Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Spherical symmetry in the kilonova AT2017gfo/GW170817

This article has been updated


The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova1,2,3,4. The kilonova’s geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta5,6,7. Previously, Sr+ was identified in the spectrum8 of the only well-studied kilonova9,10,11 AT2017gfo12, associated with the gravitational wave event GW170817. Here we combine the strong Sr+ P Cygni absorption-emission spectral feature and the blackbody nature of kilonova spectrum to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source13 also show the same sphericity independently. We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black-hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta; however, an additional process seems necessary to make the element distribution uniform.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Get just this article for as long as you need it


Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Illustration of the expanding atmospheres method for the kilonova AT2017gfo.
Fig. 2: Kilonova asymmetry index as a function of H0.
Fig. 3: Constraints on the spherical symmetry of the kilonova from the line shape.
Fig. 4: Posterior probability distributions for the luminosity distance to the kilonova AT2017gfo.

Data availability

Work in this paper was based on observations made with ESO telescopes at the Paranal Observatory under programmes 099.D-0382 (principal investigator E. Pian), 099.D-0622 (principal investigator P. D’Avanzo), 099.D-0376 (principal investigator S. J. Smartt) and 099.D-0191 (principal investigator A. Grado). The data are available at

Code availability

We use the implementation of the P Cygni profile in the Elementary Supernova from with generalizations to include variable ellipticity, inclination angle and enhancement of emission. Extensions of P Cygni code and data analysis required for generating figures can be found at

Change history

  • 06 March 2023

    In the version of this article initially published online, the author initials in ref. 30 were incorrect and the references has been amended to read: Collins, C. E. et al. Double detonations: variations in Type Ia supernovae due to different core and He shell masses – II: synthetic observables Mon. Not. R. Astron. Soc. 517, 5289–5302 (2022).


  1. Eichler, D. et al. Nucleosynthesis, neutrino bursts and γ-rays from coalescing neutron stars. Nature 340, 126–128 (1989).

    Article  ADS  Google Scholar 

  2. Barnes, J. et al. Effect of a high opacity on the light curves of radioactively powered transients from compact object mergers. Astrophys. J. 775, 18 (2013).

    Article  ADS  Google Scholar 

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

    Article  ADS  CAS  PubMed  Google Scholar 

  4. Berger, E. et al. An r-process kilonova associated with the short-hard GRB 130603B. Astrophys. J. Lett. 774, L23 (2013).

    Article  ADS  Google Scholar 

  5. Hotokezaka, K. et al. Mass ejection from the merger of binary neutron stars. Phys. Rev. D. 87, 024001 (2013).

    Article  ADS  Google Scholar 

  6. Bauswein, A. et al. Systematics of dynamical mass ejection, nucleosynthesis, and radioactively powered electromagnetic signals from neutron-star mergers. Astrophys. J. 773, 79 (2013).

    Article  Google Scholar 

  7. Rosswog, S. et al. The long-term evolution of neutron star merger remnants – I. The impact of r-process nucleosynthesis. Mon. Not. R. Astron. Soc. 439, 744–756 (2014).

    Article  ADS  CAS  Google Scholar 

  8. Watson, D. et al. Identification of strontium in the merger of two neutron stars. Nature 574, 497–500 (2019).

    Article  ADS  CAS  PubMed  Google Scholar 

  9. Abbott, B. P. et al. Gravitational waves and gamma-rays from a binary neutron star merger: GW170817 and GRB 170817A. Astrophys. J. Lett. 848, L13 (2017).

    Article  ADS  Google Scholar 

  10. Pian, E. et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature 551, 67–70 (2017).

    Article  ADS  CAS  PubMed  Google Scholar 

  11. Smartt, S. J. et al. A kilonova as the electromagnetic counterpart to a gravitational-wave source. Nature 551, 75–79 (2017).

    Article  ADS  CAS  PubMed  Google Scholar 

  12. Coulter, D. A. et al. The optical counterpart to a gravitational wave source. Science 358, 1556–1558 (2017).

    Article  ADS  CAS  PubMed  Google Scholar 

  13. Mooley, K. P. et al. Optical superluminal motion measurement in the neutron-star merger GW170817. Nature 610, 273–276 (2022).

    Article  ADS  CAS  PubMed  Google Scholar 

  14. Gillanders, J. H. et al. Modelling the spectra of the kilonova AT2017gfo – I. The photospheric epochs. Mon. Not. R. Astron. Soc. 515, 631–651 (2022).

    Article  ADS  CAS  Google Scholar 

  15. Abbott, B. P. et al. A gravitational-wave standard siren measurement of the Hubble constant. Nature 551, 85–88 (2017).

    Article  ADS  Google Scholar 

  16. Kirshner, R. P. et al. Distances to extragalactic supernovae. Astrophys. J. 228, 359 (1926).

    Google Scholar 

  17. Eastman, R. G. et al. The atmospheres of type ii supernovae and the expanding photosphere method. Astrophys. J. 466, 911 (1996).

    Article  ADS  CAS  Google Scholar 

  18. Kasen, D. et al. Opacities and spectra of the r-process ejecta from neutron star mergers. Astrophys. J. 774, 25 (2013).

    Article  ADS  CAS  Google Scholar 

  19. Aghanim, N. et al. Planck 2018 results. VI. Cosmological parameters. Astron. Astrophys. 641, A6 (2018).

  20. Mukherjee, S. et al. Velocity correction for Hubble constant measurements from standard sirens. Astron. Astrophys. 646, A65 (2021).

    Article  CAS  Google Scholar 

  21. Riess, A. G. et al. A Comprehensive measurement of the local value of the Hubble constant with 1 km s−1 Mpc−1 uncertainty from the Hubble Space Telescope and the SH0ES Team. Astrophys. J. Lett. 934, L7 (2022).

    Article  ADS  Google Scholar 

  22. Hoeflich, I. et al. Analysis of the polarization and flux spectra of SN 1993J. Astrophys. J. 459, 307 (1996).

    Article  ADS  Google Scholar 

  23. Sekiguchi, Y. et al. Dynamical mass ejection from the merger of asymmetric binary neutron stars: radiation-hydrodynamics study in general relativity. Phys. Rev. D 93, 124046 (2016).

    Article  ADS  Google Scholar 

  24. Radice, D. et al. Binary neutron star mergers: mass ejection, electromagnetic counterparts, and nucleosynthesis. Astrophys. J. 869, 130 (2018).

    Article  ADS  CAS  Google Scholar 

  25. Just, O. et al. Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers. Mon. Not. R. Astron. Soc. 448, 541–567 (2015).

    Article  ADS  CAS  Google Scholar 

  26. Foucart, F. et al. Evaluating radiation transport errors in merger simulations using a Monte Carlo algorithm. Phys. Rev. D 98, 063007 (2018).

    Article  ADS  CAS  Google Scholar 

  27. Wu, M. R. & Tamborra, I. Fast neutrino conversions: ubiquitous in compact binary merger remnants. Phys. Rev. D 95, 103007 (2017).

    Article  ADS  Google Scholar 

  28. Soker, N. et al. Supernovae Ia in 2019 (review): a rising demand for spherical explosions. New Astron. Rev. 87, 101535 (2019).

    Article  Google Scholar 

  29. Bulla, M. et al. Type Ia supernovae from violent mergers of carbon-oxygen white dwarfs: polarization signatures. Mon. Not. R. Astron. Soc. 455, 1060–1070 (2016).

    Article  ADS  CAS  Google Scholar 

  30. Collins, C. E. et al. Double detonations: variations in Type Ia supernovae due to different core and He shell masses – II: synthetic observables. Mon. Not. R. Astron. Soc. 517, 5289–5302 (2022).

  31. Mooley, K. P. et al. Superluminal motion of a relativistic jet in the neutron-star merger GW17081. Nature 561, 355–359 (2018).

    Article  ADS  CAS  PubMed  Google Scholar 

  32. Hotokezaka, K. et al. A Hubble constant measurement from superluminal motion of the jet in GW170817. Nat. Astron. 3, 940–944 (2019).

    Article  ADS  Google Scholar 

  33. Baade, W. Über eine Möglichkeit, die Pulsationstheorie der δ Cephei-Veränderlichen zu prüten. Astron. Nachr. 193, 27–36 (1974).

    Google Scholar 

  34. Gall, C. et al. Lanthanides or dust in kilonovae: lessons learned from GW170817. Astrophys. J. Lett. 849, L29 (2017).

    Article  Google Scholar 

  35. Ghisellini, G. Radiative Processes in High Energy Astrophysics (Lecture Notes in Physics) vol. 873 (Springer, 2013).

  36. Rees, M. J. Studies in radio source structure – I. A relativistically expanding model for variable quasi-stellar radio sources. Mon. Not. R. Astron. Soc. 135, 345 (1967).

    Article  ADS  Google Scholar 

  37. Drout, M. R. et al. Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis. Science 358, 1570–1574 (2017).

    Article  ADS  CAS  PubMed  Google Scholar 

  38. Gall, E. et al. Applying the expanding photosphere and standardized candle methods to Type II-Plateau supernovae at cosmologically significant redshifts. Astron. Astrophys. 592, A129 (2016).

    Article  Google Scholar 

  39. Dessart, L. & Hillier, D. J. Distance determinations using type II supernovae and the expanding photosphere method. Astron. Astrophys. 439, 671–685 (2005).

    Article  ADS  CAS  Google Scholar 

  40. Dessart, L. et al. Radiative-transfer models for supernovae IIb/Ib/Ic from binary-star progenitors. Mon. Not. R. Astron. Soc. 453, 2189–2213 (2015).

    Article  ADS  CAS  Google Scholar 

  41. Covino, S. et al. The unpolarized macronova associated with the gravitational wave event GW170817. Nat. Astron. 1, 791–794 (2017).

    Article  ADS  Google Scholar 

  42. Bulla, M. et al. The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT2017gfo. Nat. Astron. 3, 99–106 (2019).

    Article  ADS  Google Scholar 

  43. Jeffery, D. J. & Branch, D. (eds). Analysis of Supernova Spectra 6, 149 (Springer, 1990).

  44. Hutsemekers, D. & Surdej, J. Formation of P Cygni line profiles in relativistically expanding atmospheres. Astrophys. J. 361, 367 (1990).

    Article  ADS  Google Scholar 

  45. Malesani, D. et al. LIGO/Virgo G298048: optical spectral energy distribution of SSS17a. GRB Coord. Netw. 21577, 1 (2017).

    Google Scholar 

  46. Shappee, B. J. et al. Early spectra of the gravitational wave source GW170817: evolution of a neutron star merger. Science 358, 1574–1578 (2017).

    Article  ADS  CAS  PubMed  Google Scholar 

  47. Tanvir, N. R. et al. The emergence of a lanthanide-rich kilonova following the merger of two neutron stars. Astrophys. J. Lett. 848, L27 (2017).

    Article  ADS  Google Scholar 

  48. Sim, S. A. in Handbook of Supernovae (eds Alsabti, A. W. & Murdin, P.) 769 (Springer, 2017).

  49. Hjorth, J. et al. The distance to NGC 4993: the host galaxy of the gravitational-wave event GW170817. Astrophys. J. Lett. 848, L31 (2017).

    Article  ADS  Google Scholar 

  50. Howlett, C. & Davis, T. M. Standard siren speeds: improving velocities in gravitational-wave measurements of H0. Mon. Not. R. Astron. Soc. 492, 3803–3815 (2020).

    Article  ADS  CAS  Google Scholar 

  51. Nicolaou, C. et al. The impact of peculiar velocities on the estimation of the Hubble constant from gravitational wave standard sirens. Mon. Not. R. Astron. Soc. 495, 90–97 (2020).

    Article  ADS  CAS  Google Scholar 

  52. Just, O. et al. Neutrino absorption and other physics dependencies in neutrino-cooled black hole accretion discs. Mon. Not. R. Astron. Soc. 509, 1377–1412 (2021).

    Article  ADS  Google Scholar 

  53. Ito, H. et al. A global numerical model of the prompt emission in short gamma-ray bursts. Astophys. J. 918, 59 (2021).

    Article  ADS  CAS  Google Scholar 

  54. Just, O. et al. Dynamical ejecta of neutron star mergers with nucleonic weak processes – II: kilonova emission. Mon. Not. R. Astron. Soc. 510, 2820–2840 (2022).

    Article  ADS  CAS  Google Scholar 

  55. Fujibayashi, S. et al. Mass ejection from the remnant of a binary neutron star merger: viscous-radiation hydrodynamics study. Mon. Not. R. Astron. Soc. 860, 64 (2018).

    Google Scholar 

  56. Ardevol-Pulpillo, R. et al. Improved leakage-equilibration-absorption scheme (ILEAS) for neutrino physics in compact object mergers. Mon. Not. R. Astron. Soc. 485, 4754–5789 (2019).

    Article  ADS  CAS  Google Scholar 

  57. Korobkin, O. et al. On the astrophysical robustness of the neutron star merger r-process. Mon. Not. R. Astron. Soc. 426, 1940–1949 (2012).

    Article  ADS  CAS  Google Scholar 

  58. Blandford, R. D. & Znajek, R. L. Electromagnetic extraction of energy from Kerr black holes. Mon. Not. R. Astron. Soc. 179, 433–456 (1997).

    Article  ADS  Google Scholar 

  59. Thompson, T. A. et al. Magnetar spin-down, hyperenergetic supernovae, and gamma-ray bursts. Astrophys. J. 611, 380–393 (2004).

    Article  ADS  CAS  Google Scholar 

  60. Metzger, B. D. et al. A magnetar origin for the kilonova ejecta in GW170817. Astophys. J. 856, 101 (2018).

    Article  ADS  Google Scholar 

Download references


We thank A. C. Andersen, J. Selsing, R. Wojtak, K. Frantzen, C. Steinhardt and C. Vogl for useful discussions. We thank the European Space Observatory (ESO) Director General for allocating Director’s Discretionary Time to this programme, and the ESO operation staff for support. D.W. is supported in part by Independent Research Fund Denmark grant no. DFF-7014-00017. The Cosmic Dawn Centre is funded by the Danish National Research Foundation under grant no. 140. A.B. and O.J. acknowledge support from the European Research Council under the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 759253. A.B. acknowledges support from Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 279384907 – SFB 1245 and DFG – Project-ID 138713538 – SFB 881 (‘The Milky Way System’, subproject A10) and support from the State of Hesse within the Cluster Project ELEMENTS. O.J. acknowledges computational support by the HOKUSAI computer centre at RIKEN and by the VIRGO cluster at GSI. R.K. acknowledges support from the Academy of Finland (grant no. 340613). S.A.S. acknowledges funding from the UKRI STFC grant no. ST/T000198/1. D.P. acknowledges support from Israel Science Foundation grant no. 541/17.

Author information

Authors and Affiliations



A.S. and D.W. were the primary drivers of the project and wrote the main text and developed the figures. A.S. did all of the analysis and calculations, wrote most of the Methods sections, and produced Figs. 14 and Extended Data Figs. 13. O.J. performed the hydrodynamical simulations and produced Extended Data Fig. 4. O.J. and A.B. wrote the parts of the main text and methods sections related to the simulations. A.S., D.W., A.B., O.J., R.K., E.N., D.P. and S.S. were involved in interpreting and discussing the results, and commented on and/or edited the text.

Corresponding author

Correspondence to Albert Sneppen.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature thanks Elena Pian and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Additional information

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 Fig. 1 Spectral series of AT2017gfo 1.4-5.4 days after the merger.

Spectra are from the VLT/X-shooter spectrograph (grey), with best fit shown with a dashed black line, and the blackbody-only component indicated with a red dotted line and deviations from the blackbody with pink fill. Grey-shaded regions were not included in the fits. Darker shaded bars indicate telluric regions; light grey indicates overlapping noisy regions between the UVB, VIS and NIR arms of the spectrograph.

Extended Data Fig. 2 Posterior probability distributions of the luminosity distance to the kilonova AT2017gfo from epochs 1–5.

Our distance estimates based on the kilonova EPM for the spectra obtained at 1.43, 2.42, 3.41, 4.40 and 5.40 days are shown in blue, yellow, green, red, and purple histograms respectively. Filled histograms represent the full model including the blackbody continuum, Sr P Cygni, and two NIR Gaussian emission lines. Dotted histograms indicate constraints from excluding all data with wavelengths longer than 1300 nm, showing that the inclusion of the NIR Gaussian emission features do not bias the full model significantly. The dash-dotted histograms are for fits excluding the parts of the spectra with the Sr+ emission line. Distances derived from every epoch are consistent with the distances inferred from the GW standard siren plus VLBI constraints32, however the distances inferred from epochs 3–5 are sensitive to the modelling of the 1 μm emission feature. In contrast, the data from epochs 1 and 2 provide robust, tight statistical uncertainties, with no large systematic variation between different models for emission components.

Extended Data Fig. 3 Comparison of the inclination angle and luminosity distance to AT2017gfo compared to the inclination angle constraint from VLBI jet measurements.

The 1σ and 2σ constraints (dashed contours) from the combined EPM (red) and gravitational wave standard siren volumetric (blue) priors yield a tight constraint on the inclination angle, in close agreement with 1σ constraints from VLBI measurements and Hubble Space Telescope precision astrometry (grey shading13).

Extended Data Fig. 4 Numerical models of energy injection into an expanding cloud of merger ejecta.

The left panel provides color maps of the density (left) and a tracer of the original electron fraction (Ye, right) in velocity space as resulting after 1 day for four ejecta models in which different amounts of heating energy (0, 5, 10, and 30 MeV per baryon) were injected during roughly the first second of expansion. While the density distribution can be made spherical with large injection energies, the Ye stratification remains nearly unchanged. The right panel shows a model where a relativistic wind with 60° half-opening angle around the polar axis is injected. The plots display the same quantities as in the left panels for four different time steps. The wind inflates the innermost part of the ejecta, creating a hot low-density bubble, and launches a shock wave, which dissipates energy predominantly in the polar ejecta, allowing them to spread sideways and, by that, reduce the pole-to-equator variation of Ye.

Supplementary information

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Sneppen, A., Watson, D., Bauswein, A. et al. Spherical symmetry in the kilonova AT2017gfo/GW170817. Nature 614, 436–439 (2023).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing