Non-thermal emission from mildly relativistic dynamical ejecta of neutron star mergers
Gilad Sadeh, Or Guttman, Eli Waxman
Abstract
ABSTRACT Binary neutron star mergers are expected to produce fast dynamical ejecta, with mildly relativistic velocities extending to β = v/c > 0.6. We consider the radio to X-ray synchrotron emission produced by collisionless shocks driven by such fast ejecta into the interstellar medium. Analytical expressions are given for spherical ejecta with broken power-law mass (or energy) distributions, M(> γβ) ∝ (γβ)−s with s = sKN at γβ < γ0β0 and s = sft at γβ > γ0β0 (where γ is the Lorentz factor). For parameter values characteristic of merger calculation results – a ‘shallow’ mass distribution, 1 < sKN < 3, for the bulk of the ejecta (at γβ ≈ 0.2), and a steep, sft > 5, ‘fast tail’ mass distribution – our model provides an accurate (to tens of per cent) description of the evolution of the flux, including at the phase of deceleration to subrelativistic expansion. This is a significant improvement over earlier results, based on extrapolations of results valid for γβ ≫ 1 or ≪1 to γβ ≈ 1, which overestimate the flux by an order of magnitude for typical parameter values. It will enable a more reliable inference of ejecta parameters from future measurements of the non-thermal emission. For the merger event GW170817, the existence of a ‘fast tail’ is expected to produce detectable radio and X-ray fluxes over a time-scale of ∼104 d.