Subsequent Nonthermal Emission due to the Kilonova Ejecta in GW 170817

TL;DR

This study models the nonthermal emission from kilonova ejecta in GW 170817, predicting detectable signals in radio, optical, and X-ray bands depending on ambient density and ejecta properties.

Contribution

It introduces a time-dependent simulation of nonthermal emission from kilonova ejecta, incorporating observationally derived ejecta parameters and exploring conditions for detectability.

Findings

High ambient density ($ extgreater=10^{-2}~cm^{-3}$) leads to detectable signals within years.

A low-mass, high-velocity component can explain early X-ray/radio signals.

Radio flux can grow to $ extasciitilde 0.1$ mJy in low-density environments over a few years.

Abstract

The ejected material at the binary neutron star merger GW 170817 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 circumbinary medium. In this paper, we simulate the nonthermal 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 GW 170817. 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/radio counterpart detected at $\sim 10$ days after the merger, implies low ambient density. We also demonstrate that the additional low-mass ($\sim 10^{-5} M_\odot$) component with a velocity of $0.5 c$--$0.8 c$ can reproduce the early X-ray/radio counterpart. This alternative model allows a favorably high density to detect the nonthermal emission due to the kilonova ejecta. Even for a low ambient density such as $\sim 10^{-3}~\mbox{cm}^{-3}$, depending on the microscopic parameters for the electron acceleration, we can expect a growth of radio flux of $\sim 0.1$ mJy in a few years.