Quasi--Periodic Eruptions from Galaxy Nuclei

Abstract: 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.