Ultraviolet Quasiperiodic Eruptions from Star–Disk Collisions in Galactic Nuclei

• Published in: Astrophysical journal. Letters Vol. 963; no. 1; p. L1
• Main Authors: Linial, Itai, Metzger, Brian D.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.03.2024; IOP Publishing
• Subjects: Accretion disks; Black holes; Collisions; Eccentric orbits; Emission; Emissions; Galactic nuclei; Low temperature; Luminosity; Parameter identification; Star mergers; Stars; Stellar flares; Supermassive black holes; Tidal disruption; Ultraviolet astronomy; Ultraviolet transient sources; Wavelengths; X-ray astronomy; X-ray transient sources; X-rays
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad2464

Abstract “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 M • ∼ 10 5.5 M ⊙ and QPE periods P ≳ 10 hr for which the predicted flares are sufficiently luminous L UV ∼ 10 41 erg s −1 to outshine the quiescent disk emission at these wavelengths. The prospects to discover such “UV QPEs” with future satellite missions such as ULTRASAT and Ultraviolet Explorer depend on the prevalence of very low-mass SMBHs and the occurrence rate of stellar EMRIs onto them. For gaseous disks produced by the tidal disruption of stars, we predict that X-ray QPEs will eventually shut off, only to later reappear as UV QPEs as the accretion rate continues to drop.