| Abstract |
The Milky Way, while extensively studied, still conceals much about its “fossil” stellar population: white dwarfs, neutron stars, and black holes—remnants left behind at the end of stars' lives. These remnants no longer generate light from nuclear fusion like "living" stars and often remain invisible to electromagnetic telescopes, making their discovery and study challenging. However, when stellar remnants form short-period binary systems, they emit detectable gravitational-wave radiation, providing an additional messenger that allows us to search for them using both electromagnetic and gravitational-wave detectors. This proposal aims to leverage this multi-messenger approach to uncover the Milky Way’s stellar fossil population, helping us piece together the Galaxy’s stellar history, much like palaeontologists reconstruct the history of life on Earth. It will capitalise on the data from the Gaia mission that will become available in 2026 while preparing for future discoveries with the Laser Interferometer Space Antenna (LISA), which will explore a new millihertz range of the gravitational-wave spectrum in the 2030s. This proposal focuses on modelling gravitational-wave observations and their electromagnetic counterparts for Galactic stellar remnant binaries, particularly those accessible to LISA. Some of these binaries are already observable with current facilities, making them guaranteed multi-messenger sources for LISA. Galactic double white dwarfs (DWDs), by far the most common among stellar remnant binaries, offer numerous opportunities across astronomy. My past research has shown how DWDs trace the Galactic stellar mass distribution, tested their role as progenitors of Type Ia supernovae, and explored how they provide a means to study sub-stellar-mass objects like brown dwarfs and exoplanets. At sub-millihertz frequencies, DWDs are so numerous that their overlapping signals this is morecreate an unresolved stochastic foreground, impacting the detection of other LISA sources. Studying DWDs is a top priority for LISA preparation. The main objective of this research proposal is to harness the rapid growth of time-domain optical observations leading up to the LISA mission to improve theoretical models of the Galactic compact object population and to lay the groundwork for future multi-messenger studies once LISA becomes operational. To achieve this, I will focus on three key pillars: From Gaia to LISA: Data mine the Gaia survey to identify Galactic binaries containing electromagnetically-dark components, enhancing our understanding of the Milky Way’s stellar content and improving predictions for LISA. Broadening to Neutron Stars and Black Holes : Extend theoretical studies to binaries with neutron stars and black holes, exploring multi-messenger synergies across radio to gamma-ray wavebands, focusing on processes governing their evolution and developing new synergies with LISA. Probing the Galactic Centre: Model populations of millisecond pulsars and extremely large mass-ratio inspirals (Sgr A* + brown dwarf) in the Galactic Centre to address unexplained ?-ray excesses and deepen our understanding of the Galactic Centre by making predictions that are testable in both the electromagnetic and GW data. Progress on these pillars has the potential to impact fields such as Galactic and high-energy astrophysics, with indirect implications for dark matter and early universe research. Each pillar contributes to a final deliverable: a comprehensive mock catalogue of Galactic gravitational-wave sources and their electromagnetic counterparts. This catalogue, analogous to the Gaia Universe Model Snapshot, will be crucial for preparing the LISA mission, aiding in designing data processing, data interpretation, and electromagnetic follow-up. This proposal is well-timed to align with the UK contribution to the LISA mission. |
