Discovering and studying X-ray quasi-periodic eruptions

X-ray Quasi-Periodic Eruptions (QPEs) are a newly discovered class of extragalactic X-ray transients, associated with massive black holes in low-mass galaxies. They consist of sharp high-amplitude bursts of X-ray radiation recurring every few hours, in sources which have otherwise no detected signal indicating nuclear activity at any other wavebands. Since their discovery less than four years ago (Miniutti et al., Nature 2019), only a handful of QPEs has been found so far. Half of them were found through a project I am leading, which makes use of the all-sky survey data of the eROSITA X-ray telescope (e.g. Arcodia et al., Nature 2021, for the first two cases). I will give a brief overview of the multi-wavelength observational properties of QPEs and of the models proposed to explain them. I will describe our detection method with eROSITA, which is to date the only blind and systematic search that can find more QPEs. I will conclude with the most recent highlights which indicate how these seemingly simple transients are rather complex in terms of timing variability and long-term evolution. QPEs are not only a newly-discovered type of X-ray transients, they also provide a new channel to activate the nuclei of low-mass galaxies and a new way to find nuclear black holes of less than a million solar masses.