Quasi-Periodic Eruptions (QPEs) are high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central supermassive black holes in galactic nuclei. They provide a new channel to study how low-mass supermassive black holes are activated in low-mass galaxies. Previously, only two such sources were known, classified as hosting an actively accreting supermassive black hole. I will present the detection of QPEs in two further galaxies, obtained with a blind and systematic search during the first year of eROSITA operations (Arcodia et al., Nature 2021). The optical spectra of these galaxies show no signature of black hole activity, indicating that a pre-existing accretion flow typical of active nuclei is not required to trigger these events. I will give an overview of their multi-wavelength observational properties and I will discuss a few possible scenarios for their origin. What we currently suggest is that QPEs might be driven by the presence of one (or more) orbiting body (-ies) and the observed properties of QPEs indicate that the mass of the secondary object(s) needs to be much smaller than the main body. This could make QPEs a viable candidate for the electromagnetic counterparts of the so-called extreme mass-ratio inspirals, with considerable implications for the future of multi-messenger astrophysics and cosmology.
X-ray bursts from two previously quiescent galaxies: massive black holes awakening?