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 .