Spherical symmetry in the kilonova AT2017gfo/GW170817

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

Research output: Contribution to journalArticlepeer-review

18 Citations (Scopus)
31 Downloads (Pure)


The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova . 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 ejecta . Previously, Sr was identified in the spectrum of the only well-studied kilonova AT2017gfo , 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 source 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. [Abstract copyright: © 2023. The Author(s), under exclusive licence to Springer Nature Limited.]
Original languageEnglish
Pages (from-to)436-439
Number of pages4
Issue number7948
Publication statusPublished - 15 Feb 2023


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