Synchrotron Radiation from the Fast Tail of Dynamical Ejecta of Neutron Star Mergers

We find, using high-resolution numerical relativistic simulations, that the tail of the dynamical ejecta of neutron star mergers extends to mildly relativistic velocities faster than 0.7c. The kinetic energy of this fast tail is similar to 10(47)-10(49) erg, depending on the neutron star equation of state and on the binary masses. The synchrotron flare arising from the interaction of this fast tail with the surrounding interstellar medium (ISM) can power the observed nonthermal emission that followed GW170817, provided that the ISM density is similar to 10(-2) cm(-3), the two neutron stars had roughly equal masses and the neutron star equation of state is soft (small neutron star radii). One of the generic predictions of this scenario is that the cooling frequency crosses the X-ray band on a timescale of a few months to a year, leading to a cooling break in the X-ray light curve. While the recent observation of the superluminal motion resolved by very long baseline interferometry (VLBI) rules out the dynamical ejecta scenario, the model described in this paper is generic and can be applied for future neutron star merger events.