The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo

TL;DR

This paper models the polarization in kilonovae from neutron star mergers, showing how polarization signals depend on ejecta properties and viewing angles, and applies this to interpret observations of GW170817/AT 2017gfo.

Contribution

It provides detailed 3D radiative transfer simulations of kilonova polarization, linking polarization levels to ejecta composition and viewing angle, and constrains the observer's orientation for AT 2017gfo.

Findings

Polarization can reach ~1% shortly after merger in optical bands.

Polarization signals depend strongly on ejecta composition and viewing angle.

The observer of AT 2017gfo was within ~65° of the polar axis.

Abstract

The Gravitational Wave (GW) event GW 170817 was generated by the coalescence of two neutron stars (NS) and produced an electromagnetic transient, labelled AT 2017gfo, that was target of a massive observational campaign. Polarimetry, a powerful diagnostic tool for probing the geometry and emission processes of unresolved sources, was obtained for this event. The observed linear polarization was consistent with being mostly induced by intervening dust, suggesting that the intrinsic emission was weakly polarized ($P < 0.4-0.5$ %). In this paper, we present and discuss a detailed analysis of the linear polarization expected from a merging NS binary system by means of 3D Monte Carlo radiative transfer simulations assuming a range of possible configurations, wavelengths, epochs and viewing angles. We find that polarization originates from the non-homogeneous opacity distribution within the ejecta and can reach levels of $P\sim1$ % at early times (1$-$2 days after the merger) and in the optical R band. Smaller polarization signals are expected at later epochs and/or different wavelengths. From the viewing-angle dependence of the polarimetric signal, we constrain the observer orientation of AT 2017gfo within $\sim$65$^\circ$ from the polar direction. The detection of non-zero polarization in future events will unambiguously reveal the presence of a lanthanide-free ejecta component and unveil its spatial and angular distribution.