Polarized kilonovae from black hole–neutron star mergers

Abstract

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 $0.1\!-\!2\, \mu$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 disc wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching $\sim 1\!-\!6{{\ \rm per\ cent}}$ at $2\, \mu$m 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 $0.5\!-\!2\, \mu$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.

Coalescence of black hole–neutron star binaries

“…Because the expansion occurs rapidly due to the high velocity, the emission from the dynamical ejecta is likely to be observed directly irrespective of the disk outflow. The polarization degree of $ 1% for the wavelength of J0:7lm might still be expected in a wide range of the viewing angle at 1-2 d after merger for a combination of ejecta components (Bulla et al 2021, see also Li and Shen 2019 for an early-time polarization with hypothetical existence of free neutrons).…”

“…Furthermore, simulations with weak interactions including neutrino-radiation transfer give us the distributions of thermodynamic quantities such as the electron fraction in the ejecta, which determine the nucleosynthetic yield and hence microscopic properties (iii) and (iv). Precise multiband light curves have been calculated by photon-radiation-transfer simulations based on the ejecta properties derived by numerical-relativity simulations of black hole-neutron star binaries (Tanaka et al 2014;Ferna ´ndez et al 2017;Kawaguchi et al 2020b;Bulla et al 2021;Darbha et al 2021). In the following, we briefly review current understanding of the kilonova/macronova with particular emphasis on features specific to black hole-neutron star binaries.…”

“…If Thomson scattering contributes appreciably to the opacity, a deformed photosphere produces net linear polarization along the short principal axis. Although the ejecta dominated by r-process elements with large atomic numbers tend to reduce the polarization due to the small number of scattering electrons and depolarization via bound-bound transitions compared to those consisting of elements up to the iron peak (Kyutoku et al 2015), photonradiation-transfer simulations have suggested that the linear polarization of a few percent may be expected at 1-2 d after merger in near-infrared bands (Bulla et al 2021).…”

Multi-Messenger Constraints on the Hubble Constant through Combination of Gravitational Waves, Gamma-Ray Bursts and Kilonovae from Neutron Star Mergers

“…KNe accompanying BH-NS mergers might be better targets in this respect since the ejecta distribution is more strongly asymmetric compared to the BNS case. Indeed, Bulla et al [157] found that polarization levels up to 6 % may be achieved in these system for favourable viewing angles and when observing at near-IR wavelengths ( 1µm).…”

Radiation hydrodynamics modeling of kilonovae with SNEC

“…This model was later used by Breschi et al (2021) to perform a joint electromagnetic, gravitational wave parameter estimation for GW170817. More advanced models use multi-frequency Monte Carlo radiative transfer calculations (Kasen et al 2013;Wollaeger et al 2018;Kawaguchi et al 2018;Korobkin et al 2021;Bulla et al 2021). Surrogate models that can interpolate detailed Monte Carlo calculations have also been proposed (Coughlin et al 2018).…”