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.
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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 http://archive.eso.org.
We use the implementation of the P Cygni profile in the Elementary Supernova from https://github.com/unoebauer/public-astro-tools 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 https://github.com/Sneppen/Kilonova-analysis.
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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.
The authors declare no competing interests.
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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.
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Sneppen, A., Watson, D., Bauswein, A. et al. Spherical symmetry in the kilonova AT2017gfo/GW170817. Nature 614, 436–439 (2023). https://doi.org/10.1038/s41586-022-05616-x
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