Quantum mechanics

Magnetic field

Gamma-ray burst

Galaxy

Light curve

Nuclear physics

Astrophysics

LIGO

Plasma

Afterglow

Jet (fluid)

Thermodynamics

Physics

Active galactic nucleus

Astronomy

Gravitational wave

Astrophysical jet

Neutron star

Flare

Toroid

Kilonova

Monthly Notices of the Royal Astronomical Society

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.

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