HISAKI Project Team
Profile Information
- Affiliation
- Professor, Institute of Space and Astronautical Science, Japan Aerospace Exploration AgencyProfessor, Graduate School of Science Department of Physics, The University of TokyoVisiting Professor, College of Science and Engineering, Aoyama Gakuin UniversityVisiting Professor, National Institute for Fusion Science
- Degree
- PhD(Mar, 2008, Kyoto University)
- Contact information
- yamaguchi.hiroya
jaxa.jp - Researcher number
- 00513467
- J-GLOBAL ID
- 201801007525282778
- researchmap Member ID
- B000340695
- External link
主な研究テーマ・プロジェクト業務
宇宙プラズマ現象、Ｘ線連星、Ia型超新星、超新星残骸、銀河団、原子過程、実験室宇宙物理学
XRISM プロジェクトサイエンティスト
下記の業績リストは不完全です。「論文」は主要論文のみ、「講演・口頭発表等」は最近の招待講演のみリストしています。
座右の銘
努力して運を待て（仁科芳雄） 努力を続けていれば運が向いた時に見逃さないって意味だと思う。運は誰にでも同じ頻度で巡ってくる。それに気づき活かせるかは本人の努力次第。
疑行無名、疑事無功（『戦国策』） 後から迷走しないように達成目標と実施計画を明確にすべしって意味だと思う。目的意識もなく仕事に取り掛かるから、行き当たりばったりになり良い成果が挙がらない。
人、城を頼らば、城、人を捨せん（織田信長） プロジェクトがあれば何とかなると考えていたらプロジェクトが頓挫したときに自力で起き上がれないって意味だと思う。この懸念は2016年に現実のものとなった。
Research Interests
10Research Areas
2Research History
13-
Apr, 2026 - Present
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Jun, 2025 - Present
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May, 2025 - Present
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Apr, 2023 - Mar, 2026
Education
3-
Apr, 2003 - Mar, 2008
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Apr, 1999 - Mar, 2003
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Apr, 1996 - Mar, 1999
Committee Memberships
9-
Apr, 2026 - Present
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Apr, 2023 - Present
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Apr, 2022 - Present
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May, 2021 - Present
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Oct, 2018 - Present
Awards
9Papers
71-
The Astrophysical Journal Letters, 1006(2) L61-L61, Jul 28, 2026 Peer-reviewedAbstract Accretion disks in X-ray binaries regulate mass transfer onto compact objects and drive radiative and kinetic feedback to their surroundings. Here we report X-ray spectroscopy of the eclipsing neutron star low-mass X-ray binary AX J1745.6−2901 with XRISM/Resolve. The phase-averaged Fe xxvi Ly α absorption profile exhibits two absorption minima with relative depths that are inconsistent with the theoretical Ly α 1 /Ly α 2 doublet ratio expected from a single velocity component. We demonstrate that this profile is well described by two discrete velocity components: a blueshifted component at v ≃ −160 km s −1 and a redshifted component at v ≃ +590 km s −1 . The significance of the redshifted component is more than 3 σ based on a Monte–Carlo calculation. This velocity structure persists across orbital phases, disfavoring a localized origin such as a bulge or dip. The blueshifted component, well below the outer-disk escape velocity, is consistent with a slow outflow or disk atmosphere. The redshifted absorber can be explained either by infalling gas from a failed wind or by a gravitational redshift, and the present data cannot rule out either possibility. Regardless of its origin, the redshifted component is kinematically separate from the disk atmosphere and outflow. The absence of absorption at intermediate velocities further indicates a genuinely bimodal velocity distribution rather than the two ends of a single continuous flow, offering a new view of the absorbing-gas kinematics.
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Publications of the Astronomical Society of Japan, 78(4) 1268-1283, Jun 8, 2026 Peer-reviewedLast authorAbstract We observed the dwarf nova SS Cyg with the X-ray microcalorimeter onboard XRISM both in quiescence and outburst. The quiescence spectrum is explained with a multi-temperature optically thin thermal plasma emission model, characterized by He$\alpha$ and Ly$\alpha$ emission lines of Fe and Ly$\alpha$ lines of Si and S. We discovered redshifts of the Fe He$\alpha$ and Ly$\alpha$ lines that are larger than Ly$\alpha$ lines from Si and S. This indicates that the boundary layer (BL) plasma accretes on to the white dwarf (WD) accompanied by radiative cooling. Its radial accretion velocity estimated from the Fe redshift is 210–280 km s$^{-1}$. Since a 6.4 keV Fe emission line is much narrower ($\sigma _v = 561$ km s$^{-1}$) than that expected from the inner accretion disk rotation, we believe that it is emitted from the surface of the WD. The outburst spectrum can also be explained with the multi-temperature optically thin thermal plasma emission model, but is much softer than in quiescence and is dominated by He$\alpha$ lines. The Fe He$\alpha$ and Ly$\alpha$ lines and the 6.4 keV line are much broader than in quiescence, with Gaussian sigma values of 2390 and 3040 km s$^{-1}$, respectively. This leads us to the idea that the hot plasma is a corona generated through magnetic activity in the optically thick BL that has strong differential rotation, and is anchored to the BL with the magnetic field. This picture is supported by the facts that the plasma follows the universal correlation between the temperature and the emission measure of the stellar flares, and the relation between the X-ray luminosity and the photospheric luminosity of low-mass stars. The fluorescent 6.4 keV line is emitted via fluorescence from the optically thick BL and/or from the equatorial accretion belt formed temporarily during the outburst.
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Publications of the Astronomical Society of Japan, 78(3) 1141-1153, May 4, 2026 Peer-reviewedAbstract Cygnus X-3, hosting a Wolf–Rayet (WR) star whose dense wind produces various spectral lines due to photoionization by X-rays from a compact object, provides an ideal laboratory for studying wind dynamics and density structure. We measured the orbital modulations of the Fe, Ca, Ar, and S Ly$\alpha$ lines observed with the X-ray microcalorimeter (Resolve) onboard the XRISM, taking account of both emission and absorption lines of the Ly$\alpha$ complexes. The modulations of Doppler shifts of the Fe, Ca, Ar, and S Ly$\alpha$ lines showed amplitudes of $500$ km s$^{-1}$ and phase offsets of 0.04, 0.09, 0.11, and 0.17, respectively, in units of an orbital period ($4.8$ h) relative to the orbital motion of the compact object. This result indicated that H-like Fe most closely follows the compact object’s motion. The line widths ranged from 400 to $1000$ km s$^{-1}$. The intensities of both emission and absorption lines reached their minima around orbital phase 0.0 and their maxima around phase 0.5. The absorption peaks, however, did not align exactly with phase 0.5, suggesting inhomogeneous structures such as an accretion wake and/or a bow shock. We compared the observed modulations with calculations based on a stellar wind model, accelerated by ultraviolet radiation from the WR star. One of the calculations qualitatively reproduced the observed trend that H-like Fe ions were concentrated near the compact object, whereas H-like S was distributed across the binary system, with H-like Ca and Ar showing intermediate spatial distributions. From this comparison, we estimated that the mass-loss rate of the stellar wind was approximately $5 \times 10^{-6}$–$1 \times 10^{-5}\, M_{\odot }$ yr$^{-1}$.
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Nature, 651(8107) 909-913, Mar 25, 2026 Peer-reviewed
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Publications of the Astronomical Society of Japan, 78(2) 735-744, Mar 6, 2026 Peer-reviewedAbstract The Fe K$\alpha$ fluorescence line at 6.4 keV is a powerful probe of cold matter surrounding X-ray sources and has been widely used in various astrophysical contexts. The X-ray microcalorimeter spectrometer onboard XRISM can measure line shifts with unprecedented precision of ${\sim }$0.2 eV, equivalent to a line-of-sight velocity of ${\sim }$10 km s$^{-1}$. At this level of accuracy, however, several factors that influence the line energy must be carefully considered prior to astrophysical interpretation. One such important factor is the ionization degree, Fe$^{q+}$. The K$\alpha$ line shifts redward by ${\sim }$4 eV as q increases from 0 (neutral) to 8 (Ar-like). Additionally, the accompanying Fe K$\beta$ line at 7.06 keV shifts blueward by ${\sim }$30 eV from $q=0$ to 8. We demonstrate that this effect is actually observable in the XRISM data of the high-mass X-ray binary Centaurus X-3 (Cen X-3). We advocate that the differential energy shift between the K$\alpha$ and K$\beta$ line provides a robust estimate of q by decoupling from other effects that shift the two lines in the same direction. We derived $q \sim 5$ (Sc-like) for the fluorescing matter by comparing the observation with atomic structure calculations of our own and in the literature. By accounting for the derived charge state and the corresponding shift in the rest-frame line energy, we made corrections for this effect and reached a consistent residual shift among the K$\alpha$, K$\beta$, and the optical measurement attributable to the systemic velocity of the system. Consequently, we obtained a new constraint on the location of the cold matter. This ionization effect needs to be assessed in all use cases of the Fe K$\alpha$ line shift beyond Cen X-3, and the proposed metric is generally applicable to all of them.
Misc.
9-
NASA/GSFC press release, Apr, 2015
Books and Other Publications
2Presentations
51-
The 16th Asian International Seminar on Atomic and Molecular Physics, Oct 28, 2026 Invited
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2nd Plasma Astrophysics School and Workshop (PASW26), May 12, 2026 Invited
Teaching Experience
5-
物理学ゼミナール (東京大学)
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高エネルギー天文学特論 (東京大学 天文学専攻)
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Advanced Plasma Physics (The University of Tokyo)
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Advanced Space Physics and Astronomy (The University of Tokyo)
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JAXA宇宙科学技術講義 (立教大学)
Professional Memberships
2Research Projects
9-
科学研究費助成事業, 日本学術振興会, Apr, 2026 - Mar, 2030
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所外施設利用共同研究 研究コア提案型共同研究, 核融合科学研究所, Apr, 2025 - Mar, 2028
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OPEN MIX LAB, 自然科学研究機構, Apr, 2026 - Mar, 2027
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, Apr, 2023 - Mar, 2027
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OPEN MIX LAB, 自然科学研究機構, Apr, 2024 - Mar, 2026
● 指導学生等の数
1-
Fiscal Year2026年度(FY2026)Doctoral program3Master’s program5Students under Cooperative Graduate School System4Students under Commissioned Guidance Student System4Students under Skills Acquisition System3
● 指導学生の表彰・受賞
4-
Student NameLeo HirataStudent affiliationThe University of TokyoAward高エネルギー加速器研究機構 測定器開発センター 第15回測定器開発優秀修士論文賞Date2025/9/17
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Student NameLeo HirataStudent affiliationThe University of TokyoAward22nd International Conference on Atomic Processes in Plasmas Best Student Poster AwardDate2025/7/24
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Student NameDaiki MiuraStudent affiliationThe University of TokyoAwardXRISM Science Workshop for Young Researchers 2024 Best Poster AwardDate2024/9/27
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Student NameYuken OshiroStudent affiliationThe University of TokyoAwardXRISM Science Workshop for Young Researchers 2024 Best Poster AwardDate2024/9/27
● 指導学生の顕著な論文
2-
Student nameYuken OhshiroStudent affiliation東京大学Author(s), journal, volume number, pagination (year of publication)Ohshiro et al., ApJL 913, L34, 7 (2021)TitleDiscovery of a Highly Neutronized Ejecta Clump in the Type Ia Supernova Remnant 3C 397DOIhttps://doi.org/10.3847/2041-8213/abff5b
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Student nameDaiki MiuraStudent affiliationThe University of TokyoAuthor(s), journal, volume number, pagination (year of publication)Daiki Miura, Shogo Kobayashi, Hiroya YamaguchiTitlePhase-dependent Spectral Shape Changes in the Ultraluminous X-Ray Pulsar NGC 5907 ULX1DOI10.3847/1538-4357/ad4451
● 専任大学名
1-
Affiliation (university)東京大学（University of Tokyo)
