The complex time and energy evolution of quasi-periodic eruptions in eRO-QPE1

Abstract

Quasi-periodic eruptions (QPEs) are recurrent X-ray bursts found in the nuclei of low-mass galaxies. Their trigger mechanism is still unknown, but recent models involving one or two stellar-mass companions around the central massive (≈105 − 106 M⊙) black hole have gathered significant attention. While these models have been compared only qualitatively with observations, the phenomenology of QPEs is developing at a fast pace, and has the potential to reveal new insights. Here we report two new observational results found in eRO-QPE1, the brightest QPE source discovered so far. First, the eruptions in eRO-QPE1 sometimes occur as single isolated bursts, and at others as chaotic mixtures of multiple overlapping bursts with very different amplitudes. Second, we confirm that QPEs peak at later times and are broader at lower energies than at higher energies, while we find for the first time that QPEs also start earlier at lower energies. Furthermore, eruptions appear to undergo a counterclockwise hysteresis cycle in a plane of hardness ratio versus total count rate. The first behavior has not been found in any other QPE source, and implies that if a common trigger mechanism is in place for all QPEs, it must be able to produce both types of timing properties, regular and complex. The second result implies that the X-ray emitting component does not have an achromatic evolution even during the start of QPEs, and that the rise is harder than the decay at a given total count rate. This specific energy dependence could be qualitatively compatible with inward radial propagation during the rise within a compact accretion flow, the presence of which is suggested by the stable quiescence spectrum observed in general for QPE sources.