Late time kilonova light curves and implications to GW 170817
  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$^\pm$ 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 $d\log L/d\log
t\simeq-2.8$, 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$^\pm$ energy. The late time IR and optical measurements of
GW 170817 are consistent with this expected behavior provided that the
ionization loss time exceeds the expansion time at $t>t_\varepsilon\approx
7$~d, as predicted based on the early (few day) electromagnetic emission.
