Self-consistent 3D radiative transfer for kilonovae: directional spectra from merger simulations

Luke J. Shingles*, Christine E. Collins, Vimal Vijayan, Andreas Flörs, Oliver Just, Gerrit Leck, Zewei Xiong, Andreas Bauswein, Gabriel Martínez-Pinedo, Stuart A. Sim

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)
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Abstract

We present 3D radiative transfer calculations for the ejecta from a neutron star merger that include line-by-line opacities for tens of millions of bound–bound transitions, composition from an r-process nuclear network, and time-dependent thermalization of decay products from individual α and β− decay reactions. In contrast to expansion opacities and other wavelength-binned treatments, a line-by-line treatment enables us to include fluorescence effects and associate spectral features with the emitting and absorbing lines of individual elements. We find variations in the synthetic observables with both the polar and azimuthal viewing angles. The spectra exhibit blended features with strong interactions by Ce iii, Sr ii, Y ii, and Zr ii that vary with time and viewing direction. We demonstrate the importance of wavelength calibration of atomic data using a model with calibrated Sr, Y, and Zr data, and find major differences in the resulting spectra, including a better agreement with AT2017gfo. The synthetic spectra for a near-polar inclination show a feature at around 8000 Å, similar to AT2017gfo. However, they evolve on a more rapid timescale, likely due to the low ejecta mass (0.005 M) as we take into account only the early ejecta. The comparatively featureless spectra for equatorial observers gives a tentative prediction that future observations of edge-on kilonovae will appear substantially different from AT2017gfo. We also show that 1D models obtained by spherically averaging the 3D ejecta lead to dramatically different direction-integrated luminosities and spectra compared to full 3D calculations.

Original languageEnglish
Article numberL41
Number of pages11
JournalAstrophysical Journal Letters
Volume954
Issue number2
DOIs
Publication statusPublished - 08 Sept 2023

Keywords

  • Neutron stars
  • R-process
  • Radiative transfer simulations
  • Transient sources
  • Gravitational wave astronomy
  • Nuclear astrophysics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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