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Quasi-periodic eruptions (QPEs) are recurrent X-ray bursts found so far inthe nuclei of low-mass galaxies. Their trigger mechanism is still unknown, butrecent models involving one or two stellar-mass companions around the centralmassive ($\approx10^5-10^6$ solar masses) black hole have gathered significantattention. While these have been compared only qualitatively with observations,the phenomenology of QPEs is developing at a fast pace, with the potential toreveal new insights. Here we report two new observational results found ineRO-QPE1, the brightest QPE source discovered so far: i) the eruptions ineRO-QPE1 occur sometimes as single isolated bursts, and at others as chaoticmixtures of multiple overlapping bursts with very different amplitudes; ii) weconfirm that QPEs peak at later times and are broader at lower energies, withrespect to higher energies while, for the first time, we find that QPEs alsostart earlier at lower energies. Furthermore, eruptions appear to undergo ananti-clockwise hysteresis cycle in a plane of hardness ratio versus total countrate. Behavior i) was not found before in any other QPE source and implies thatif a common trigger mechanism is in place for all QPEs, it must be able toproduce both types of timing properties, regular and complex. Result ii)implies that the X-ray emitting component does not have an achromatic evolutioneven during the start of QPEs, and that the rise is harder than the decay at agiven total count rate. This specific energy dependence could be qualitativelycompatible with inward radial propagation during the rise within a compactaccretion flow, the presence of which is suggested by the stable quiescencespectrum observed in general for QPE sources.

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