Chandra X-ray Observations of the Neutron Star Merger GW170817: Thermal X-Ray Emission From a Kilonova Remnant?

TL;DR

This study analyzes Chandra X-ray data of GW170817 to investigate thermal and non-thermal emission mechanisms, exploring the possibility of a kilonova remnant emitting thermal X-rays and providing insights into nucleosynthesis products.

Contribution

It introduces thermal plasma models for X-ray emission from a kilonova remnant, expanding beyond previous non-thermal models and analyzing potential line emissions.

Findings

Thermal bremsstrahlung can explain the X-ray emission.

Evidence of line emission near 1.3 keV and 2.2 keV.

Lower plasma temperatures or intrinsic absorption improve model fits.

Abstract

The recent discovery of the neutron star merger and multi-messenger event GW170817 (also known as kilonova GRB170817A) provides an unprecedented laboratory in which to study these mysterious objects, as well as an opportunity to test cutting-edge theories of gravity in the strong field regime. Before this event, such tests of our understanding of the nature of gravity were not possible. In this study, we analyze the X-ray observations of GW170817 obtained with NASA's Chandra X-ray Observatory following the 17 August 2017 detection of the event by the Laser Interferometer Gravitational Wave Observatory (LIGO). Motivated by understanding the emission mechanism for X-ray light and the outcome of the merger event, we fit the Chandra data with both non-thermal (as done previously in the literature) and thermal models. We specifically explore thermal plasma models that would be expected from a kilonova remnant (KNR). We reproduced the non-thermal results which were recently published by Nynka et al. (2018). We also find that thermal bremsstrahlung emission from hot plasma can account for the X-ray emission from this source. Furthermore, we consider models allowing for an intrinsic absorption from the merger event, yielding a softer power-law model photon index than previously published, or a lower plasma temperature. We also report on evidence for line emission, or excess above the continuum model, near 1.3 keV and 2.2 keV which sheds new light on the interpretation of the KNR and its nucleosynthesis products. We discuss the feasibility for the KNR as the origin for thermal X-ray emission at this stage of the kilonova evolution.