arXiv subjects

Kazuki Ampuku

Publications and source records attributed to Kazuki Ampuku.

4 recordsLinked to original sources

arXiv2026

Probing the All-sky Distribution of Soft X-ray Emission Associated with the Milky Way

We present an all-sky analysis of soft X-ray emission associated with the Milky Way using HaloSat and ROSAT data, extending our previous study of five Galactic-plane regions to 330 fields across the sky. We performed uniform spectral fitting with a model including an unabsorbed Local Hot Bubble and two additional absorbed thermal components, representing emission from warm-hot and hot plasma, while accounting for solar wind charge exchange and the cosmic X-ray background. The warm hot component is often interpreted as emission from the circumgalactic medium, whereas the origin of the hot component remains uncertain. Individual-field fits generally favored the inclusion of the hot component, and the field-to-field temperature distributions had medians of kT = 0.19 and 0.74 keV, with 16th-84th percentile ranges of 0.17-0.21 and 0.61-0.90 keV, respectively. The corresponding 16th-84th percentile EM ranges were (1.0-8.2) x 10-2 and (0.5-4.1) x 10-3 cm-6 pc. Stacked spectra from fields within 60 deg. <= l <= 180 deg. supported the individual-field results. After excluding bright X-ray sources and large-scale Galactic structures, we modeled the EM distributions of 178 fields using a hybrid spatial framework comprising stellar emission and disk-like and halo-like diffuse-plasma components. The warm-hot emission was described mainly by the disk-like term, whereas the hot emission was described mainly by stellar emission with an additional halo-like term. These results support a widespread two temperature description and suggest a composite origin for the hot emission.

astro-ph.GA↗

arXiv2026

The First Insights into an Ultraluminous X-ray Pulsar with XRISM: Phase-Resolved High-Resolution Spectroscopy of the Fe K-shell Band of M82 X-2

During the performance verification phase, XRISM observed the M82 galaxy for a net exposure of 207.7 ks, with the ultraluminous X-ray pulsar (ULXP) X-2 included in the field of view. A pulsation search identified a candidate signal with a period close to the previously known value, 1.38727 s, at a significance of $3.15σ$ based on Monte Carlo simulations. Using this candidate period, phase-resolved spectral analysis with the high spectral resolution of Resolve was performed. The spectra suggest that, if the candidate pulsation is real, the Fe K$α$ emission line in the pulse peak phase has a larger width ($36^{+60}_{-13}$ eV) than that in the remaining phase at a significance exceeding $3σ$. This suggests that at least a fraction of the Fe K$α$ emission is associated with the ULXP system. The observed width corresponds to a velocity dispersion of $(1.7^{+2.8}_{-0.6})\times10^3$ km s$^{-1}$, which is too large to be explained by motions in the companion star atmosphere. The rise time of the pulsation constrains the line-emitting region to be smaller than $6.3\times10^4$ km, suggesting an origin in the accretion flow. This work demonstrates the capability of XRISM Resolve for pulsation-resolved high-resolution spectroscopy of ULX pulsars.

astro-ph.HE↗

arXiv2026

Development of Electroformed X-ray Optics Bridging Synchrotron Technology and Space Astronomy

We have developed X-ray telescope mirrors using an original electroforming replication technique established through the fabrication of millimeter-aperture, ultra-short-focal-length nanofocusing mirrors for synchrotron X-ray microscopy. This paper presents detailed results of X-ray illumination tests of a 60-mm-diameter, full-circumference, double-reflection monolithic electroformed nickel mirror and its Mirror Module Assembly (MMA). The experiments were conducted at the 1-km beamline BL29XUL at SPring-8. To simulate a parallel X-ray beam from celestial sources, we constructed a dedicated evaluation system, the High-Brilliance X-ray Kilometer-long Large-Area Expanded-beam Evaluation System (HBX-KLAEES). Owing to the high photon flux and the quasi-point-like source with a small divergence provided by HBX-KLAEES, the imaging performance was evaluated with high fidelity, resolving both the sharp core and large-angle components of the Point Spread Function (PSF). The results show an extremely sharp core with a Full Width at Half Maximum (FWHM) of 0.7 arcsec and a Half Power Diameter (HPD) of 14 arcsec, even after integration into the MMA. In addition, a positive correlation was found between angular resolution and axial figure error in both the primary and secondary mirror sections, indicating that axial figure errors contribute to image degradation. Based on these results, the MMA was selected as one of the hard X-ray optics for the FOXSI-4 sounding rocket experiment, which performs high-resolution soft and hard X-ray imaging spectroscopy of solar flares and was successfully launched. These results demonstrate the potential for further improvements in angular resolution and the development of high-resolution, ultra-short focal length X-ray optics for small satellites, including CubeSats.

astro-ph.IM↗

arXiv2023

Soft X-ray Energy Spectra in the Wide-Field Galactic Disk Area Revealed with HaloSat

We analyzed data from HaloSat observations for five fields in the Galactic disk located far away from the Galactic center (135$^{\circ}$ $<$ $l$ $<$ 254$^{\circ}$) to understand the nature of soft X-ray energy emission in the Galactic disk. The fields have 14$^{\circ}$ diameter and were selected to contain no significant high-flux X-ray sources. All five HaloSat soft X-ray energy spectra (0.4--7 keV with energy resolution of $<$100 eV below 1 keV) show a possibility of the presence of unresolved high-temperature plasma in the Galactic disk (UHTPGD) with a temperature of 0.8--1.0 keV and an emission measure of (8--11)$\times10^{-4} \rm cm^{-6} pc$ in addition to the soft X-ray diffuse background components mainly studied at higher galactic latitudes (solar wind charge exchange emission, local hot bubble, Milky Way halo emission, and the cosmic X-ray background). This suggests that the UHTPGD is present across the whole Galactic disk. We also observed UHTPGD emission in a region with no bright sources in an {\it XMM-Newton} field contained within one of the {\it HaloSat} fields. The temperature and emission measure are consistent with those measured with {\it HaloSat}. Moreover, the stacked spectra of the X-ray point-like sources and NIR-identified point sources such as stars in the {\it XMM-Newton} field also show a spectral feature similar to the UHTPGD emission. This suggests that the UHTPGD may partly originate from point-like sources such as stars.

astro-ph.HE↗