Constraining the ejecta for the nonthermal emission from GW170817

TL;DR

This paper models the nonthermal emission from GW170817's ejecta, constraining the energy distribution and velocities, and concludes that different ejecta components produce thermal and nonthermal emissions at different angles.

Contribution

It introduces a simple ejecta model with a power-law energy distribution to interpret multi-band observations of GW170817's nonthermal emission.

Findings

Constrained the energy distribution index to approximately 6.7.

Estimated the total kinetic energy to be between 0.3 and 5 x 10^{51} erg.

Determined the slowest ejecta velocity to be about 0.7-0.8c.

Abstract

We consider a simple model for the nonthermal emission from GW170817, in which a quasi-spherical ejecta is released in the merger event, with the kinetic energy distributed over the momentum as $E(>\gamma\beta)\propto(\gamma\beta)^{-k}$. The ejecta drives a shock into the medium and gives rise to synchrotron radiation. Using multi-band observations we constrain that $k\approx6.7$; (assuming medium density of $\sim10^{-2}\rm cm^{-3}$ and postshock magnetic field carries a fraction $10^{-5}-10^{-3}$ of the postshock internal energy) the total kinetic energy is $(0.3-5)\times10^{51}$erg; the slowest ejecta velocity is $\sim(0.7-0.8)c$; and the fastest ejecta has a Lorentz factor of $\sim4-7$. We conclude that the sub-relativistic dynamical ejecta responsible to the kilonova cannot produce the nonthermal emission. The co-existence of the nonthermal and thermal kilonova emission implies that two corresponding ejecta are ejected at different angles.