Abstract
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 semimajor 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 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.
Highlights
Quasi-periodic eruptions (QPEs) are a newly discovered class of short X-ray bursts that originate in spatial coincidence with galactic nuclei, both active and otherwise inactive
Close flybys tidally strip gas from one or both RLOFing stars, briefly enhancing the mass-transfer rate onto the supermassive black hole (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 remarkably similar to the “quasi-periodic eruptions” (QPEs) recently observed from galactic nuclei hosting low-mass SMBHs
They last for a duration τQPE hrs, recur with periods, TQPE, that range from hours to almost a day between different sources, and exhibit peak luminosities at least an order of magnitude above the quiescent level (Miniutti et al 2019; Giustini et al 2020; Arcodia et al 2021)
Summary
Quasi-periodic eruptions (QPEs) are a newly discovered class of short X-ray bursts that originate in spatial coincidence with galactic nuclei, both active and otherwise inactive They last for a duration τQPE hrs, recur with periods, TQPE, that range from hours to almost a day between different sources, and exhibit peak luminosities at least an order of magnitude above the quiescent level (Miniutti et al 2019; Giustini et al 2020; Arcodia et al 2021). Archival X-ray detections of RX J1301.9+2747 and GSN 0691 show these nuclei have been active for at least 18.5 (Giustini et al 2020) and 11 (Miniutti et al 2019) years, respectively They may not have been generating QPEs this entire time, with XMM Slew Survey observations of GSN069 and eRO-QPE2 ruling out QPE emission as recently as ∼2014 and ∼2010 at the level of the present-day quiescent flux
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