TY - JOUR

T1 - Spherical symmetry in the kilonova AT2017gfo/GW170817

AU - Sneppen, Albert

AU - Watson, Darach

AU - Bauswein, Andreas

AU - Just, Oliver

AU - Kotak, Rubina

AU - Nakar, Ehud

AU - Poznanski, Dovi

AU - Sim, Stuart

PY - 2023/2/16

Y1 - 2023/2/16

N2 - 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(9-11) 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.

AB - 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(9-11) 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.

KW - SUPERLUMINAL MOTION

KW - II SUPERNOVAE

KW - MERGER

KW - NUCLEOSYNTHESIS

KW - SPECTRA

KW - IDENTIFICATION

KW - POLARIZATION

KW - COUNTERPART

KW - ABSORPTION

KW - EVOLUTION

U2 - 10.1038/s41586-022-05616-x

DO - 10.1038/s41586-022-05616-x

M3 - Journal article

C2 - 36792736

VL - 614

SP - 436

EP - 439

JO - Nature

JF - Nature

SN - 0028-0836

IS - 7948

ER -