Interacting Stellar EMRIs as Sources of Quasi-periodic Eruptions in Galactic Nuclei

A star that approaches a supermassive black hole (SMBH) on a circular extreme mass ratio inspiral (EMRI) can undergo Roche lobe overflow (RLOF), resulting in a phase of long-lived mass transfer onto the SMBH. If the interval separating consecutive EMRIs is less than the mass-transfer timescale driven by gravitational wave emission (typically similar to 1-10 Myr), the semimajor axes of the two stars will approach each another on scales of less than or similar to hundreds to thousands of gravitational radii. Close flybys tidally strip gas from one or both RLOFing stars, briefly enhancing the mass-transfer rate onto the SMBH and giving rise to a flare of transient X-ray emission. If both stars reside in a common orbital plane, these close interactions will repeat on a timescale as short as hours, generating a periodic series of flares with properties (amplitudes, timescales, sources lifetimes) remarkably similar to the quasi-periodic eruptions (QPEs) recently observed from galactic nuclei hosting low-mass SMBHs. A cessation of QPE activity is predicted on a timescale of months to years, due to nodal precession of the EMRI orbits out of alignment by the SMBH spin. Channels for generating the requisite coplanar EMRIs include the tidal separation of binaries (Hills mechanism) or Type I inward migration through a gaseous AGN disk. Alternative stellar dynamical scenarios for QPEs, that invoke single stellar EMRIs on an eccentric orbit undergoing a runaway sequence of RLOF events, are strongly disfavored by formation rate constraints.

Automated summary

When a star approaches a supermassive black hole in a circular extreme mass ratio inspiral, it can undergo Roche lobe overflow, resulting in mass transfer onto the black hole. Close interactions between the stars can generate periodic flares of X-ray emission. These interactions can be repeated on short timescales and have similar properties to quasi-periodic eruptions observed in galactic nuclei hosting low-mass black holes.