大川　博督 (オオカワ　ヒロタダ)

写真a

所属

附属機関・学校 高等研究所

職名

准教授（任期付）
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  • 理工学術院   先進理工学部

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  • 素粒子、原子核、宇宙線、宇宙物理にする理論   一般相対論

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  • 計算科学

  • 宇宙物理学

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  • Gravitational waves from hierarchical triple systems with Kozai-Lidov oscillation

    Priti Gupta, Haruka Suzuki, Hirotada Okawa, Kei Ichi Maeda

    Physical Review D   101 ( 10 )  2020年05月  [査読有り]

    　概要を見る

    © 2020 American Physical Society. We study gravitational waves from a hierarchical three-body system up to first-order post-Newtonian approximation. Under certain conditions, the existence of a nearby third body can cause periodic exchange between eccentricity of an inner binary and relative inclination, known as Kozai-Lidov oscillations. We analyze features of the waveform from the inner binary system undergoing such oscillations. We find that variation caused due to the tertiary companion can be observed in the gravitational waveforms and energy spectra, which should be compared with those from isolated binaries and coplanar three-body system. The detections from future space-based interferometers will make possible the investigation of gravitational wave spectrum in mHz range and may fetch signals by sources addressed.

    DOI

  • Fast collective neutrino oscillations inside the neutrino sphere in core-collapse supernovae

    M. Delfan Azari, S. Yamada, T. Morinaga, H. Nagakura, S.Furusawa, A. Harada, H. Okawa, W. Iwakami, K. Sumiyoshi

    Physical Review D    2020年01月

  • Cumulative shift of periastron time of binary pulsar with Kozai–Lidov oscillation

    Suzuki H, Gupta P, Okawa H, Maeda K.-I

    Monthly Notices of the Royal Astronomical Society: Letters   486 ( 1 ) L52 - L57  2019年06月  [査読有り]

    　概要を見る

    © 2019 The Author(s). We study a hierarchical triple system with the Kozai–Lidov mechanism, and analyse the cumulative shift of periastron time of a binary pulsar by the emission of gravitational waves. Time evolution of the osculating orbital elements of the triple system is calculated by directly integrating the first-order post-Newtonian equations of motion. The Kozai–Lidov mechanism will bend the evolution curve of the cumulative shift when the eccentricity becomes large. We also investigate the parameter range of mass and semimajor axis of the third companion with which the bending of the cumulative-shift curve could occur within 100 yr.

    DOI

  • Linear analysis of fast-pairwise collective neutrino oscillations in core-collapse supernovae based on the results of Boltzmann simulations

    Milad Delfan Azari, Shoichi Yamada, Taiki Morinaga, Wakana Iwakami, Hirotada Okawa, Hiroki Nagakura, Kohsuke Sumiyoshi

    Physical Review D   99 ( 10 )  2019年05月  [査読有り]

    　概要を見る

    © 2019 American Physical Society. Neutrinos are densely populated deep inside the core of massive stars after their gravitational collapse to produce supernova explosions and form compact stars such as neutron stars and black holes. It has been considered that they may change their flavor identities through so-called fast-pairwise conversions induced by mutual forward scatterings. If that is really the case, the dynamics of supernova explosion will be influenced, since the conversion may occur near the neutrino sphere, from which neutrinos are effectively emitted. In this paper, we conduct a pilot study of such possibilities based on the results of fully self-consistent, realistic simulations of a core-collapse supernova explosion in two spatial dimensions under axisymmetry. As we solved the Boltzmann equations for neutrino transfer in the simulation not as a postprocess but in real time, the angular distributions of neutrinos in momentum space for all points in the core at all times are available, a distinct feature of our simulations. We employ some of these distributions extracted at a few selected points and times from the numerical data and apply linear analysis to assess the possibility of the conversion. We focus on the vicinity of the neutrino sphere, where different species of neutrinos move in different directions and have different angular distributions as a result. This is a pilot study for a more thorough survey that will follow soon. We find no positive sign of conversion unfortunately at least for the spatial points and times we studied in this particular model. We hence investigate rather in detail the condition for the conversion by modifying the neutrino distributions rather arbitrarily by hand.

    DOI

  • Gravitational collapse of a massless scalar field in a periodic box

    Chul Moon Yoo, Taishi Ikeda, Hirotada Okawa

    Classical and Quantum Gravity   36 ( 7 )  2019年03月  [査読有り]

    　概要を見る

    © 2019 IOP Publishing Ltd. Gravitational collapse of a massless scalar field with the periodic boundary condition in a cubic box is reported. This system can be regarded as a lattice universe model. We construct the initial data for a Gaussian-like profile of the scalar field taking the integrability condition associated with the periodic boundary condition into account. For a large initial amplitude, a black hole is formed after a certain period of time. While the scalar field spreads out in the whole region for a small initial amplitude. It is shown that the expansion law in a late time approaches that of the radiation dominated universe and the matter dominated universe for the small and large initial amplitude cases, respectively. For the large initial amplitude case, the horizon is initially a past outer trapping horizon, whose area decreases with time, and after a certain period of time, it turns to a future outer trapping horizon with the increasing area.

    DOI

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