The unpolarized macronova associated with the gravitational wave event GW170817

TL;DR

This paper reports the detection of low optical polarization in the macronova associated with GW170817, indicating a symmetric emission region and providing insights into the ejecta composition and geometry of neutron star mergers.

Contribution

It presents the first polarization measurement of a macronova associated with a gravitational wave event, constraining the ejecta geometry and composition.

Findings

Low polarization suggests symmetric ejecta geometry.

Polarization consistent with scattering by Galactic dust.

Supports lanthanide-rich macronova interpretation.

Abstract

The merger of two dense stellar remnants including at least one neutron star (NS) is predicted to produce gravitational waves (GWs) and short duration gamma ray bursts (GRBs). In the process, neutron-rich material is ejected from the system and heavy elements are synthesized by r-process nucleosynthesis. The radioactive decay of these heavy elements produces additional transient radiation termed "kilonova" or "macronova". We report the detection of linear optical polarization P = (0.50 +/- 0.07)% at 1.46 days after detection of the GWs from GW170817, a double neutron star merger associated with an optical macronova counterpart and a short GRB. The optical emission from a macronova is expected to be characterized by a blue, rapidly decaying, component and a red, more slowly evolving, component due to material rich of heavy elements, the lanthanides. The polarization measurement was made when the macronova was still in its blue phase, during which there is an important contribution from a lanthanide-free outflow. The low degree of polarization is consistent with intrinsically unpolarized emission scattered by Galactic dust, suggesting a symmetric geometry of the emitting region and low inclination of the merger system. Stringent upper limits to the polarization degree from 2.45 - 9.48 days post-burst are consistent with the lanthanides-rich macronova interpretation.