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

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 suchUV 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.