Observable features of GW170817 kilonova afterglow

Abstract

ABSTRACT The neutron star merger, GW170817, was followed by an optical-infrared transient (a kilonova) which indicated that a substantial ejection of mass at trans-relativistic velocities occurred during the merger. Modelling of the kilonova is able to constrain the kinetic energy of the ejecta and its characteristic velocity but, not the high-velocity distribution of the ejecta. Yet, this distribution contains crucial information on the merger dynamics. In this work, we assume a power-law distribution of the form E(> βΓ) ∝ (βΓ)−α for the energy of the kilonova ejecta and calculate the non-thermal signatures produced by the interaction of the ejecta with the ambient gas. We find that ejecta with minimum velocity β0 ≃ 0.3 and energy E ∼ 1051 erg, as inferred from kilonova modelling, has a detectable radio, and possibly X-ray, afterglow for a broad range of parameter space. This afterglow component is expected to dominate the observed emission on a time-scale of a few years post-merger and peak around a decade later. Its light curve can be used to determine properties of the kilonova ejecta and, in particular, the ejecta velocity distribution α, the minimum velocity β0, and its total kinetic energy E. We also predict that an afterglow rebrightening, that is associated with the kilonova component, will be accompanied by a shift of the centroid of the radio source towards the initial position of the explosion.