Origin of the transient harder X-ray component near QPE peaks in eRO-QPE4

Determine the physical origin of the transient harder X-ray spectral component observed near the peak of quasi-periodic eruptions in the galaxy eRO-QPE4 with XMM-Newton, and establish whether this harder emission is an intrinsic astrophysical component of the eruption or due to non-astrophysical effects.

Background

The authors report, for the first time in the QPE population, evidence of a faint harder X-ray component appearing close to the peak of the eruptions in eRO-QPE4. They model this additional emission as Comptonization of the thermal QPE spectrum, finding it to be much fainter than the dominant soft thermal component (flux ratio ≈ 1:94 in 0.2–2.0 keV), and note that its spectral slope is largely unconstrained.

This harder component is not observed throughout all phases of the eruption and has not been seen in other QPE sources, raising uncertainty about its nature. The paper highlights the need to clarify whether the feature is a genuine part of the source’s emission mechanism or an artifact (e.g., instrumental pile-up), as discussed further in the appendix.