a radio source coincident with NGC 4993. GCN Circ. 21559 (2017)

  29. 29

    Evans, P. A. et al. Swift and NuSTAR observations of GW170817: detection of a blue kilonova. Science http://doi.org/10.1126/science.aap9580 (2017)

    CAS  ADS  PubMed  PubMed Central  Google Scholar 

  30. 30

    Fong, W. & Berger, E. Hubble Space Telescope observations of short gamma-ray burst host galaxies: morphologies, offsets, and local environments. Astrophys. J. 708, 9–25 (2010)

    CAS  ADS  Google Scholar 

  31. 31

    Belczynski, K. et al. A study of compact object mergers as short gamma-ray burst progenitors. Astrophys. J. 648, 1110–1116 (2006)

    CAS  ADS  Google Scholar 

  32. 32

    Levan, A. J. et al. The environment of the binary neutron star merger GW170817. Astrophys. J. 848, https://doi.org/10.3847/2041-8213/aa905f (2017)

    ADS  Google Scholar 

  33. 33

    Chincarini, G. et al. The last born at La Silla: REM, the Rapid Eye Mount. Messenger 113, 40–44 (2003)

    ADS  Google Scholar 

  34. 34

    Melandri, A. et al. LIGO/Virgo G298048: REM optical/NIR observations of candidate in NGC 4993. GCN Circ. 21532 (2017)

  35. 35

    Melandri, A. et al. LIGO/Virgo G298048: REM optical/NIR observations. GCN Circ. 21556 (2017)

  36. 36

    Pian, E. et al. LIGO/Virgo G298048: GRAWITA VLT/X-shooter observations and tentative redshift of SSS17a. GCN Circ. 21592 (2017)

  37. 37

    D’Avanzo, P. et al. LIGO/Virgo G298048: ESO/VLT optical observations. GCN Circ. 21653 (2017)

  38. 38

    Grado, A. et al. LIGO/VIRGO G298048: INAF VST-ESO PARANAL observations. GCN Circ. 21598 (2017)

  39. 39

    Grado, A. et al. LIGO/VIRGO G298048: INAF VST-ESO PARANAL observations of NGC4993. GCN Circ. 21703 (2017)

  40. 40

    Bertin, E. & Arnouts, S. SExtractor: software for source extraction. Astron. Astrophys. Suppl. Ser. 117, 393–404 (1996)

    ADS  Google Scholar 

  41. 41

    Capaccioli, M. & Schipani, P. The VLT survey telescope opens to the sky: history of a commissioning. Messenger 146, 2–7 (2011)

    ADS  Google Scholar 

  42. 42

    Kuijken, K. OmegaCAM: ESO’s newest imager. Messenger 146, 8–11 (2011)

    ADS  Google Scholar 

  43. 43

    Grado, A., Capaccioli, M., Limatola, L. & Getman, F. VST processing facility: first astronomical applications. Mem. Soc. Astron. It. Suppl. 19, 362 (2012)

    Google Scholar 

  44. 44

    Tody, D. IRAF in the Nineties. In Astronomical Data Analysis Software and Systems II (eds Hanisch, R. J. et al. .) 173–183 (ASP Conf. Ser. Vol. 52, 1993)

    Google Scholar 

  45. 45

    Vernet, J. et al. X-shooter, the new wide band intermediate resolution spectrograph at the ESO Very Large Telescope. Astron. Astrophys. 536, A105 (2011)

    Google Scholar 

  46. 46

    Freudling, W. et al. Automated data reduction workflows for astronomy. The ESO Reflex environment. Astron. Astrophys. 559, A96 (2013)

    Google Scholar 

  47. 47

    Modigliani, A. et al. The X-shooter pipeline. Proc. SPIE 7737, http://doi.org/10.1117/12.857211 (2010)

  48. 48

    Moehler, S. et al. Flux calibration of medium-resolution spectra from 300 nm to 2500 nm: model reference spectra and telluric correction. Astron. Astrophys. 568, A9 (2014)

    Google Scholar 

  49. 49

    Noll, S. et al. An atmospheric radiation model for Cerro Paranal. I. The optical spectral range. Astron. Astrophys. 543, A92 (2012)

    Google Scholar 

  50. 50

    Jones, A. et al. An advanced scattered moonlight model for Cerro Paranal. Astron. Astrophys. 560, A91 (2013)

    Google Scholar 

  51. 51

    Smette, A. et al. Molecfit: a general tool for telluric absorption correction. I. Method and application to ESO instruments. Astron. Astrophys. 576, A77 (2015)

    Google Scholar 

  52. 52

    Kausch, W. et al. Molecfit: a general tool for telluric absorption correction. II. Quantitative evaluation on ESO-VLT/X-Shooter spectra. Astron. Astrophys. 576, A78 (2015)

    Google Scholar 

  53. 53

    Horne, W. An optimal extraction algorithm for CCD spectroscopy. Publ. Astron. Soc. Pac. 98, 609–617 (1986)

    CAS  ADS  Google Scholar 

  54. 54

    Fried, D. L. Limiting resolution looking down through the atmosphere. J. Opt. Soc. Am. 56, 1380–1384 (1966)

    ADS  Google Scholar 

  55. 55

    Poznanski, D., Prochaska, J. X. & Bloom, J. S. An empirical relation between sodium absorption and dust extinction. Mon. Not. R. Astron. Soc. 426, 1465–1474 (2012)

    CAS  ADS  Google Scholar 

  56. 56

    Munari, U. & Zwitter, T. Equivalent width of Na I and K I lines and reddening. Astron. Astrophys. 318, 269–274 (1997)

    ADS  Google Scholar 

  57. 57

    Munari, U. et al. Diffuse interstellar bands in RAVE survey spectra. Astron. Astrophys. 488, 969–973 (2008)

    CAS  ADS  Google Scholar 

  58. 58

    Schlafly, E. F. & Finkbeiner, D. P. Measuring reddening with Sloan Digital Sky Survey stellar spectra and recalibrating SFD. Astrophys. J. 737, 103 (2011)

    ADS  Google Scholar 

  59. 59

    Mazzali, P., Valenti, S. & Della Valle, M. The metamorphosis of supernova SN 2008D/XRF 080109: a link between supernovae and GRBs/hypernovae. Science 321, 1185–1188 (2008)

    CAS  ADS  PubMed  PubMed Central  Google Scholar 

  60. 60

    Harutyunyan, A. et al. ESC supernova spectroscopy of non-ESC targets. Astron. Astrophys. 488, 383–399 (2008)

    ADS  Google Scholar 

  61. 61

    Pan, Y.-C. The old host-galaxy environment of SSS17a, the first electromagnetic counterpart to a gravitational-wave source. Astrophys. J. 848, https://doi.org/10.3847/2041-8213/aa9116 (2017)

    ADS  Google Scholar 

  62. 62

    Levan, A. J. et al. LIGO/Virgo G298048: MUSE Integral field observations. GCN Circ. 21681 (2017)

  63. 63

    Hallinan, G. et al. A radio counterpart to a neutron star merger. Science http://doi.org/10.1126/science.aap9855 (2017)

    CAS  ADS  PubMed  PubMed Central  Google Scholar 

  64. 64

    Alexander, K. D. et al. The electromagnetic counterpart of the binary neutron star merger LIGO/VIRGO GW170817. VI. Radio constraints on a relativistic jet and predictions for late-time emission from the kilonova ejecta. Astrophys. J. 848, https://doi.org/10.3847/2041-8213/aa905d (2017)

    ADS  Google Scholar 

  65. 65

    Goldstein, A. et al. LIGO/Virgo G298048: update on Fermi/GBM GRB 170817A analysis. GCN Circ. 21528 (2017)

  66. 66

    D’Avanzo, P. et al. A complete sample of bright Swift short gamma-ray bursts. Mon. Not. R. Astron. Soc. 442, 2342–2356 (2014)

    ADS  Google Scholar 

  67. 67

    Berger, E. Short-duration gamma-ray bursts. Annu. Rev. Astron. Astrophys. 52, 43–105 (2014)

    ADS  Google Scholar 

  68. 68

    Ghisellini, G. et al. Are GRB980425 and GRB031203 real outliers or twins of GRB060218? Mon. Not. R. Astron. Soc. 372, 1699–1709 (2006)

    ADS  Google Scholar 

  69. 69

    Salafia, O. S., Ghisellini, G., Pescalli, A., Ghirlanda, G. & Nappo, F. Light curves and spectra from off-axis gamma-ray bursts. Mon. Not. R. Astron. Soc. 461, 3607–3619 (2016)

    CAS  ADS  Google Scholar 

  70. 70

    Wanderman, D. & Piran, T. The rate, luminosity function and time delay of non-collapsar short GRBs. Mon. Not. R. Astron. Soc. 448, 3026–3037 (2015)

    CAS  ADS  Google Scholar 

  71. 71

    Ghirlanda, G. et al. Short gamma-ray bursts at the dawn of the gravitational wave era. Astron. Astrophys. 594, A84 (2016)

    Google Scholar 

  72. 72

    Fong, W. et al. The afterglow and early-type host galaxy of the short GRB 150101B at z = 0.1343. Astrophys. J. 833, 151 (2016)

    ADS  Google Scholar 

  73. 73

    Ghirlanda, G. et al. Gamma-ray bursts in the comoving frame. Mon. Not. R. Astron. Soc. 420, 483–494 (2012)

    CAS  ADS  Google Scholar 

  74. 74

    Liang, E.-W. et al. A comprehensive study of gamma-ray burst optical emission. II. Afterglow onset and late re-brightening components. Astrophys. J. 774, 13 (2013)

    ADS  Google Scholar 

  75. 75

    Haggard, D. et al. A deep Chandra X-ray study of neutron star coalescence GW170817. Astrophys. J. 848, https://doi.org/10.3847/2041-8213/aa8ede (2017)

    ADS  Google Scholar 

  76. 76

    Moldon, J. & Beswick, R. LIGO/Virgo G298048: e-MERLIN upper limits on 5 GHz compact emission from SSS17a. GCN Circ. 21804 (2017)

  77. 77

    Mooley, K. et al. LIGO/VIRGO G298048: MeerKAT observations of SSS17a. GCN Circ. 21891 (2017)

  78. 78

    van Eerten, H. J., Leventis, K., Meliani, Z., Wijers, R. A. M. J. & Keppens, R. Gamma-ray burst afterglows from transrelativistic blast wave simulations. Mon. Not. R. Astron. Soc. 403, 300–316 (2010)

    ADS  Google Scholar 

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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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Correspondence to E. Pian.

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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.

Extended Data Table 1 Log of photometric observations
Extended Data Table 2 Log of spectroscopic observations

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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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