Compact radio emission indicates a structured jet was produced by a binary neutron star merger

TL;DR

High-resolution radio observations of GW170817 reveal a structured relativistic jet, ruling out isotropic outflow and indicating that at least 10% of neutron star mergers produce such jets.

Contribution

This study provides direct imaging evidence of a structured jet in GW170817, using VLBI to distinguish it from isotropic outflows, and estimates the occurrence rate of such jets.

Findings

Source size constrained to <2.5 mas, ruling out isotropic outflow.

Detected a structured relativistic jet in GW170817.

At least 10% of neutron star mergers produce jets.

Abstract

The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High spatial resolution measurements of the source size and displacement can discriminate between these scenarios. We present Very Long Baseline Interferometry observations, performed 207.4 days after the merger, using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milliarcseconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.