Synchrotron radiation from the fast tail of dynamical ejecta of neutron star mergers

TL;DR

This paper uses high-resolution simulations to show that neutron star mergers produce a fast, energetic ejecta tail capable of generating observable synchrotron emission, explaining some features of GW170817's afterglow.

Contribution

It demonstrates that the fast tail of neutron star merger ejecta can produce observable synchrotron flares, linking merger dynamics to electromagnetic signals.

Findings

Fast ejecta tail extends to >0.7c velocity.

Kinetic energy of fast tail is 10^{47}–10^{49} erg.

Synchrotron emission can explain GW170817 afterglow.

Abstract

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 $\sim 10^{47}$--$10^{49}$ 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 ISM can power the observed non-thermal emission that followed GW170817, provided that the ISM density is $\sim 10^{-2}\,{\rm cm^{-3}}$, 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 time scale of a few months to a year, leading to a cooling break in the X-ray light curve. If this dynamical ejecta scenario is correct, we expect that the synchrotron radio flare from the ejecta that have produced the macronova/kilonova emission will be observable on time scales of $10^3$ to $10^5$ days. Further multi-frequency observations will confirm or rule out this dynamical ejecta scenario.