A long-lived neutron star merger remnant in GW170817: constraints and clues from X-ray observations

TL;DR

This paper suggests that GW170817's remnant is likely a long-lived, magnetized neutron star with gravitational wave-dominated spin-down, supported by X-ray observations indicating possible magnetic activity and a structured jet.

Contribution

It provides evidence supporting a long-lived neutron star remnant in GW170817, highlighting the role of gravitational wave losses and magnetic dissipation in the post-merger evolution.

Findings

A magnetized neutron star model fits the electromagnetic data.

X-ray features suggest possible magnetic reactivation of the remnant.

Energy injection into the jet is negligible, consistent with off-axis structured jet observations.

Abstract

Multi-messenger observations of GW170817 have not conclusively established whether the merger remnant is a black hole (BH) or a neutron star (NS). We show that a long-lived magnetized NS with a poloidal field $B\approx 10^{12}$G is fully consistent with the electromagnetic dataset, when spin down losses are dominated by gravitational wave (GW) emission. The required ellipticity $\epsilon\gtrsim 10^{-5}$ can result from a toroidal magnetic field component much stronger than the poloidal component, a configuration expected from a NS newly formed from a merger. Abrupt magnetic dissipation of the toroidal component can lead to the appearance of X-ray flares, analogous to the one observed in gamma-ray burst (GRB) afterglows. In the X-ray afterglow of GW170817 we identify a low-significance ($\gtrsim 3\sigma$) temporal feature at 155 d, consistent with a sudden reactivation of the central NS. Energy injection from the NS spin down into the relativistic shock is negligible, and the underlying continuum is fully accounted for by a structured jet seen off-axis. Whereas radio and optical observations probe the interaction of this jet with the surrounding medium, observations at X-ray wavelengths, performed with adequate sampling, open a privileged window on to the merger remnant.