A year in the life of GW170817: the rise and fall of a structured jet from a binary neutron star merger

TL;DR

This study presents a year-long observation of GW170817, revealing the evolution of its afterglow consistent with a structured jet viewed at an angle, challenging previous choked jet models.

Contribution

It provides detailed late-time multi-wavelength data supporting the structured jet model for GW170817, with implications for short GRB properties.

Findings

Afterglow peaked at 160 days with X-ray luminosity ~5e39 erg/s

The light curve shows a transition from shallow rise to rapid decline

Properties align with those of cosmological short GRBs

Abstract

We present the results of our year-long afterglow monitoring of GW170817, the first binary neutron star (NS) merger detected by advanced LIGO and advanced Virgo. New observations with the Australian Telescope Compact Array (ATCA) and the Chandra X-ray Telescope were used to constrain its late-time behavior. The broadband emission, from radio to X-rays, is well-described by a simple power-law spectrum with index ~0.585 at all epochs. After an initial shallow rise ~t^0.9, the afterglow displayed a smooth turn-over, reaching a peak X-ray luminosity of ~5e39 erg/s at 160 d, and has now entered a phase of rapid decline ~t^(-2). The latest temporal trend challenges most models of choked jet/cocoon systems, and is instead consistent with the emergence of a relativistic structured jet seen at an angle of ~22 deg from its axis. Within such model, the properties of the explosion (such as its blastwave energy E_K~2E50 erg, jet width theta_c~4 deg, and ambient density n~3E-3 cm^(-3)) fit well within the range of properties of cosmological short GRBs.