Origin of the soft X-ray excess in active galactic nuclei

Determine the physical origin of the soft X-ray excess observed in active galactic nuclei, distinguishing among warm-corona emission, relativistically ionized reflection, and hybrid explanations.

Background

The paper notes that many active galactic nuclei exhibit a soft X-ray excess below 2 keV relative to an extrapolated power-law continuum. Although several physical explanations have been proposed—including emission from an additional warm corona, relativistic ionized reflection, or a combination of both—the paper states that the origin of this component remains unresolved. This issue provides general astrophysical context but is not addressed by the paper’s timing analysis of Mrk 1044.

References

The origin of the soft excess remains under debate.

— Sudden emergence of a low-frequency hard X-ray lag in the Seyfert 1 galaxy Mrk 1044  (2608.17292 - Kang et al., 18 Aug 2026) in Section 1, Introduction

RE~J1034+396 exhibits a prominent soft X-ray excess, whose physical origin remains under debate.

— First Observational Evidence for QPO-like Coevolution between Characteristic Damping Timescales and X-ray Time Lags among AGNs  (2608.19610 - Zhang et al., 20 Aug 2026) in Section 4.1, Possible Physical Origins of the Hard- and Soft-band CDTs

We also applied a relativistic reflection model to representative observations; however, given the moderate spectral resolution and limited high-energy coverage of Swift/XRT, the current data do not allow us to robustly determine whether reflection provides a superior or physically preferred description.

— Long-term X-ray variability of the NLS1 Ark 564  (2609.30793 - R et al., 25 Sep 2026) in Abstract