# Late time kilonova light curves and implications to GW 170817

## TL;DR

This paper models late-time kilonova light curves from neutron star mergers, showing a universal decay rate of approximately t^-2.8, consistent with observations of GW 170817 if ionization losses dominate after about a week.

## Contribution

It introduces a universal model for late-time kilonova emission based on ionization energy loss, applicable across different compositions and radioactive decay rates.

## Key findings

- Late-time luminosity decays approximately as t^-2.8.
- IR and optical data of GW 170817 align with the model if ionization losses dominate after 7 days.
- The decay rate is nearly independent of composition and decay energy release.

## 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$^\pm$ 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.