Bulla, M.M.BullaKyutoku, K.K.KyutokuTanaka, M.M.TanakaCOVINO, StefanoStefanoCOVINOBruten, J. R.J. R.BrutenMatsumoto, T.T.MatsumotoMaund, J. R.J. R.MaundTESTA, VincenzoVincenzoTESTAWiersema, K.K.Wiersema2022-03-292022-03-2920210035-8711http://hdl.handle.net/20.500.12386/32047We 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. The polarization spectra of the two models are significantly different. The model lacking a disk wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching $\sim1\%-6\%$ at $2\,\mu$m depending on the orientation. The model with a disk-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 disk-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disk-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 $\sim48$ hours and in the $0.5-2\,\mu$m 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.STAMPAenArticle10.1093/mnras/staa37962-s2.0-85100248606http://arxiv.org/abs/2009.07279v2https://academic.oup.com/mnras/article/501/2/1891/6030051?login=true2021MNRAS.501.1891BFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_11 Relativistic astrophysics