Spherical symmetry in the kilonova AT2017gfo/GW170817

Albert Sneppen (), Darach Watson, Andreas Bauswein, Oliver Just, Rubina Kotak, Ehud Nakar, Dovi Poznanski and Stuart Sim
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Albert Sneppen: Cosmic Dawn Center (DAWN)
Darach Watson: Cosmic Dawn Center (DAWN)
Andreas Bauswein: GSI Helmholtzzentrum für Schwerionenforschung
Oliver Just: GSI Helmholtzzentrum für Schwerionenforschung
Rubina Kotak: University of Turku
Ehud Nakar: Tel-Aviv University
Dovi Poznanski: Tel-Aviv University
Stuart Sim: Astrophysics Research Centre, Queen’s University Belfast

Nature, 2023, vol. 614, issue 7948, 436-439

Abstract: Abstract 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.

Date: 2023
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DOI: 10.1038/s41586-022-05616-x

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