Polarized kilonovae from black hole-neutron star mergers

M. Bulla, K. Kyutoku, M. Tanaka, S. Covino, J. R. Bruten, T. Matsumoto, J. R. Maund, V. Testa, K. Wiersema

Research output: Contribution to journalArticlepeer-review

14 Citations (Scopus)


We predict linear polarization for a radioactively powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3D Monte Carlo radiative transfer simulations for two different models, both featuring a lanthanide-rich dynamical ejecta component from numerical-relativity simulations while only one including an additional lanthanide-free disc-wind component. We calculate polarization spectra for nine different orientations at 1.5, 2.5, and 3.5 d after the merger and in the 0.1-2 μ wavelength range. We find that both models are polarized at a detectable level 1.5 d after the merger while show negligible levels thereafter. The polarization spectra of the two models are significantly different. The model lacking a disc wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching ∼ 1-6 per cent at 2 μ depending on the orientation. The model with a disc-wind component, instead, features a characteristic 'double-peak' polarization spectrum with one peak in the optical and the other in the infrared. Polarimetric observations of future events will shed light on the debated neutron richness of the disc-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disc-wind component, while polarization increasing from zero in the optical to a peak in the infrared would suggest a lanthanide-rich composition for the whole ejecta. Future polarimetric campaigns should prioritize observations in the first ∼48 h and in the 0.5-2 μ range, where polarization is strongest, but also explore shorter wavelengths/later times where no signal is expected from the kilonova and the interstellar polarization can be safely estimated.

Original languageEnglish
Pages (from-to)1891-1899
Number of pages9
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Publication statusPublished - 2021 Feb 1


  • gravitational waves
  • methods: numerical
  • neutron star mergers
  • opacity
  • polarization
  • radiative transfer
  • transients: black hole

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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