“Quasiperiodic eruptions” (QPE) are recurrent nuclear transients with periods of several hours to almost a day, which thus far have been detected exclusively in the X-ray band. We have shown that many of the key properties of QPE flares (period, luminosity, duration, emission temperature, alternating long-short recurrence time behavior, and source rates) are naturally reproduced by a scenario involving twice-per-orbit collisions between a solar-type star on a mildly eccentric orbit, likely brought into the nucleus as an extreme mass-ratio inspiral (EMRI), and the gaseous accretion disk of a supermassive black hole (SMBH). The flare is generated by the hot shocked debris expanding outwards from either side of the disk midplane, akin to dual miniature supernovae. Here, we consider the conditions necessary for disk–star collisions to generate lower-temperature flares that peak in the ultraviolet (UV) instead of the X-ray band. We identify a region of parameter space at low SMBH mass
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.
more » « less- Award ID(s):
- 2009255
- NSF-PAR ID:
- 10362811
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 926
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 101
- Size(s):
- ["Article No. 101"]
- Sponsoring Org:
- National Science Foundation
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