Did Binary Neutron Star Merger GW170817 Leave Behind a Long-lived Neutron Star?

Abstract We consider the observational implications of the binary neutron star merger GW170817, which may have left behind a rapidly rotating massive neutron star that launches a relativistic, equatorial outflow as well as a jet. We show that if the equatorial outflow (ring) is highly beamed in the equatorial plane, its luminosity can be “hidden” from view until late times, even if carrying a significant fraction of the spin-down energy of the merger remnant. This hidden ring reveals itself as a rebrightening in the light curve once it slows down enough for the Earth to be within the ring’s relativistic beaming solid angle. We compute semianalytic light curves using this model and find they are in agreement with the observations thus far, and we provide predictions for the ensuing afterglow.