Late-time Kilonova Light Curves and Implications to GW170817

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 $d\mathrm{log}L/d\mathrm{log}t\simeq -2.8$, 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 > tε ≈ 7 days, as predicted based on the early (few day) electromagnetic emission.