The complex time and energy evolution of quasi-periodic eruptions in
  eRO-QPE1
  Quasi-periodic eruptions (QPEs) are recurrent X-ray bursts found so far 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 ($\approx10^5-10^6$ solar masses) black hole have gathered significant
attention. While these have been compared only qualitatively with observations,
the phenomenology of QPEs is developing at a fast pace, with the potential to
reveal new insights. Here we report two new observational results found in
eRO-QPE1, the brightest QPE source discovered so far: i) the eruptions in
eRO-QPE1 occur sometimes as single isolated bursts, and at others as chaotic
mixtures of multiple overlapping bursts with very different amplitudes; ii) we
confirm that QPEs peak at later times and are broader at lower energies, with
respect to higher energies while, for the first time, we find that QPEs also
start earlier at lower energies. Furthermore, eruptions appear to undergo an
anti-clockwise hysteresis cycle in a plane of hardness ratio versus total count
rate. Behavior i) was not found before 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. Result ii)
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.
