Quasi--Periodic Eruptions from Galaxy Nuclei

I consider quasi–periodic eruptions (QPEs) from galaxy nuclei. All the known cases fit naturally into a picture of accretion from white dwarfs (WDs) in highly eccentric orbits about the central black holes which decay through gravitational wave emission. I argue that ESO 243-39 HLX–1 is a QPE source at an earlier stages of this evolution, with correspondingly a longer period, more extreme eccentricity, and significantly more massive WD donor. I show explicitly that mass transfer in QPE systems is always highly stable, despite recent claims to the contrary in the literature. This stability may explain the alternating long–short eruptions seen in some QPE sources. As the WD orbit decays, the eruptions occupy larger fractions of the orbit and become brighter, making searches for quasi–periodicities in bright low–mass galaxy nuclei potentially fruitful.