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Spherical symmetry in the kilonova AT2017gfo/GW170817
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Journal Article
Spherical symmetry in the kilonova AT2017gfo/GW170817
2023
Overview
The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova
1
–
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 ejecta
5
–
7
. Previously, Sr
+
was identified in the spectrum
8
of the only well-studied kilonova
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–
11
AT2017gfo
12
, 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
13
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.
Spectra taken after the kilonova associated with GW170817 show a high degree of spherical symmetry and a line shape is found that is consistent with a completely spherical expansion to within a few per cent.
