Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817

TL;DR

Late-time observations of GW170817's afterglow reveal a structured, collimated relativistic jet, confirming the connection between binary neutron star mergers and short gamma-ray bursts, despite the initial faint gamma-ray detection.

Contribution

This study demonstrates that structured jets with energetic cores explain the afterglow brightness evolution, revealing the jet geometry and confirming the link between BNS mergers and short GRBs.

Findings

The afterglow brightening constrains jet structure and orientation.

GW170817 produced a relativistic jet with a core and wings.

Most BNS mergers may produce observable short GRBs if aligned properly.

Abstract

The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multi-wavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma-rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubts on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multi-wavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30 degrees away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.