Tomoya Suzuguchi, Hidetoshi Omiya, Hiroki Takeda

Recent X-ray observations have discovered a class of periodic X-ray flares in galactic nuclei known as quasi-periodic eruptions (QPEs). A promising explanation of QPEs is an emission produced when a stellar-mass object orbiting a central supermassive black hole crosses an accretion disk. If the companion is a compact object, such systems would be a prospective multi-messenger target for the space-based observatory LISA and its successors. Here we quantify the prospects for joint X-ray and GW detection of QPEs with orbital frequency in the mHz band using a minimal flare-emission model. Our analysis shows that X-ray observations are most effective at orbital frequencies up to roughly 1 mHz, whereas LISA is sensitive chiefly above about 1 mHz. Because the optimal sensitivity windows overlap only marginally, we predict at most one joint detection during LISA's nominal mission lifetime. Extending GW sensitivity into the sub-millihertz regime (< 0.1 mHz) would raise the possibility of the joint detection by an order of magnitude, enabling QPEs as an interesting multi-messenger target.