Milli-Hertz Gravitational Wave Background Produced by Quasi-Periodic Eruptions

TL;DR

This paper explores the potential gravitational wave background generated by quasi-periodic eruptions (QPEs) involving white dwarf-black hole binaries, which could influence future gravitational wave detection and astrophysical studies.

Contribution

It introduces models of QPEs as sources of gravitational waves, demonstrating that eccentric WD-MBH binaries can produce a detectable background in the LISA frequency band.

Findings

QPEs are generally too weak to be individually resolved by LISA.

Eccentric WD-MBH binaries produce a broad GW spectrum overlapping into a detectable background.

The GW background from QPEs could affect searches for seed black holes and stellar-mass black hole binaries.

Abstract

Extreme-mass-ratio inspirals (EMRIs) are important targets for future space-borne gravitational-wave (GW) detectors, such as the Laser Interferometer Sapce Antenna (LISA). Recent works suggest that EMRI may reside in a population of newly discovered X-ray transients called "quasi-periodic eruptions" (QPEs). Here we follow this scenario and investigate the detectability of the five recently discovered QPEs by LISA. We consider two specific models in which the QPEs are made of either stellar-mass objects moving on circular orbits around massive black holes (MBHs) or white dwarfs (WDs) on eccentric orbits around MBHs. We find that in either case each QPE is too weak to be resolvable by LISA. However, if QPEs are made of eccentric WD-MBH binaries, they radiate GWs in a wide range of frequencies. The broad spectra overlap to form a background which, between $0.003-0.02$ Hz, exceeds the background known to exist due to other types of sources. Presence of this GW background in the LISA band could impact the future search for the seed black holes at high redshift as well as the stellar-mass binary black holes in the local universe.