ESO Expanding Horizons White Paper: Electromagnetic characterisation of millihertz gravitational-wave sources in the Milky Way

TL;DR

This white paper emphasizes the importance of electromagnetic follow-up observations to fully characterize millihertz gravitational-wave sources in the Milky Way, highlighting current gaps and future needs for maximizing scientific returns.

Contribution

It identifies the lack of dedicated electromagnetic facilities and coordinated surveys necessary for comprehensive follow-up of millihertz gravitational-wave sources.

Findings

Millihertz gravitational-wave sources include thousands of compact binaries in the Milky Way.

Electromagnetic characterization is essential to determine binary component properties.

No current dedicated facilities are planned for electromagnetic follow-up at the required scale.

Abstract

The millihertz band is densely populated by continuous gravitational-wave signals from Galactic compact binaries, dominated by double white dwarfs (DWDs; binaries of two white dwarfs) with contributions from systems containing neutron stars and black holes (Amaro-Seoane et al. 2023). As these binaries inspiral due to gravitational-wave radiation, they can reach contact and begin mass transfer in the millihertz band. Gravitational-wave detectors like LISA will survey such compact binaries across the Milky Way, yielding samples numbering in the tens of thousands, with essentially complete sensitivity to orbital periods shorter than ~10-20 min (e.g. Lamberts et al. 2019). Assessing the nature of the binary components - and deriving masses, temperatures and compositions - requires systematic electromagnetic characterisation that breaks gravitational-wave degeneracies and enables full atmospheric and orbital solutions. At present, no dedicated facility or coordinated survey is planned to deliver electromagnetic follow-up at the scale necessary to maximise the science return of the millihertz gravitational-wave data; this white paper discusses the need and requirements of such a capability.