The Very Late Time Afterglow of GW170817 Favors a Wobbling Jet

TL;DR

The paper suggests that the very late-time afterglow of GW170817 is best explained by a wobbling, ring-shaped jet, which challenges previous collimated jet models and indicates a significant disk tilt.

Contribution

It introduces a wobbling jet model for GW170817's afterglow, supported by Bayesian analysis, and highlights its potential prevalence in GRBs.

Findings

Wobbling jet model fits the late afterglow data better than collimated jet models.

Bayesian analysis favors a ring-shaped jet over a collimated jet at 4.8σ significance.

The inferred wobbling angle is approximately 27°, indicating a significant disk tilt.

Abstract

GW170817 remains the only binary neutron star merger detected through multimessenger emission. Its afterglow has been monitored for nearly a decade, offering an unprecedented opportunity to probe the properties of the outflow. The shallow decay of the very late-time afterglow challenges the prediction of a collimated structured jet. Motivated by recent general-relativistic magnetohydrodynamic simulations, we propose that the GW170817 afterglow is powered by a wobbling jet that drags a ring on the sky. This structure predicts a post-break decay rate shallower than that of a collimated jet, as observers will see a progressively longer emitting arc after the break. A misaligned ring-shaped jet can therefore self-consistently explain the multimessenger data without invoking any extra component. Through a Bayesian analysis of the multimessenger data, we find a ring-shaped jet is favored over a collimated jet at a significance level of 4.8$\sigma$. Our results imply a wobbling angle of $\sim 27^\circ$. Such a large angle points to a significant disk tilt, potentially arising from disk-infalling gas interaction or asymmetric angular momentum ejection. Similar shallow decays have also been found in other GRB afterglows, raising the possibility that wobbling jets are common among GRBs.