Afterglow light curves of the non-relativistic ejecta mass in a stratified circumstellar medium

TL;DR

This paper models the afterglow light curves from non-relativistic ejecta in stratified media, predicting their evolution during different phases and constraining parameters using multi-wavelength observations from neutron star mergers.

Contribution

It provides a detailed theoretical framework for afterglow light curves in stratified environments and applies it to real GW event data for parameter constraints.

Findings

Predicted light curves during free-coasting and Sedov-Taylor phases.

Constraints on kilonova afterglow parameters from GW170817 and S190814bv.

Highlighting the need for long-term observations to refine models.

Abstract

We present the afterglow light curves produced by the deceleration of the non-relativistic ejecta mass in a stratified circumstellar medium with a density profile $n(r)\propto r^{-k}$ with $k=0$, $1$, $1.5$, $2$ and $2.5$. Once the ejecta mass is launched with equivalent kinetic energy parametrized by $E(>\beta)\propto \beta^{-\alpha}$ (where beta is the ejecta velocity) and propagates into the surrounding circumstellar medium, it first moves with constant velocity (the free-coasting phase), and later it decelerates (the Sedov-Taylor expansion). We present the predicted synchrotron and synchrotron-self Compton light curves during the free-coasting phase, and the subsequent Sedov-Taylor expansion. In particular cases, we show the corresponding light curves generated by the deceleration of several ejecta masses with different velocities launched during the coalescence of binary compact objects and the core-collapse of dying massive stars which will contribute at distinct timescales, frequencies, and intensities. Finally, using the multi-wavelength observations and upper limits collected by a large campaign of orbiting satellites and ground telescopes, we constrain the parameter space of both the KN afterglow in GW170817 and the possibly generated KN afterglow in S190814bv. Further observations on timescales of years post-merger are needed to derive tighter constraints.