SRG/eROSITA No. 5: Discovery of quasi-periodic eruptions every ~3.7 days from a galaxy at z>0.1

Abstract: Quasi-periodic eruptions (QPEs) are repeating soft X-ray bursts from the nuclei of galaxies, tantalizingly proposed to be extreme mass ratio inspirals. Here, we report the discovery of a new galaxy showing X-ray QPEs, the fifth found through a dedicated blind search in the \emph{SRG}/eROSITA all-sky survey data, hereafter named eRO-QPE5. Its QPE duration ($t_{\rm dur}\sim0.6$\,d), recurrence time ($t_{\rm recur}\sim3.7\,$d), integrated energy per eruption ($\sim3.4 \times 10^{47}\,$erg), and black hole mass ($M_{\rm BH}\sim 2.8 \times 10^7\,M_{\odot}$) sit at the high end of the known population. Like other eROSITA or X-ray-discovered QPEs, no previous or concurrent optical-IR transient is found in archival photometric datasets, and the optical spectrum looks almost featureless. With a spectroscopic redshift of $0.1155$, eRO-QPE5 is the most distant QPE source discovered to date. Given the number of recent discoveries, we test for possible correlations and confirm a connection between $t_{\rm dur}$ and $t_{\rm recur}$, while we do not find any significant correlation involving either $M_{\rm BH}$ or the QPE temperature. The slope of the $t_{\rm dur}-t_{\rm recur}$ relation ($1.14\pm0.16$) is roughly consistent with predictions from star-disk collision models, with a preference for those that suggest that QPEs are powered by stellar debris streams around the orbiter. Considering this and previous discoveries, eROSITA has proved extremely successful in finding many QPE candidates given its grasp, namely its sensitivity and large field of view, and scanning capabilities over the full sky. We advocate the need of sensitive wide-area and time-domain oriented surveys from future-generation soft X-ray missions.