Unraveling the structure of the stratified ultra-fast outflows in PDS 456 with XRISM
Yerong Xu, Luigi Gallo, Kouichi Hagino, J. N. Reeves, Francesco Tombesi, Misaki Mizumoto, A. Luminari, A G Gonzalez, Ehud Behar, Rozenn Boissay-Malaquin, V. Braito, Pierpaolo Condó, Chris Done, Aiko Miyamoto, Ryuki Mizukawa, Hirokazu Odaka, Riki Sato, A. Tanimoto, Makoto S Tashiro, Tahir Yaqoob, Satoshi Yamada
Abstract
Abstract Multiple clumpy wind components ($v_\mathrm{out}\sim 0.2\!-\!0.3c$) in the luminous quasar PDS 456 have recently been resolved in the Fe K band for the first time, thanks to the high-resolution X-ray spectrometer Resolve onboard XRISM. 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, characterized by a scaling relation between wind velocity and ionization parameter $v_\mathrm{out}\propto \xi ^{(0.14\pm 0.04)}$. To evaluate the screening effect in photoionization modeling, we tested all possible order permutations of six 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 $v_\mathrm{out}\sim 0.27c$) and the lowest-ionized hard X-ray UFO ($\log \xi \sim 4.1$ and $v_\mathrm{out}\sim 0.23c$) 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. 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 XRISM with an open gate valve 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 active galactic nucleus feedback.