An upper-limit on the linear polarization fraction of the GW170817 radio continuum

TL;DR

This study provides an upper limit on the linear polarization of radio emission from GW170817, constraining magnetic field configurations in the merger's ejecta and informing models of neutron star merger afterglows.

Contribution

First to set an observational upper limit on the linear polarization fraction of GW170817's radio emission, constraining magnetic field structures in the ejecta.

Findings

Linear polarization fraction of GW170817 is below ~12% at 2.8 GHz.

Results are consistent with low polarization levels in long gamma-ray burst afterglows.

Constraints support structured jet models with specific magnetic field configurations.

Abstract

We present late-time radio observations of GW170817, the first binary neutron star merger discovered through gravitational waves by the advanced LIGO and Virgo detectors. Our observations, carried out with the Karl G. Jansky Very Large Array, were optimized to detect polarized radio emission, and thus to constrain the linear polarization fraction of GW170817. At an epoch of ~244 days after the merger, we rule out linearly polarized emission above a fraction of ~12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived upper-limit on the radio continuum linear polarization fraction strongly constrains the magnetic field configuration in the shocked ejecta. We show that our results for GW170817 are compatible with the low level of linear polarization found in afterglows of cosmological long gamma-ray bursts. Finally, we discuss our findings in the context of future expectations for the study of radio counterparts of binary neutron star mergers identified by ground-based gravitational-wave detectors.