Extreme Mass Ratio Inspirals in Light of Quasi-periodic Eruptions: Milli-Hertz Gravitational Wave Background
Quasi-periodic eruptions (QPEs) are repeated X-ray bursts originating in galactic nuclei. Of the many proposed models, the favored model is the disk-collision model in which a stellar mass orbiter collides with a disk formed from a tidal disruption event, generating flares twice per orbit. In this model QPEs are tracers of circular extreme mass ratio inspirals (EMRIs) and can be used to infer the EMRI formation rate and estimate their contribution to the stochastic gravitational wave background (SGWB) in the Laser Interferometer Space Antenna (LISA) band. Whether the secondary is a stellar-mass black hole or a main sequence star is still debated and leads to different results for the EMRI rate and SGWB. We obtain fiducial rates -- subject to systematic uncertainties -- of $R_{\rm SE} = 2.88\times10^{-6}$ per galaxy per year for stellar EMRIs and $R_{\rm BHE} = 6.07\times10^{-6}$ per galaxy per year for black hole EMRIs, then estimate their contribution to the SGWB. We find that only black hole EMRIs contribute to the 1 - 10 milliHertz band resolvable by LISA, and depending on the secondary mass and formation radius can contribute from just below the LISA sensitivity curve to roughly two orders of magnitude above it. Stellar EMRIs, being tidally disrupted before reaching the 1 - 10 milliHertz band, only contribute to sub-milliHertz frequencies and remain below the LISA sensitivity curve.