GW170817: A Neutron Star Merger in a Mass-Transferring Triple System

TL;DR

This paper proposes that the bright, blue light curve of GW170817 resulted from ejecta reheated by interaction with a mass-transferring outer star in a hierarchical triple system, offering a new explanation for electromagnetic counterparts.

Contribution

It introduces a novel model where a hierarchical triple system explains the bright, blue kilonova emission through ejecta reheating by the outer star.

Findings

Reheating occurs when ejecta collides with the evolved outer star.

Mass transfer in triples can produce coplanar systems like PSR J0337+1715.

This mechanism may explain electromagnetic signals from black hole mergers.

Abstract

The light curve of GW170817 is surprisingly blue and bright. Assuming that the event is a binary neutron star merger, we argue that blueness and brightness of the light curve is the result of ejecta that contains an substantial amount of thermal energy. To achieve this, the ejecta must be reheated at a substantial distance (1 to 2000 solar radii) from the merger to avoid losing the energy to adiabatic cooling. We show that this reheating can occur if the merger occurs in a hierarchical triple system where the outer star has evolved and filled its Roche lobe. The outer star feeds mass to the inner binary, forming a circumbinary disc, driving the inner binary to merge. Because the outer star fills its Roche lobe, a substantial fraction of the dynamical ejecta collides with the evolved star, reheating the ejecta in the process. We suggest that the process of mass transfer in heirarchical triples tends to form coplanar triple systems such as PSR J0337+1715, and may provide electromagnetic counterparts to binary black hole mergers.