研究者業績
基本情報
- 所属
- 国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 准教授総合研究大学院大学 先端学術院 宇宙科学コース 准教授
- 学位
- 博士(理学)(1998年3月 京都大学)修士(理学)(1995年3月 京都大学)
- 研究者番号
- 80342624
- ORCID ID
https://orcid.org/0000-0002-9099-5755- J-GLOBAL ID
- 202001011170717781
- researchmap会員ID
- R000011919
経歴
7-
2026年4月 - 現在
-
2007年4月 - 2026年3月
-
2003年10月 - 2007年3月
-
2001年7月 - 2003年9月
-
2001年4月 - 2001年6月
学歴
3-
1995年4月 - 1998年3月
-
1993年4月 - 1995年4月
-
1989年4月 - 1993年3月
論文
273-
Monthly Notices of the Royal Astronomical Society 527(3) 7121-7135 2024年1月 査読有りABSTRACT We analyse the XMM–Newton RGS spectra of Wolf–Rayet (WR) 140, an archetype long-period eccentric WR+O colliding wind binary. We evaluate the spectra of O and Fe emission lines and find that the plasmas emitting these lines have the largest approaching velocities with the largest velocity dispersions between phases 0.935 and 0.968 where the inferior conjunction of the O star occurs. This behaviour is the same as that of the Ne line-emission plasma presented in our previous paper. We perform a diagnosis of the electron number density ne using the He-like triplet lines of O and Ne-like Fe–L lines. The former results in a conservative upper limit of ne ≲ 1010–1012 cm−3 on the O line-emission site, while the latter cannot impose any constraint on the Fe line-emission site because of statistical limitations. We calculate the line-of-sight velocity and its dispersion separately along the shock cone. By comparing the observed and calculated line-of-sight velocities, we update the distance of the Ne line-emission site from the stagnation point. By assuming radiative cooling of the Ne line-emission plasma using the observed temperature and the local stellar wind density, we estimate that the line-emission site extends along the shock cone by at most ±58 per cent (phase 0.816) of the distance from the stagnation point. In this framework, the excess of the observed velocity dispersion over that calculated is ascribed to turbulence in the hot-shocked plasma at earlier orbital phases of 0.816, 0.912, and 0.935, with the largest velocity dispersion of 340-630 km s−1 at phase 0.912.
-
Optics for EUV, X-Ray, and Gamma-Ray Astronomy XI 2023年10月5日
-
Nuclear Instruments and Methods in Physics Research A 1048 167975-167975 2023年3月 査読有り
-
Journal of Low Temperature Physics 209(5-6) 971-979 2022年12月 査読有り
-
Journal of Astronomical Telescopes, Instruments, and Systems 8(04) 044001 2022年10月 査読有り
-
Monthly Notices of the Royal Astronomical Society 513(4) 6074-6087 2022年7月 査読有り<jats:title>Abstract</jats:title> <jats:p>We analyze a series of XMM-Newton RGS data of the binary Wolf-Rayet star WR140 that encompasses one entire orbit. We find that the RGS detects X-rays from optically-thin thermal plasma only during orbital phases when the companion O star is on the near side of the WR star. Although such X-rays are believed to be emitted from the shock cone formed through collision of the stellar winds, temperature and density profiles of the plasma along the cone have not been measured observationally. We find that the temperature of the plasma producing Ne emission lines is 0.4-0.8 keV, using the intensity ratio of Kα lines from He-like and H-like Ne. We also find, at orbital phases 0.816 and 0.912, that the electron number density in the Ne line-emission site is approximately 1012 cm−3 from the observed intensity ratios f/r and i/r of the He-like triplet. We calculated the shock cone shape analytically, and identify the distance of the Ne line-emission site from the shock stagnation point to be 0.9-8.9× 1013 cm using the observed ratio of the line-of-sight velocity and its dispersion. This means that we will be able to obtain the temperature and density profiles along the shock cone with emission lines from other elements. We find that the photo-excitation rate by the O star is only 1.3-16.4 per cent of that of the collisional excitation at orbital phase 0.816. This implies that our assumption that the plasma is collisionally excited is reasonable, at least at this orbital phase.</jats:p>
-
Publications of the Astronomical Society of Japan 74(2) 477-487 2022年4月 査読有りAbstract We report on the results of our simultaneous observations of three large stellar flares with soft X-rays (SXRs) and an Hα emission line from two binary systems of RS CVn type. The energies released in the X-ray and Hα emissions during the flares were 1036–1038 and 1035–1037 erg, respectively. This renders the set of the observations as the first successful simultaneous X-ray/Hα observations of the stellar flares with energies above 1035 erg; although the coverage of the Hα observations was limited, with $\sim\! 10\%$ of the e-folding time in the decay phase of the flares, that of the SXR ones was complete. Combining the obtained physical parameters and those in literature for solar and stellar flares, we obtained a good proportional relation between the emitted energies of X-ray and Hα emissions for a flare energy range of 1029–1038 erg. The ratio of the Hα-line to bolometric X-ray emissions was ∼0.1, where the latter was estimated by converting the observed SXR emission to that in the 0.1–100 keV band according to the best-fitting thin thermal model. We also found that the e-folding times of the SXR and Hα light curves in the decaying phase of a flare are in agreement for a time range of 1–104 s. Even very large stellar flares with energies of six orders of magnitude larger than the most energetic solar flares follow the same scaling relationships with solar and much less energetic stellar flares. This fact suggests that their physical parameters can be estimated on the basis of the known physics of solar and stellar flares.
-
Proceedings of SPIE - The International Society for Optical Engineering 12181 2022年
-
Publications of the Astronomical Society of Japan 73(5) 1418-1428 2021年10月 査読有り<title>Abstract</title> We present our analysis of the Suzaku data of SS Cygni (SS Cyg) from 2005 both in quiescence and outburst. A fluorescent iron Kα line bears significant information about the geometry of an X-ray-emitting hot plasma and a cold reflector, such as the surfaces of the white dwarf (WD) and the accretion disk (AD). Our reflection simulation has revealed that the X-ray-emitting hot plasma is located either very close to the WD surface in the boundary layer (BL), with an upper limit radial position of &lt;1.004 times the white dwarf radius (RWD), or near the entrance of the BL where the optically thick AD is truncated at a distance of 1.14–1.27 RWD for the assumed WD mass of 1.19 M⊙ in quiescence. In the latter configuration, the plasma torus is located just above the inner edge of the AD. The result suggests that the accreting matter is heated up close to the maximum temperature immediately after the matter enters the BL. The matter probably expands precipitously at the entrance of the BL and leaves the disk plane to reach a height comparable to the radial distance of the plasma torus from the center of the WD. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface. In addition, the plasma distributes above the disk like coronae, as suggested by a previous study, and the 90% upper limit of the coronae radial position is 1.2 RWD.
-
Journal of Astronomical Telescopes, Instruments, and Systems 7(2) 028005 2021年4月 査読有り
-
Journal of Geophysical Research (Space Physics) 126(4) e28886 2021年4月 査読有り
-
The Astrophysical Journal 910(1) 25-25 2021年3月 査読有り<jats:title>Abstract</jats:title> <jats:p>We report that the RS CVn–type star GT Mus (HR 4492, HD 101379+HD 101380) was the most active star in the X-ray sky in the last decade in terms of the scale of recurrent energetic flares. We detected 11 flares from GT Mus in 8 yr of observations with the Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. The detected flare peak luminosities were 1–4 × 10<jats:sup>33</jats:sup> erg s<jats:sup>−1</jats:sup> in the 2.0–20.0 keV band for its distance of 109.6 pc. Our timing analysis showed long durations (<jats:italic>τ</jats:italic> <jats:sub>r</jats:sub> + <jats:italic>τ</jats:italic> <jats:sub>d</jats:sub>) of 2–6 days with long decay times (<jats:italic>τ</jats:italic> <jats:sub>d</jats:sub>) of 1–4 days. The released energies during the decay phases of the flares in the 0.1–100 keV band were in the range of 1–11 × 10<jats:sup>38</jats:sup> erg, which are at the upper end of the observed stellar flare. The released energies during the whole duration were in the range of 2–13 × 10<jats:sup>38</jats:sup> erg in the same band. We carried out X-ray follow-up observations for one of the 11 flares with the Neutron star Interior Composition Explorer (NICER) on 2017 July 18 and found that the flare cooled quasi-statically. On the basis of a quasi-static cooling model, the flare loop length is derived to be 4 × 10<jats:sup>12</jats:sup> cm (or 60 <jats:italic>R</jats:italic> <jats:sub>☉</jats:sub>). The electron density is derived to be 1 × 10<jats:sup>10</jats:sup> cm<jats:sup>−3</jats:sup>, which is consistent with the typical value of solar and stellar flares (10<jats:sup>10–13</jats:sup> cm<jats:sup>−3</jats:sup>). The ratio of the cooling timescales between radiative (<jats:italic>τ</jats:italic> <jats:sub>rad</jats:sub>) and conductive (<jats:italic>τ</jats:italic> <jats:sub>cond</jats:sub>) cooling is estimated to be <jats:italic>τ</jats:italic> <jats:sub>rad</jats:sub> ∼ 0.1 <jats:italic>τ</jats:italic> <jats:sub>cond</jats:sub> from the temperature; thus, radiative cooling was dominant in this flare.</jats:p>
-
Astroparticle Physics 126 102529-102529 2021年3月 査読有り
-
Publications of the Astronomical Society of Japan 73(1) 143-153 2021年2月 査読有り<title>Abstract</title> We present our analysis of the Suzaku data of U Geminorum (U Gem) from 2012 both in quiescence and outburst. Unlike SS Cygni (SS Cyg), the hard X-ray flux of U Gem is known to increase at times of optical outburst. A sophisticated spectral model and reliable distance estimate now reveal that this can be attributed to the fact that the mass accretion rate onto the white dwarf (WD) does not exceed the critical rate that causes the optically thin to thick transition of the boundary layer. From comparison of the X-ray and optical light curves, the X-ray outburst peak seems to be retarded by 2.1 ± 0.5 d, although there remains uncertainty in the X-ray peak identification, due to short data coverage. The larger delay than SS Cyg (0.9–1.4 d) also supports the lower accretion rate in U Gem. A fluorescent iron 6.4 keV emission line bears significant information about the geometry of the X-ray-emitting hot plasma and the accretion disk (AD) that reflects the hard X-ray emission. Our reflection simulation has shown that the optically thick AD is truncated at a distance of 1.20–1.25 times the white dwarf radius (RWD) in quiescence, and the accreting matter in the disk turns into the optically thin hard-X-ray-emitting plasma at this radius. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface, although disk truncation can take place in the region of &lt;1.012 RWD. From the profile of the 6.4 keV line, we have also discovered that the accreting matter is heated up close to the maximum temperature immediately after the matter enters the boundary layer at the disk truncation radius. This is consistent with the fact that the hard X-ray spectra of dwarf novae, in general, can be well represented with the cooling flow model.
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 11444 2021年1月4日 筆頭著者責任著者
-
Journal of Astronomical Telescopes, Instruments, and Systems 7(01) 014001 2021年1月 査読有り
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 11444 2020年12月30日
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 2020年12月14日
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 11444 2020年12月13日
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 11444 2020年12月13日
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 11444 2020年12月13日
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 11444 2020年12月13日
-
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray 11444 2020年12月13日
-
The Astrophysical Journal 893(1) 49 2020年4月 査読有り
-
The Astrophysical Journal 891(2) 126 2020年3月 査読有り
-
Publications of the Astronomical Society of Japan 71(5) 97 2019年10月 査読有り
-
Publications of the Astronomical Society of Japan 71(3) 50 2019年6月 査読有り
-
Monthly Notices of the Royal Astronomical Society 483(2) 1701-1721 2019年2月 査読有りHigh-resolution spectroscopy of the core of the Perseus Cluster of galaxies, using the Hitomi satellite above 2 keV and the XMM-Newton Reflection Grating Spectrometer at lower energies, provides reliable constraints on the abundances of O, Ne, Mg, Si, S, Ar, Ca, Cr, Mn, Fe, and Ni. Accounting for all known systematic uncertainties, the Ar/Fe, Ca/Fe, and Ni/Fe ratios are determined with a remarkable precision of less than 10%, while the constraints on Si/Fe, S/Fe, and Cr/Fe are at the 15% level, and Mn/Fe is measured with a 20% uncertainty. The average biases in determining the chemical composition using archival CCD spectra from XMM-Newton and Suzaku range typically from 15-40%. A simple model in which the enrichment pattern in the Perseus Cluster core and the proto-solar nebula are identical gives a surprisingly good description of the high-resolution X-ray spectroscopy results, with χ2 = 10.7 for 10 d.o.f. However, this pattern is challenging to reproduce with linear combinations of existing supernova nucleosynthesis calculations, particularly given the precise measurements of intermediate α-elements enabled by Hitomi. We discuss in detail the degeneracies between various supernova progenitor models and explosion mechanisms, and the remaining uncertainties in these theoretical models. We suggest that including neutrino physics in the core-collapse supernova yield calculations may improve the agreement with the observed pattern of α-elements in the Perseus Cluster core. Our results provide a complementary benchmark for testing future nucleosynthesis calculations required to understand the origin of chemical elements....
-
Publications of the Astronomical Society of Japan 71(1) 24 2019年1月 査読有り
-
Journal of Low Temperature Physics 193(5-6) 991-995 2018年12月 査読有り
-
Publications of the Astronomical Society of Japan 70(6) 113 2018年12月 査読有り
-
Publications of the Astronomical Society of Japan 70(3) 38 2018年6月 査読有り
-
Journal of Astronomical Telescopes, Instruments, and Systems 4(2) 021407 2018年4月 査読有り
-
Journal of Astronomical Telescopes, Instruments, and Systems 4(2) 021402 2018年4月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 13 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 14 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 17 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 15 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 9 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 10 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 16 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 11 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 12 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 19 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 21 2018年3月 査読有り
-
Publications of the Astronomical Society of Japan 70(2) 20 2018年3月 査読有り
-
14th Marcel Grossman Meeting On Recent Developments in Theoretical and Experimental General Relativity, Astrophysics and Relativistic Field Theories, Proceedings 1915-1921 2018年
-
SPACE TELESCOPES AND INSTRUMENTATION 2018: ULTRAVIOLET TO GAMMA RAY 10699 2018年
MISC
93講演・口頭発表等
8-
44th COSPAR Scientific Assembly 2022年
担当経験のある科目(授業)
7-
2025年9月 - 2026年3月宇宙理学概論(オムニパス) (総合研究大学院大学)
-
2023年9月 - 2024年3月
-
2021年4月 - 2022年3月宇宙理学概論(オムニパス) (総合研究大学院大学)
-
2019年10月 - 2020年3月宇宙理学概論(オムニパス) (総合研究大学院大学)
-
2009年10月 - 2010年3月応用物理学講義 (中央大学)
共同研究・競争的資金等の研究課題
12-
日本学術振興会 科学研究費助成事業 2023年4月 - 2027年3月
-
日本学術振興会 科学研究費助成事業 2022年4月 - 2025年3月
-
日本学術振興会 科学研究費助成事業 基盤研究(B) 2022年4月 - 2025年3月
-
日本学術振興会 科学研究費助成事業 基盤研究(A) 2020年4月 - 2023年3月
-
日本学術振興会 科学研究費助成事業 挑戦的研究(萌芽) 2019年6月 - 2022年3月