A double-edged sword: extra-galactic Fast X-ray Transients

Introduction

We will use extra-galactic Fast X-ray Transients (FXTs) to study binary neutron star (BNS) mergers. Merging neutron stars are important for the formation of r-process elements and provide standardizable signals allowing the Hubble constant (H0) to be measured, maximizing the science output of these multi-messenger events.

Research Objectives

Comparing BNS mergers with and without an FXT signal provides a way to constrain the elusive equation of state of matter at supra-nuclear densities. Unlike the highly beamed short gamma-ray burst signal associated with BNS mergers, the FXT signal is approximately isotropic, providing us with the means to quickly localize the merger, even for events out to the BNS merger detection horizon of the 3rd generation of GW detectors such as Einstein Telescope or Cosmic Explorer.

Challenges

A high-risk aspect is that the link between FXTs and BNSs is not yet 100% certain. FXTs manifest as singular short flashes of X-ray photons with durations ranging from minutes to hours.

Technological Advancements

The imminent launch of the all-sky X-ray survey satellite Einstein Probe, together with the recent deployment of premiere ground-based instrumentation, will revolutionize this field by enabling the discovery and immediate follow-up of a statistically significant sample of more than a hundred bright FXTs over the >3-year Einstein Probe mission duration.

Expected Outcomes

Complemented with a comprehensive study of the FXT host galaxies, this will lead to a breakthrough in our understanding of FXTs and enable us to measure:

1. The yield of r-process elements
2. The Hubble constant (H0)
3. The maximum mass of a neutron star using BNS mergers

In addition to our main goals, we expect serendipitous discoveries in this new research field.