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

TL;DR

This paper proposes a model where interactions of stellar EMRIs near supermassive black holes produce quasi-periodic eruptions (QPEs) in galactic nuclei, explaining observed X-ray flares through tidal interactions and mass transfer.

Contribution

It introduces a novel mechanism linking stellar EMRI interactions to QPEs, emphasizing the role of close flybys and orbital dynamics in producing periodic flares.

Findings

Close stellar flybys can cause transient X-ray flares resembling QPEs.

Coplanar EMRIs can generate periodic flares on hours timescales.

QPE activity ceases due to orbital precession over months to years.

Abstract

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 ~1-10 Myr), the semi-major axes of the two stars will approach each another on scales of <~ 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 an 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 inwards migration through a gaseous AGN disk. Alternative 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.