Distinguish the causes of high-ionization UFO absorption variability

Distinguish whether the observed strengthening of the high-ionization ultra-fast outflow absorption in PDS 456 is caused by changes in the intrinsic column density, changes in the covering fraction of the high-ionization clumps, or changes in their ionization state.

Background

The time-sliced spectra show that the high-ionization UFO absorption strengthens during the observation, as indicated by both the fitted total column density and a model-independent equivalent-width measurement.

The observed trend could result from intrinsic structural changes in the absorbing clumps or from the declining ionizing continuum, which would lower the ionization state and strengthen Fe-K absorption. The available data do not discriminate between these explanations.

References

Possible explanations for the observed strengthening of the high-ionization UFO absorption include (1) variations in the intrinsic column density or covering fraction of the high-ionization clumps and (2) changes in the ionization state of the high-ionization UFO. The overall increase in opacity from slice 2 to slice 7, as the continuum flux declines following the flare, is qualitatively consistent with a decrease in the ionization state in response to the declining ionizing flux, which would strengthen the Fe-K absorption lines. Although the column density and equivalent width in slices 1 and 2 are consistent despite the higher continuum flux in slice 2, this may be attributable to their statistical uncertainties. Variations in the intrinsic column density or covering fraction are also plausible, and the present data do not allow us to distinguish among these mechanisms.

— 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 high-ionization UFO absorption”