Spherical symmetry in the kilonova AT2017gfo/GW170817

Albert Sneppen*, Darach Watson, Andreas Bauswein, Oliver Just, Rubina Kotak, Ehud Nakar, Dovi Poznanski, Stuart Sim

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova1–4. 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 ejecta5–7. Previously, Sr+ was identified in the spectrum8 of the only well-studied kilonova9–11 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.

Original languageEnglish
Pages (from-to)436-439
Number of pages4
Issue number7948
StatePublished - 16 Feb 2023


FundersFunder number
European Space Observatory
Horizon 2020 Framework Programme759253
European Research Council
Deutsche Forschungsgemeinschaft138713538, 279384907 – SFB 1245
Danmarks Grundforskningsfond140
Academy of Finland340613
Israel Science Foundation541/17
GSI Helmholtzzentrum für Schwerionenforschung
Danmarks Frie ForskningsfondDFF-7014-00017


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