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

TL;DR

This paper presents new observational insights into quasi-periodic eruptions (QPEs) in galaxy nuclei, revealing complex timing behaviors and energy-dependent evolution that challenge existing models and suggest inward radial propagation in accretion flows.

Contribution

It reports the first observation of both isolated and overlapping QPE bursts in eRO-QPE1, and finds that QPEs start earlier at lower energies, providing new constraints on their trigger mechanisms.

Findings

QPEs can occur as isolated or overlapping bursts.

QPEs peak later and are broader at lower energies.

QPEs start earlier at lower energies, indicating energy-dependent evolution.

Abstract

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.