Determine the physical origin of the low-ionization emission component

Determine the physical origin of the variable low-ionization emission component in PDS 456, including whether it arises from the distant low-ionization ultra-fast outflow or from a compact soft-excess-emitting region near the X-ray corona.

Background

The normalization of the low-ionization emission component appears to vary rapidly, implying a compact emitting region. If the variation responds to the observed X-ray flare, the emitting region must be located within roughly 40 gravitational radii of the corona, whereas the low-ionization absorber inferred from covering-fraction variability lies at least about 4,000 gravitational radii away.

This discrepancy motivates the possibility that the fitted emission component includes an additional compact soft-excess component, potentially produced by warm Comptonization in the inner disk, a warm corona, or an optically thick wind photosphere. The paper does not resolve which physical source produces the component.

References

One possible origin of this additional component is the soft excess, whose physical origin remains uncertain. A relevant possibility for a nearly face-on super- or near-Eddington AGN is warm Comptonization: disk photons may be Comptonized in warm, optically thick material associated with a puffed-up inner-disk surface, a warm corona, or an optically thick wind photosphere. Such a region could be both compact and close to the X-ray corona, consistent with the constraints above. Determining the physical origin of emiss_low requires more detailed modeling and is beyond the scope of this paper; we leave this question for future work.

— The Geometry of Ultra-Fast Outflows Probed by Soft X-ray Variability in PDS 456  (2608.28034 - Sato et al., 28 Aug 2026) in Appendix, Section “The variability of the low-ionization emission component”