Polarized kilonovae from black hole-neutron star mergers

Understand

We predict linear polarization for a radioactively-powered kilonova following the merger of a black hole and a neutron star.

• Specifically, we perform 3-D 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 disk 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 $0.1-2\,\mu$m wavelength range.
• We find that both models are polarized at a detectable level 1.5 d after the merger while show negligible levels thereafter.