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

Multimessenger observations of GW170817 have not conclusivelyestablished whether the merger remnant is a black hole (BH) or a neutronstar (NS). We show that a long-lived magnetized NS with a poloidal fieldB ap 10$^12$ G is fully consistent with the electromagneticdataset, when spin-down losses are dominated by gravitational wave (GW)emission. The required ellipticity $epsilon$ gsim 10$^-5$ canresult from a toroidal magnetic field component much stronger than thepoloidal component, a configuration expected from an NS newly formedfrom a merger. Abrupt magnetic dissipation of the toroidal component canlead to the appearance of X-ray flares, analogous to the one observed ingamma-ray burst (GRB) afterglows. In the X-ray afterglow of GW170817, weidentify a low-significance (gsim3$sigma$) temporal feature at 155 d,consistent with a sudden reactivation of the central NS. Energyinjection from the NS spin-down into the relativistic shock isnegligible, and the underlying continuum is fully accounted for by astructured jet seen off-axis. Whereas radio and optical observationsprobe 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.