Late-time Kilonova Light Curves and Implications to GW170817

Abstract

We discuss the late-time (tens of days) emission from the radioactive ejecta of mergers involving neutron stars, when the ionization energy loss time of beta-decay electrons and positrons exceeds the expansion time. We show that if the e^± are confined to the plasma (by magnetic fields), then the time dependence of the plasma heating rate, {\dot{\varepsilon }}_d, and hence of the bolometric luminosity L={\dot{\varepsilon }}_d, are given by d{log}L/d{log}t≃ -2.8, nearly independent of the composition and of the instantaneous radioactive energy release rate, \dot{\varepsilon }. This universality of the late-time behavior is due to the weak dependence of the ionization loss rate on composition and on e^± energy. The late-time IR and optical measurements of GW170817 are consistent with this expected behavior provided that the ionization loss time exceeds the expansion time at t > t _ɛ ≈ 7 days, as predicted based on the early (few day) electromagnetic emission.