arXiv subjects

Riki Sato

Publications and source records attributed to Riki Sato.

3 recordsLinked to original sources

arXiv2026

The Geometry of Ultra-Fast Outflows Probed by Soft X-ray Variability in PDS 456

Constraining the location and geometry of ultra-fast outflows (UFOs) is essential for identifying where they are launched and how they are accelerated. We investigate the soft X-ray variability of the luminous quasar PDS 456 using simultaneous March 2024 observations with XRISM/Xtend and NuSTAR. A model-independent comparison between the flare and quiescent phases reveals spectral variability around 1 keV in the rest frame, while the hard X-ray spectral shape remains nearly unchanged. Broadband spectral fitting shows that the soft X-ray structure is well described by a partial-covering low-ionization UFO with $\log (\xi/(\mathrm{erg~cm~s^{-1}})) \simeq 3.1$ and $v_{\mathrm{out}} \simeq 0.30c$. Time-sliced spectral analysis further reveals significant variability in the covering fraction of this absorber. Interpreting this variability as transverse motion across the X-ray source, we constrain the crossing velocity to be $v_{\mathrm{cross}} \lesssim 5 \times 10^{-3}c$ and derive a lower limit on the absorber distance of $r \gtrsim 4 \times 10^3 R_{\mathrm{g}}$. This location is substantially farther out than the high-ionization UFO previously inferred at ~200-600 $R_{\mathrm{g}}$, while the two phases have comparable outflow velocities. The resulting velocity-distance structure disfavors a self-similar magnetocentrifugal wind and instead suggests either radiation-pressure acceleration following a Castor-Abbott-Klein-like velocity law or compact magnetic acceleration through magnetic reconnection. These results demonstrate that soft X-ray partial-covering variability can provide a geometrical probe of UFOs and directly connect spectral variability to wind acceleration.

astro-ph.HE↗

arXiv2025

Development of Solar Flare X-ray Polarimeter with Micro-Pixel CMOS Sensors

We are developing an X-ray polarimeter using micro-pixel CMOS sensors for solar flare X-ray polarimetry. The system consists of a 2.5-$\mu$m pixel CMOS image sensor with a 12.8$\times$12.8 mm$^2$ imaging area and a readout system based on a Zynq System-on-Chip. While previous studies have validated this concept, no realistic feasibility studies have been conducted for the solar flare X-ray polarization observation. In this work, we performed polarization sensitivity measurements at synchrotron facilities. The results show that our polarimeter is sensitive to the X-ray polarization, exhibiting a modulation factor of 5-15% at an energy range of 6-22 keV. The measurements also determined the thickness of the sensitive layer to be approximately 5 $\mu$m, and the thicknesses of the insensitive layers to be 0.8 $\mu$m (Si), 2.1 $\mu$m (SiO2), and 0.24 $\mu$m (Cu). These measured thicknesses lead to a quantum efficiency of 3-4% at 10 keV. Based on these experimental evaluations, we estimated the sensitivity of the micro-pixel CMOS polarimeter system. We found that, when combined with a telescope with an effective area of $\sim$10 cm$^2$, this system can detect X-ray polarization with a polarization degree of a few percent for M-class flares.

astro-ph.IM↗

arXiv2025

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

Multiple clumpy wind components ($v_{out}\sim0.2-0.3c$) 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 $v_{out}\propto\xi^{(0.14\pm0.04)}$. 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 ($\log\xi\sim3$ and $v_{out}\sim0.27c$) and the lowest-ionized hard X-ray UFO ($\log\xi\sim4.1$ and $v_ {out}\sim0.23c$) 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.

astro-ph.HE↗