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

James Munday; Valeriya Korol; Camilla Danielski; Na'ama Hallakoun; Astrid Lamberts; Gijs Nelemans; Anna Pala; Steven Parsons; Ingrid Pelisoli; Alberto Rebassa Mansergas; Jan van Roestel

arXiv · 2512.12075

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

James Munday·Valeriya Korol·Camilla Danielski·Na'ama Hallakoun·Astrid Lamberts·Gijs Nelemans·Anna Pala·Steven Parsons·Ingrid Pelisoli·Alberto Rebassa Mansergas·Jan van Roestel

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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.

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James Munday, Valeriya Korol, Camilla Danielski, Na'ama Hallakoun, Astrid Lamberts, Gijs Nelemans, Anna Pala, Steven Parsons, Ingrid Pelisoli, Alberto Rebassa Mansergas, Jan van Roestel. 2025-12-12. ESO Expanding Horizons White Paper: Electromagnetic characterisation of millihertz gravitational-wave sources in the Milky Way. https://arxiv.org/abs/2512.12075

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