Subsequent Non-thermal Emission due to the Kilonova Ejecta in GW170817.

The ejected material at the binary neutron star merger GW170817 was confirmed as a kilonova by UV, optical and IR observations. This event provides a unique opportunity to investigate the particle acceleration at a mildly relativistic shock propagating in the circum binary medium. In this {\it Letter}, we simulate the non-thermal emission from electrons accelerated by the shock induced by the kilonova ejecta with a time-dependent method. The initial velocity and mass of the ejecta in the simulations are obtained from the kilonova observations in GW170817. If the ambient density is high enough ($\geq 10^{-2}~\mbox{cm}^{-3}$), radio, optical/IR, and X-ray signals will be detected in a few years, though the off-axis short gamma-ray burst models, accounting for the X-ray counterpart detected at $\sim 10$ days after the merger, implies low ambient density. Even for low ambient density as $\sim 10^{-3}~\mbox{cm}^{-3}$, depending on the microscopic parameters for the electron acceleration, we can expect a growth of radio flux as $\sim \mu$Jy in a few years.