arXiv subjects

Toshiya Iwata

Publications and source records attributed to Toshiya Iwata.

7 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↗

arXiv2026

XRISM Discovery of Multiple Ionized Fe-K Emission and Absorption Components in Centaurus A

We present the first clear detection of ionized Fe-K emission and absorption components in the nearby radio galaxy Centaurus A, revealed by the high-resolution XRISM/Resolve detector. In the 6.5-6.9 keV band, XRISM reveals multiple Fe XXV and Fe XXVI emission components. One is a broad (with a width of sigma = 3000 km/s) and redshifted (+3400 km/s) component, originating at D = 0.02 pc from the central black hole. The other two components are narrow (with a width of sigma = 500 km/s) and exhibit redshifted and blueshifted velocities (+2600 km/s and -1500 km/s), originating from more distant regions (D = 0.1 pc). The photo-ionized model explains the broader component, while the two narrower components can be explained by either photo-ionization or collisional ionization. One interpretation is that the broader component is an outflow at ~10^2 R_S (R_S; Schwarzschild radius) and the narrow component is a shock-heated plasma close to the torus, with a possible connection to the JWST-discovered outflow outside the torus. Two blueshifted absorption lines are detected at ~7.1 keV (~10^4 km/s) and ~10.6 keV (~10^5 km/s). The line significance of the 10.6 keV line is above 98%. The absorption line components might be attributed to the broad emission component. These results demonstrate the high potential of XRISM/Resolve to characterize ionized emission and absorption features in the Fe-K band. Our findings establish a new benchmark in the study of circumnuclear environments in low-luminosity radio galaxies, thereby contributing to a broader understanding of AGN unification.

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.

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arXiv2025

XRISM analysis of the complex Fe K$\alpha$ line in Centaurus A

We analyze the high-resolution XRISM/Resolve spectrum of the Fe K$\alpha$ emission line of the nearest active galactic nucleus, in Centaurus A. The line features two narrow and resolved peaks of Fe K$\alpha_1$, and Fe K$\alpha_2$ with a FWHM of $(4.8\pm0.2)\times10^2$ km/s each. A broad line with a FWHM of $(4.3\pm0.3)\times10^3$ km/s, and with a flux similar to the two narrow line cores, is also required. This broad component is not observed in the optical or IR spectra of Cen A. The line shape requires the existence of an emission region that extends from $\sim10^{-3}$ pc to $\sim10^1$ pc. Assuming that the emissivity follows a radial power-law profile of $r^{-q}$, we find $q\approx2$. This may indicate an extended corona, an emitting region that bends towards the corona, or a non-uniform density. When assuming $q=3$, the line shape can only be reproduced by including three emitting components in the model. The measured best-fit inclination is $24^{+13}_{-7}$ degrees, but higher inclinations are only slightly disfavored. A single blurred MYTorusL line profile can describe the line shape, but requires a large relative normalization. This could be due to past variability, modified abundances, or differing geometries. The line shape can be reproduced from the radii measured by reverberation mapping, but only if an additional extended emitting region at small radii is included.

astro-ph.HE↗

arXiv2024

Imaging reconstruction method on X-ray data of CMOS polarimeter combined with coded aperture

X-ray polarization is a powerful tool for unveiling the anisotropic characteristics of high-energy celestial objects. We present a novel imaging reconstruction method designed for hard X-ray polarimeters employing a Si CMOS sensor and coded apertures, which function as a photoelectron tracker and imaging optics, respectively. Faced with challenges posed by substantial artifacts and background noise in the coded aperture imaging associated with the conventional balanced correlation method, we adopt the Expectation-Maximization (EM) algorithm as the foundation of our imaging reconstruction method. The newly developed imaging reconstruction method is validated with imaging polarimetry and a series of X-ray beam experiments. The method demonstrates the capability to accurately reproduce an extended source comprising multiple segments with distinct polarization degrees. Comparative analysis exhibits a significant enhancement in imaging reconstruction accuracy compared to the balanced correlation method, with the background noise levels reduced to 17%. The outcomes of this study enhance the feasibility of Cube-Sat imaging polarimetry missions in the hard X-ray band, as the combination of Si CMOS sensors and coded apertures is a promising approach for realizing it.

astro-ph.IM↗

arXiv2024

Multi-epoch X-ray spectral analysis of Centaurus A: revealing new constraints on iron emission line origins

We conduct X-ray reverberation mapping and spectral analysis of the radio galaxy Centaurus A to uncover its central structure. We compare the light curve of the hard X-ray continuum from Swift Burst Alert Telescope observations with that of the Fe K$α$ fluorescence line, derived from the Nuclear Spectroscopic Telescope Array (NuSTAR), Suzaku, XMM-Newton, and Swift X-ray Telescope observations. The analysis of the light curves suggests that a top-hat transfer function, commonly employed in reverberation mapping studies, is improbable. Instead, the relation between these light curves can be described by a transfer function featuring two components: one with a lag of $0.19_{- 0.02}^{+ 0.10}~\mathrm{pc}/c$, and another originating at $r > 1.7~\mathrm{pc}$ that produces an almost constant light curve. Further, we analyze the four-epoch NuSTAR and six-epoch Suzaku spectra, considering the time lag of the reflection component relative to the primary continuum. This spectral analysis supports that the reflecting material is Compton-thin, with $N_{\mathrm{H}} = 3.14_{-0.74}^{+0.44} \times 10^{23}~ \mathrm{cm}^{-2}$. These results suggest that the Fe K$α$ emission may originate from Compton-thin circumnuclear material located at sub-parsec scale, likely a dust torus, and materials at a greater distance.

astro-ph.HE↗

arXiv2024

Development of the X-ray polarimeter using CMOS imager: polarization sensitivity of a $1.5~{\rm μm}$ pixel CMOS sensor

We are developing an imaging polarimeter by combining a fine-pixel CMOS image sensor with a coded aperture mask as part of the cipher project, aiming to achieve X-ray polarimetry in the energy range of $10$$\unicode{x2013}$$30~\mathrm{keV}$. A successful proof-of-concept experiment was conducted using a fine-pixel CMOS sensor with a $2.5~\mathrm{μm}$ pixel size. In this study, we conducted beam experiments to assess the modulation factor (MF) of the CMOS sensor with a $1.5~\mathrm{μm}$ pixel size manufactured by Canon and to determine if there was any improvement in the MF. The measured MF was $8.32\% \pm 0.34\%$ at $10~\mathrm{keV}$ and $16.10\% \pm 0.68\%$ at $22~\mathrm{keV}$, exceeding those of the $2.5~\mathrm{μm}$ sensor in the $6$$\unicode{x2013}$$22~\mathrm{keV}$ range. We also evaluated the quantum efficiency of the sensor, inferring a detection layer thickness of $2.67 \pm 0.48~{\rm μm}$. To develop a more sensitive polarimeter, a sensor with a thicker detection layer, smaller pixel size, and reduced thermal diffusion effect is desirable.

astro-ph.IM↗