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

Abstract

Multiple clumpy wind components ($v_{out}\sim 0.2-0.3c$) 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 \texttt{PION}. Our results reveal a stratified ionization structure, characterized by a relation between wind velocity and ionization parameter $v_{out}\propto {\xi }^{(0.14\pm 0.04)}$. To evaluate the impact of the screening effect, we tested all possible order permutations of six \texttt{PION} components. We find that highly ionized UFOs ($\log \xi >4.5$) are insensitive to their relative positions, whereas the soft X-ray UFO ($\log \xi \sim 3$ and $v_{out}\sim 0.27c$) and the lowest-ionized hard X-ray UFO ($\log \xi \sim 4.1$ and $v_{out}\sim 0.23c$) 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.

22 pages, 18 figures, and 2 tables, accepted for publication in PASJ for the XRISM special issue

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