Unraveling the structure of the stratified ultra-fast outflows in PDS 456 with XRISM

Abstract

Multiple clumpy wind components (vₒᵤₜ ~0.2-0.3c) in the luminous quasar PDS 456 have recently been resolved by XRISM in the Fe-K band for the first time. In this paper, we investigate the structure of ultra-fast outflows (UFOs) using coordinated observations from XRISM, XMM-Newton, and NuSTAR, along with the self-consistently calculated photoionization model PION. Our results reveal a stratified ionization structure likely driven by the radiation field, characterized by a relation between wind velocity and ionization parameter vₒᵤₜ ∝ ξ*. To evaluate the impact of the screening effect, we tested all possible order permutations of six PION components. We find that highly ionized UFOs (log ξ > 4.5) are insensitive to their relative positions, whereas the soft X-ray UFO (log ξ ~ 3 and vₒᵤₜ ~ 0.27c) and the lowest-ionized hard X-ray UFO (log ξ ~ 4.1 and vₒᵤₜ~ 0.23c) are statistically favored -- based on the evidence from both the C-statistic and Bayesian analysis -- to occupy the middle and innermost layers, respectively. This suggests a possible trend where slower UFOs are launched from regions closer to the supermassive black hole (SMBH). The soft X-ray UFO is found to be thermally unstable, regardless of its relative position. However, its location remains unclear. Our sequence analysis and its similarity to hard X-ray UFOs suggest that they may be co-spatial, while variability constraints support its location within the broad-line region at sub-parsec scales. Simulations with the gate-valve opened XRISM show that high-resolution soft X-ray data can enhance the reliability of our results. Furthermore, simulations with the future X-ray mission NewAthena demonstrate its capability to resolve the absorber sequence and spatial distributions, enabling the determination of UFO structures and their roles in AGN feedback.

* = superscript (0.38±0.06)