Towards inferring the geometry of kilonovae

Christine E. Collins*, Luke J. Shingles, Andreas Bauswein, Stuart A. Sim, Theodoros Soultanis, Vimal Vijayan, Andreas Flörs, Oliver Just, Gerrit Leck, Georgios Lioutas, Gabriel Martínez-Pinedo, Albert Sneppen, Darach Watson, Zewei Xiong

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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Recent analysis of the kilonova, AT2017gfo, has indicated that this event was highly spherical. This may challenge hydrodynamics simulations of binary neutron star mergers, which usually predict a range of asymmetries, and radiative transfer simulations show a strong direction dependence. Here we investigate whether the synthetic spectra from a 3D kilonova simulation of asymmetric ejecta from a hydrodynamical merger simulation can be compatible with the observational constraints, suggesting a high degree of sphericity in AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line profile model leads to a value for the photospheric velocity that is consistent with the value obtained from the expanding photosphere method. We would infer that our kilonova simulation is highly spherical at early times, when the spectra resemble a blackbody distribution. The two independently inferred photospheric velocities can be very similar, implying a high degree of sphericity, which can be as spherical as inferred for AT2017gfo, demonstrating that the photosphere can appear spherical even for asymmetrical ejecta. The last-interaction velocities of radiation escaping the simulation show a high degree of sphericity, supporting the inferred symmetry of the photosphere. We find that when the synthetic spectra resemble a blackbody, the expanding photosphere method can be used to obtain an accurate luminosity distance (within 4-7 per cent).

Original languageEnglish
Pages (from-to)1333-1346
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date22 Feb 2024
Publication statusPublished - 01 Apr 2024

Bibliographical note

Publisher Copyright:
© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.


  • methods: numerical
  • neutron star mergers
  • radiative transfer

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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