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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 massM•∼ 105.5M⊙and QPE periodsP≳ 10 hr for which the predicted flares are sufficiently luminousLUV∼ 1041erg s−1to 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.
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This content will become publicly available on April 3, 2026
Radiation Transport Simulations of Quasiperiodic Eruptions from Star–Disk Collisions
Abstract Periodic collisions between a star on an inclined orbit around a supermassive black hole and its accretion disk offer a promising explanation for X-ray “quasiperiodic eruptions” (QPEs). Each passage through the disk midplane shocks and compresses gas ahead of the star, which subsequently re-expands above the disk as a quasi-spherical cloud. We present spherically symmetric Monte Carlo radiation transport simulations that follow the production of photons behind the radiation-mediated shock, Comptonization by hot electrons, and the eventual escape of the radiation through the expanding debris. Such 1D calculations are approximately justified for thin disks (scale-heighth ≲ few × R⋆), through which the star of radiusR⋆passes more quickly than the shocked gas can flow around the star. For collision speedsvcoll ≳ 0.15cand disk surface densities Σ ∼ 103g cm−2characteristic of those encountered by stellar orbits consistent with QPE recurrence times, the predicted transient light curves exhibit peak luminosities ≳1042erg s−1and Comptonized quasi-thermal (Wien-like) spectra that peak at energieshν ∼ 100 eV, which is broadly consistent with QPE properties. For these conditions, gas and radiation are out of equilibrium, rendering the emission temperature harder than the blackbody value due to inefficient photon production behind the radiation-mediated shock. The predicted eruptions execute counterclockwise loops in hardness–luminosity space, qualitatively similar to QPE observations. Reproducing the observed eruption properties (duration, luminosity, temperature) requires a large radiusR⋆ ≳ 10R⊙, which may point to inflation of the star’s atmosphere from repeated collisions.
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- Award ID(s):
- 2009255
- PAR ID:
- 10623927
- Publisher / Repository:
- IoP
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 983
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 40
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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