OKAWA, Hirotada



Affiliated organization, Waseda Institute for Advanced Study

Job title

Associate Professor(non-tenure-track)

Concurrent Post 【 display / non-display

  • Faculty of Science and Engineering   School of Advanced Science and Engineering


Research Areas 【 display / non-display

  • Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics   General Relativity

Research Interests 【 display / non-display

  • Computational Science

  • Astrophysics

Papers 【 display / non-display

  • Multidimensional Boltzmann Neutrino Transport Code in Full General Relativity for Core-collapse Simulations

    Ryuichiro Akaho, Akira Harada, Hiroki Nagakura, Kohsuke Sumiyoshi, Wakana Iwakami, Hirotada Okawa, Shun Furusawa, Hideo Matsufuru, Shoichi Yamada

    Astrophysical Journal   909 ( 2 )  2021.03

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    We develop a neutrino transfer code for core-collapse simulations that directly solves the multidimensional Boltzmann equations in full general relativity. We employ the discrete ordinate method, which discretizes the 6D phase space. The code is an extension of our special relativistic code coupled to a Newtonian hydrodynamics code, which is currently employed for core-collapse supernova simulations. In order to demonstrate our code's capability to treat general relativistic effects, we conduct some tests. We first compute the free streaming of neutrinos in the Schwarzschild and Kerr spacetimes and compare the results with the geodesic curves; in the Schwarzschild case, we deploy not only a 1D grid in space under spherical symmetry but also a 2D spatial mesh under axisymmetry in order to assess the capability of the code to compute the spatial advection of neutrinos. Second, we calculate the neutrino transport in a fixed matter background, which is taken from a core-collapse supernova simulation with our general relativistic but spherically symmetric Boltzmann hydrodynamics code, to obtain a steady neutrino distribution; the results are compared with those given by the latter code.


  • Post-Newtonian Kozai-Lidov mechanism and its effect on cumulative shift of periastron time of binary pulsar

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

    Monthly Notices of the Royal Astronomical Society   500 ( 2 ) 1645 - 1665  2021.01

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    We study the Kozai-Lidov mechanism in a hierarchical triple system in detail by the direct integration of the first-order post-Newtonian equations of motion. We analyse a variety of models with a pulsar to evaluate the cumulative shift of the periastron time of a binary pulsar caused by the gravitational wave emission in a hierarchical triple system with Kozai-Lidov mechanism. We compare our results with those by the double-averaging method. The deviation in the eccentricity, even if small, is important in the evaluation of the emission of the gravitational waves. We also calculate the cumulative shift of the periastron time by using obtained osculating orbital elements. If Kozai-Lidov oscillations occur, the cumulative shift curve will bend differently from that of the isolated binary. If such a bending is detected through the radio observation, it will be the first indirect observation of gravitational waves from a triple system.


  • Simulations of the early postbounce phase of core-collapse supernovae in three-dimensional space with full boltzmann neutrino transport

    Wakana Iwakami, Hirotada Okawa, Hiroki Nagakura, Akira Harada, Shun Furusawa, Kosuke Sumiyoshi, Hideo Matsufuru, Shoichi Yamada

    Astrophysical Journal   903 ( 2 )  2020.11

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    We report on the core-collapse supernova simulation we conducted for a 11.2M⊙ progenitor model in threedimensional space up to 20 ms after bounce, using a radiation-hydrodynamics code with full Boltzmann neutrino transport. We solve the six-dimensional Boltzmann equations for three neutrino species and the three-dimensional compressible Euler equations with Furusawa and Togashi's nuclear equation of state. We focus on the prompt convection at ∼10 ms after bounce and investigate how neutrinos are transported in the convective matter. We apply a new analysis based on the eigenvalues and eigenvectors of the Eddington tensor and make a comparison between the Boltzmann transport results and the M1 closure approximation in the transition regime between the optically thick and thin limits. We visualize the eigenvalues and eigenvectors using an ellipsoid, in which each principal axis is parallel to one of the eigenvectors and has a length proportional to the corresponding eigenvalue. This approach enables us to understand the difference between the Eddington tensor derived directly from the Boltzmann simulation and the one given by the M1 prescription from a new perspective. We find that the longest principal axis of the ellipsoid is almost always nearly parallel to the energy flux in the M1 closure approximation, whereas in the Boltzmann simulation it becomes perpendicular in some transition regions, where the mean free path is ∼0.1 times the radius. In three spatial dimensions, the convective motions make it difficult to predict where this happens and to possibly improve the closure relation there.


  • The Boltzmann-radiation-hydrodynamics Simulations of Core-collapse Supernovae with Different Equations of State: The Role of Nuclear Composition and the Behavior of Neutrinos

    Akira Harada, Hiroki Nagakura, Wakana Iwakami, Hirotada Okawa, Shun Furusawa, Kohsuke Sumiyoshi, Hideo Matsufuru, Shoichi Yamada

    Astrophysical Journal   902 ( 2 )  2020.10

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    Using the Boltzmann-radiation-hydrodynamics code, which solves the Boltzmann equation for neutrino transport, we present the results of the simulations with the nuclear equations of state (EOSs) of Lattimer and Swesty (LS) and Furusawa and Shen (FS). We extend the simulation time of the LS model and conduct thorough investigations, though our previous paper briefly reported some of the results. Only the LS model shows the shock revival. This seems to originate from the nuclear composition: the different nuclear composition results in the different energy loss by photodissociation and hence the different strength of the prompt convection and the later neutrino-driven convection. The protoneutron star seen in the FS model is more compact than that in the LS model because the existence of multinuclear species softens the EOS. For the behavior of neutrinos, we examined the flux and the Eddington tensor of neutrinos. In the optically thick region, the diffusion of neutrinos and the dragging by the motion of matter determine the flux. In the optically thin region, the free-streaming determines it. The Eddington tensor is compared with that obtained from the M1-closure relation. The M1-closure scheme overestimates the contribution from the velocity-dependent terms in the semitransparent region.


  • Threshold of primordial black hole formation in nonspherical collapse

    Chul Moon Yoo, Tomohiro Harada, Hirotada Okawa

    Physical Review D   102 ( 4 )  2020.08

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    We perform (3+1)-dimensional simulations of primordial black hole (PBH) formation starting from the spheroidal superhorizon perturbations. We investigate how the ellipticity (prolateness or oblateness) affects the threshold of PBH formation in terms of the peak amplitude of curvature perturbation. We find that, in the case of the radiation-dominated universe, the effect of ellipticity on the threshold is negligibly small for large amplitude of perturbations expected for PBH formation.


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