Ultraviolet Quasi-periodic Eruptions from Star-Disk Collisions in Galactic Nuclei

TL;DR

This paper explores the conditions under which star-disk collisions in galactic nuclei produce ultraviolet quasi-periodic eruptions, expanding understanding of QPEs beyond X-ray observations and predicting new observable phenomena.

Contribution

It introduces a model for UV QPEs from star-disk collisions around low-mass SMBHs, highlighting conditions for UV flares and their potential observability with future missions.

Findings

UV QPEs can outshine quiescent disk emission at low SMBH masses.

Predicted UV flares have luminosities around 10^{41} erg/s.

X-ray QPEs may transition to UV QPEs as accretion rates decline.

Abstract

``Quasi-periodic 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, 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 which peak in the ultraviolet (UV) instead of the X-ray band. We identify a region of parameter space at low SMBH mass $M_{\bullet} \sim 10^{5.5}M_{\odot}$ and QPE periods $P \gtrsim 10$ hr for which the predicted flares are sufficiently luminous $L_{\rm UV} \sim 10^{41}$ 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 UVEX depends on the prevalence of very low-mass SMBH 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.