HISAKI Project Team
Profile Information
- Affiliation
- Professor, Institute of Space and Astronautical Science, High Energy Astrophysics, Japan Aerospace Exploration AgencyProfessor, Graduate Institute for Advanced Studies, Space and Astronautical Science program, The Graduate University for Advanced Studies, SOKENDAIVisiting Professor, School of Science, Institute of Science TokyoVisiting professor, Graduate School of Science and Technology, Kwansei Gakuin University
- Degree
- PhD(The University of Tokyo)
- J-GLOBAL ID
- 200901025041369206
- researchmap Member ID
- 1000144439
My research field is X-ray astronomy, especially observational studies of X-ray binaries including neutron stars or black holes, and development of X-ray CCD cameras for X-ray astronomy satellites.
Research Interests
6Research Areas
2Awards
1-
1996
Papers
252-
The Astrophysical Journal, 1003(2) 156-156, May 22, 2026Abstract The supermassive binary system, η Carinae, is experiencing enormous wind-driven mass loss at a rate unparalleled in the rest of the Galaxy. Their wind–wind collision (WWC) continuously produces shock heated, X-ray-emitting plasmas. The XRISM X-ray observatory observed the system in 2023 and 2024 when the X-ray emission began to increase toward periastron passage in 2025. This paper reports unprecedentedly high-resolution X-ray spectra in the Fe K α band between 6.2 and 7.1 keV, obtained with the Resolve X-ray microcalorimeter. The hydrogen-like (Ly α ) and helium-like (He α ) lines reveal three velocity components. Two of them are broadened with maximum velocities of 2000–3000 km s −1 , likely originating from the postshock companion wind. The other is relatively narrow, with a Gaussian broadening of only ∼290 km s −1 in 1 σ , which may originate from the postshock companion wind at the WWC stagnation point or penetrating the primary wind. The Fe fluorescent lines exhibit a moderate blueshift and broadening with velocities at 100–200 km s −1 , consistent with the primary wind’s velocity field. The spectra also confirm a Compton shoulder of the He α line complex for the first time. Both fluorescing and scattering spectral profiles indicate that the binary system is seen from the companion side during these observations. The flux ratio of the Compton-scattering emission to the fluorescent line suggests substantial hydrogen depletion of the primary wind, expected from CNO-cycled hydrogen nuclear fusion gas.
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Publications of the Astronomical Society of Japan, Apr 9, 2026Abstract We report the presence of a highly ionized absorber in the transient, eclipsing low-mass X-ray binary AX J1745.6-2901, observed from 2024 February 26–29 with XRISM’s Resolve and Xtend instruments. During a soft/high state without dips, Resolve’s high spectral resolution ($E/\Delta E \sim 1000$, full width at half-maximum) revealed narrow velocity widths ($\sigma \sim 110~{\rm km~s^{-1 } }$) for Fe xxvi and Ni xxviii lines, even with low photon statistics. These widths are consistent with binary orbital motion. The observed modest blueshift velocity (${\sim }160~{\rm km~s^{-1 } }$) indicates that the absorber is located sufficiently far from the neutron star (${>}10^9$ cm), so that gravitational redshift effects are not dominant. On the other hand, broad-band spectral analysis using a photoionized plasma model applied to the Xtend data constrains the absorber to lie within a radius of ${\lesssim }10^{9.5}$ cm, as inferred from the upper limits of the best-fitting ionization parameter ($\log \xi \sim 4.4$) and the large column density (${\sim }1.6\times 10^{24}~{\rm cm^{-2 } }$). At this distance, the observed outward velocity of the absorber is about an order of magnitude smaller than the escape velocity from the neutron star.
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Astronomy & Astrophysics, 708 A130-A130, Apr 1, 2026Context. The origin of accretion-disc winds remains disputed. High-inclination, dipping, neutron-star (NS) low-mass X-ray binaries (LMXBs) provide an excellent testbed for studying the launching mechanism of such winds due to them persistently accreting and showing a nearly ubiquitous presence of highly ionised plasmas. Aims. We aim to establish or rule out the presence of a wind in the high-inclination LMXB 4U 1624−49, for which a highly ionised plasma has been repeatedly observed in X-ray spectra by Chandra and XMM-Newton, and a thermal–radiative pressure wind is expected. Methods. We leveraged the exquisite spectral resolution of the X-ray Imaging and Spectroscopy Mission (XRISM) to perform phase-resolved spectroscopy of the full binary orbit to characterise the highly ionised plasma at all phases except during absorption dips. Results. An outflow is clearly detected via phase-resolved spectroscopy of the source with XRISM Resolve. Based on analysis of the radial-velocity curve, we determine an average velocity of ∼200−320 km s −1 and a column density above 10 23 cm −2 . The line profiles are generally narrow, spanning ∼50−100 km s −1 , depending on the orbital phase; this points to a low-velocity sheer or turbulence of the highly ionised outflow and a potential increase of turbulence as the absorption dip is approached, likely due to turbulent mixing. Conclusions. The line profiles, together with the derived launching radius and wind velocity, are consistent with a wind being launched from the outskirts of the disc and without stratification, pointing to a thermal-radiative pressure origin.
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Nature Astronomy, Mar 31, 2026
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Nature, 651(8107) 909-913, Mar 25, 2026
Misc.
207Professional Memberships
4-
Jan, 2000 - Present
Research Projects
15-
Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C), Japan Society for the Promotion of Science, Apr, 2019 - Mar, 2023
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Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), Japan Society for the Promotion of Science, Jun, 2012 - Mar, 2017
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, Apr, 2012 - Mar, 2015
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Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B), Japan Society for the Promotion of Science, Apr, 2010 - Mar, 2013
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科学研究費助成事業 特別研究員奨励費, 日本学術振興会, 2005 - 2007
● 指導学生等の数
1-
Fiscal Year2021年度(FY2021)Master’s program3Students under Cooperative Graduate School System3JSPS Research Fellowship (Young Scientists)1
● 専任大学名
1-
Affiliation (university)総合研究大学院大学（SOKENDAI)
