Details of the Researcher

PHOTO

Tomoki Wada
Section
Frontier Research Institute for Interdisciplinary Sciences
Job title
Degree

  • Ph.D. (Kyoto University)

Research History 6

  • 2025/04 - Present
    National Chung Hsing University　Visiting researcher

  • 2025/04 - Present
    Tohoku University　Frontier Research Institute for Interdisciplinary Sciences　Postdoctral Fellow (JSPS PD)

  • 2025/01 - 2025/03
    National Chung Hsing University　Postdoctral Fellow

  • 2024/04 - 2024/12
    Tohoku University　Frontier Research Institute for Interdisciplinary Sciences　Postdoctral Fellow

  • 2022/04 - 2024/03
    東京大学宇宙線研究所　特任研究員

  • 2020/04 - 2022/03
    京都大学大学院 基礎物理学研究所 日本学術振興会特別研究員

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Education 2

  • 京都大学大学院　理学研究科

    2017/04 - 2022/03

  • 京都大学 理学部

    2013/04 - 2017/03

Professional Memberships 2

  • 日本天文学会

  • 日本物理学会

Research Interests 4

  • マグネター

  • High Energy Astrophysics

  • Gravitational wave

  • Fast Radio Burst

Research Areas 2

  • Natural sciences / Astronomy /

  • Natural sciences / Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics /

Papers 11

  1. Statistical Inference of Fast Radio Burst Environments Using Galaxy Number Density

    Vignesh V. V. Rao, Tetsuya Hashimoto, Tomotsugu Goto, Shotaro Yamasaki, Mohanraj Madheshwaran, Sridhar Gajendran, Tomoki Wada, Simon C.-C. Ho, Terry Long Phan, Yuri Uno, Amos Y.-A. Chen, Hiroto Masaka

    Publications of the Astronomical Society of the Pacific　2026/08/01

    DOI： 10.1088/1538-3873/ae918e 　

  2. Constraints on the Physical Association between ICECAT1 Neutrinos and Fast Radio Bursts Using the Second CHIME/FRB Catalogue

    Hiroto Masaoka, Tetsuya Hashimoto, Shotaro Yamasaski, Yuhei Iwata, Tomoki Wada, Tomotsugu Goto, Shintaro Yoshiura, Kazuaki Hashiyama, Mareki Honma, Takuya Akahori, Kohei Kurahara

    2026/03/26

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    We present a search for neutrino counterparts to fast radio bursts (FRBs) using temporal and spatial cross-matching between the Second CHIME/FRB catalogue and the IceCube high-energy alert-track catalogue ICECAT1. Because current FRB--neutrino models do not provide a unique consensus on emission ordering, our primary significance test adopts a two-sided, order-agnostic temporal hypothesis. The analysis accounts for declination-dependent CHIME/FRB exposure and the look-elsewhere effect across multiple trials. No statistically significant FRB--neutrino association is found. The most significant pair is FRB\,20190630C--IC\,190629A, with a post-trial probability of $p=0.076$ ($1.43σ$), consistent with a chance coincidence. Within our statistical framework, a detectable physical association would require a time offset shorter than $\sim256$~s at $3σ$ or $\sim63$~ms at $5σ$. Using a population-level stacking analysis, we derive 90\% upper limits on the neutrino-to-radio luminosity ratio of FRBs, $ξ\lesssim 10^{8}-10^{11}$ for neutrino power-law spectral indices $γ=1.0-3.0$. These limits improve upon previous constraints by approximately two orders of magnitude and represent the most stringent bounds from FRB--neutrino coincidence searches to date. Although the current limits remain above the predictions of most magnetar-based models, they begin to constrain scenarios involving exceptionally efficient hadronic energy dissipation.

  3. Unveiling Hidden Clustering: An Unsupervised Machine Learning Study of Repeating FRB 20220912A

    An-Chieh Hsu, Tetsuya Hashimoto, Tomotsugu Goto, Tomoki Wada, Bjorn Jasper Raquel

    Publications of the Astronomical Society of the Pacific　2026/02/01

    DOI： 10.1088/1538-3873/ae3986 　

  4. Spectral Shapes of Pair Annihilation Line Emission in Magnetar Giant Flares

    Tomoki Wada, Shigeo S Kimura

    The Astrophysical Journal Letters　2026/01/20

    DOI： 10.3847/2041-8213/ae3289 　

  5. Vacuum polarization effects in baryon-loaded magnetar bursts and implications for x-ray polarization

    Tomoki Wada

    Physical Review D　2025/12/15

    DOI： 10.1103/k3hz-k6v4 　

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    Magnetars provide natural laboratories for strong-field quantum electrodynamics processes, such as vacuum polarization, which gives rise to vacuum resonance together with the plasma response. We develop a general framework to describe vacuum resonance in a three-component plasma consisting of ions, electrons, and positrons, as expected in baryon-loaded magnetar bursts. By introducing a parametrization of the plasma composition, we establish the general criterion for the occurrence of vacuum resonance in such plasmas. Our analysis encompasses both Mikheyev-Smirnov-Wolfenstein-like adiabatic mode conversion and nonadiabatic eigenmode transition, highlighting their dependence on the plasma composition. Applying this framework to baryon-loaded fireballs in magnetar bursts, we estimate the characteristic X-ray polarization signatures. Detection of these polarizations will provide observational signatures of vacuum polarization as well as baryon loading in magnetar fireballs.

  6. On Acceleration of Highest-energy Cosmic Rays in a Novel Scenario of Magnetar Transients

    Jiro Shimoda, Tomoki Wada

    The Astrophysical Journal　2025/06/20

    Publisher: arXiv

    DOI： 10.3847/1538-4357/add33c 　

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    Transient phenomena in magnetars have been considered as possible acceleration sites of ultrahigh-energy cosmic-rays (CRs), whose energy reaches ~200 EeV, such as the Amaterasu particle. However, the process of CR acceleration and the trigger mechanism of magnetar transients remains unclear. A recently suggested scenario for the activity predicts that the magnetar's rotation axis suddenly flips due to the `Dzhanibekov effect,' resulting in a sudden rise of the Euler force. The material in the outer layer plastically flows due to the force and finally fractures in this scenario. We study the possibilities of ion acceleration along with this scenario. If the degenerate electrons burst open from the fractured region like a balloon burst, the pair plasma formation can be ignited inside the crust. We find that such pair plasma can emit photons similar to the observed bursts from magnetars. We also find that the electron stream at the beginning of the burst phenomenon possibly induces a strong electric field for a moment, resulting in the acceleration of ~1 ZeV ion within a timescale of ~1 ps. The nuclear spallation reactions limit this timescale, and therefore, high-energy CR `neutrons' from the parenteral nuclei become proper observational predictions of this scenario: their arrival time and direction will be correlated with the bursting photon emissions of the host magnetars. The nuclear spallation of ~ZeV nuclei is preferred to explain $\gtrsim$10 PeV neutrino events observed by IceCube and KM3Net.

  7. Radiative Acceleration and X-ray Spectrum of an Outflowing Pure Electron–Positron Pair Fireball in Magnetar Bursts

    Tomoki Wada, Katsuaki Asano

    Progress of Theoretical and Experimental Physics　2025/03/08

    DOI： 10.1093/ptep/ptaf033 　

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    One X-ray short burst accompanied by a Galactic fast radio burst has been detected so far, and its X-ray cut-off energy was significantly higher than that of other X-ray short bursts. Such X-ray bursts are thought to be emitted from a fireball in the magnetosphere of magnetars. If a fireball is formed around a magnetic pole, it expands and is accelerated under its radiation pressure, later producing photon emission and plasma outflow, which can be responsible for radio bursts. We numerically study the radiative acceleration of this outflowing fireball, which consists of electron-positron pairs and radiation, and obtain the spectrum of the escaped X-ray photons. We consistently take into account cyclotron resonant scattering, which enhances the scattering cross section resulting in a strong radiative force and high optical depth. Our results show that similar spectra to the observed X-ray spectrum in the Galactic fast radio burst are realized in outflowing fireballs and that the plasma outflow is simultaneously accelerated to a high Lorentz factor owing to cyclotron resonant scattering.

  8. The Dzhanibekov Effect as a Possible Source of Magnetar Activity

    Tomoki Wada, Jiro Shimoda

    The Astrophysical Journal　2024/09/01

    DOI： 10.3847/1538-4357/ad6847 　

  9. Expanding fireball in magnetar bursts and fast radio bursts

    Tomoki Wada, Kunihito Ioka

    Monthly Notices of the Royal Astronomical Society　519　(3)　4094-4109　2023/01/09

    DOI： 10.1093/mnras/stac3681 　

    ISSN： 0035-8711

    eISSN： 1365-2966

  10. Binary Comb Models for FRB 121102

    Tomoki Wada, Kunihito Ioka, Bing Zhang

    The Astrophysical Journal　920　(1)　54-54　2021/10/01

    DOI： 10.3847/1538-4357/ac127a 　

    ISSN： 0004-637X

    eISSN： 1538-4357

  11. Analytic properties of the electromagnetic field of binary compact stars and electromagnetic precursors to gravitational waves

    Tomoki Wada, Masaru Shibata, Kunihito Ioka

    Progress of Theoretical and Experimental Physics　2020　(10)　2020/10/09

    DOI： 10.1093/ptep/ptaa126 　

    ISSN： 2050-3911

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Research Projects 4

  1. マグネターのX線偏光で探る超強磁場の物理現象

    和田 知己

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 若手研究

    Institution: 東北大学

    2025/04/01 - 2028/03/31

  2. マルチメッセンジャー観測に向けたマグネター爆発現象に関する理論的研究

    和田 知己

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 特別研究員奨励費

    Institution: 東北大学

    2025/04/01 - 2028/03/31

  3. Origin of fast radio bursts from multi-wavelength and multi-time domain

    Offer Organization: Japan Society for the Promotion of Science

    System: Grants-in-Aid for Scientific Research Grant-in-Aid for Research Activity Start-up

    Category: Grant-in-Aid for Research Activity Start-up

    Institution: The University of Tokyo

    2022/08 - 2024/03

  4. 中性子星連星の合体前電磁波対応天体に関する理論的研究

    和田 知己

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業 特別研究員奨励費

    Category: 特別研究員奨励費

    Institution: 京都大学

    2020/04/24 - 2022/03/31

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    本年度は、まず中性子星を含むコンパクト星連星が合体直前に軌道運動を由来として放射する電磁波の光度および 周囲の電磁場の解析的な計算を、ニュートン重力のもとで行った。その結果、典型的な 10 の 12 乗ガウス程度の 大きさの磁場を持つ中性子星の合体において、主に次の二つの場合に電波放射の光度が高速電波バーストの光度 と同程度になることがわかった。一つは、ブラックホールのスピンは小さいが、合体に伴う全電磁波光度の 10% が電波放射として放射される場合である。もう一つは全電磁波光度の電波への変換効率が 0.1%程度と小さいが、 ブラックホールのスピンが大きく、中性子星が合体の直前まで壊されないような場合である。これらの場合には コンパクト連星合体由来の重力波と高速電波バーストが同時に観測される可能性がある。 また、真空中を中性子星が公転運動する際に作られる電磁場の解析解を用いて、公転運動によって形成される 磁気圏の粒子数密度を解析的に評価した。この磁気圏は先行研究の数値シミュレーションから合体直前に形成さ れることが既に明らかになっていたが、我々の導出した解析解を使うことで、その磁気圏のプラズマ数密度のパ ラメータ依存性を導出した。その結果、合体直前は軌道運動由来の磁気圏の粒子数密度が高く、プラズマの multiplicity が大きい場合か、高速電波バーストが星の近傍で形成されると我々には観測できなくなることも明 らかになった。 2020年に発見された高速電波バーストFRB 121102の周期的活動性に関する研究も行った。初めて周期的 活動性が報告された FRB 180916 に対して考案されたモデルの一つである binary comb model を FRB 121102 に対 して適用した。