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A multiscale study of turbulent heating in hot accretion flows

2019年11月18日(月) 14:00 - 15:00

川面 洋平 (東北大学 助教)

Recently, the Event Horizon Telescope (ETH) collaboration revealed the stunning picture of radiation from the vicinity of the black hole. For accurate interpretation of the observation, it is crucial to understand the nature of plasma in the accretion disk. The disks that EHT is observing are called radiatively inefficient accretion flows, in which the plasma is hot and dilute, and consequently collisionless. In collisionless plasma, ions and electrons can have different temperatures as they do not thermally relax through Coulomb interaction. The ion-to-electron temperature ratio is the key to interpreting the observation because we can measure only the electrons' energy via radiation. To study ion and electron heating, kinetic treatment, rather than hydrodynamic treatment, is necessary. However, kinetic plasma turbulence is an extremely challenging subject. Therefore, we utilized gyrokinetics that is widely used in magnetic confinement fusion research. Our new multiscale approach treats a "large scale" where turbulence is driven by magnetorotational instability via MHD, and a "small scale" where turbulence is dissipated via gyrokinetics. Using this approach, we formulated a prescription of ion-to-electron heating ratio. In my talk, I will also present basic knowledge that is necessary to study collisionless turbulent heating.

会場: セミナー室 (132号室)

イベント公式言語: 英語

Dark matter search in extended dwarf spheroidal galaxies with CTA

2019年10月11日(金) 14:00 - 15:00

廣島 渚 (理化学研究所 数理創造プログラム (iTHEMS) 特別研究員)

The nature of dark matter (DM) is still a big mystery. Among the varieties of candidates, Weakly Interacting Massive Particle (WIMP) is one of the most promising ones. Gamma-ray observations of dwarf spheroidal galaxies (dSphs) by Fermi satellites put the strongest constraints at mDM<~ a few hundreds of GeV. In the near future, Cherenkov Telescope Array (CTA) starts its operations and expect to probe WIMP of mDM>~O(1)TeV. Different from previous experiments, spatial distributions of DM in dSphs are resolved with CTA. In this talk, I explain how it affects our accessibility to DM annihilation cross-section.

会場: セミナー室 (132号室)

イベント公式言語: 英語

Angular power spectrum analysis on current and future high-energy neutrino data

2019年9月18日(水) 14:00 - 15:00

Ariane Dekker (Faculty of Science, University of Amsterdam, Netherlands)

The astrophysical neutrino events that have been measured in the last couple of years show an isotropic distribution on the sky. To constrain the contribution of source populations to the observed neutrino sky, we consider isotropic and anisotropic components of the diffuse neutrino data. We simulate through-going muon neutrino events by applying statistical distributions for the fluxes of extra-galactic sources and investigate the sensitivities of current (IceCube) and future (IceCube-Gen2 and KM3NeT) experiments. I will show that the angular power spectrum is a powerful probe to assess the angular characteristics of neutrino data and demonstrate that we are already constraining rare and bright sources with current IceCube data. In addition, I will investigate the decay and annihilation of very heavy dark matter as a potential neutrino source suggested by the excess in HESE data. We apply our angular power spectrum analysis to HESE data for different channels, allowing us to interpret the observed neutrino sky and perform a sensitivity forecast.

会場: 研究本館 1階 160号室

イベント公式言語: 英語

ABBL/iTHEMS/r-EMU Joint Seminar: Towards systematic and consistent nuclear data inputs for astrophysical r-process with Bayesian approaches

2019年6月28日(金) 14:00 - 15:00

ハオジヤオ・リアン (理化学研究所 仁科加速器科学研究センター (RNC) 量子ハドロン物理学研究室 専任研究員)

This is his 2nd seminar talk for non-experts of nuclear physics, following the 1st one on 18th Jan. 2019. Abstract: In this interdisciplinary talk, I will start with some basic concepts as well as some frontiers of nuclear physics, and then introduce the roles of nuclear data inputs for the study of astrophysical rapid neutron-capture process (r-process), which is responsible for the creation of approximately half the abundances of the atomic nuclei heavier than iron. Recent progress in nuclear physics focuses on improving the accuracy of crucial nuclear inputs, such as nuclear masses, beta-decay half-lives. Nevertheless, in most of the studies these inputs are investigated individually. One of our ongoing attempts is to organize the crucial nuclear inputs in a systematic and consistent way, together with Bayesian and/or machine learning approaches, which are able to provide not only the theoretical results but also the corresponding uncertainties.

会場: 研究本館224-226室

イベント公式言語: 英語

Using combined Particles-in-MHD-Cells to model particle acceleration in astrophysical shocks

2019年6月17日(月) 14:00 - 15:00

Allard Jan van Marle (Research Professor, Ulsan National Institute of Science and Technology, Republic of Korea)

Astrophysical shocks can accelerate charged particles through diffusive shock acceleration. This process involves repeated shock crossings where the particle gains energy from collisions with the electromagnetic field. Eventually, these particles will reach relativistic speeds and can be observed as cosmic rays. In order to simulate this process, we need a method that can handle both the large-scale structure of astrophysical shocks, as well as the behaviour of individual particles. We achieve this by combining the classical magnetohydrodynamics (MHD) and particle-in-cell (PIC) methods. This allows us to describe the thermal plasma of the shock through MHD, while simultaneously using PIC to follow the movement of non-thermal particles as they are accelerated. Our results show a complicated interaction that destabilizes the shock, reducing the efficiency with which particles can be accelerated.

会場: セミナー室 (132号室)

イベント公式言語: 英語

ABBL-iTHEMS Joint Seminars (Following r-EMU seminar 13:30-15:00)

2019年5月31日(金) 15:15 - 17:35

15:15-15:35 Hiroyuki Sagawa (ICRR, U.Tokyo) “Recent TA Results and the extension status for the study of the highest-energy cosmic rays” 15:35-16:30 Igor Tkachev (Russian Academy of Sciences) “Axion stars: from birth to death” 16:30-16:40 Break 16:40-17:35 Peter Tinyakov (U. Libre de Bruxelles) “Solar mass black holes and dark matter"

会場: 研究本館 435-437号室

イベント公式言語: 英語

ABBL, iTHEMS, r-EMU Joint Seminar: Impacts of New Carbon Fusion Cross Sections on Type Ia Supernovae

2019年4月12日(金) 15:00 - 16:00

森 寛治

会場: セミナー室 (132号室)

イベント公式言語: 英語

The Origin of the X-ray Clumpy Ejecta in Type Ia Supernova Remnants

2019年4月3日(水) 14:00 - 15:00

佐藤 寿紀 (理化学研究所 開拓研究本部 (CPR) 玉川高エネルギー宇宙物理研究室 基礎特別研究員)

X-ray-emitting clumpy structures are generally observed in young Type Ia supernova remnants although the origin is still obscure. There are two candidates for explaining the formation of clumps; initial clumpiness in ejecta at the explosion (i.e., clumpy ejecta model) or hydrodynamic instabilities made from smooth ejecta profile (i.e., smooth ejecta model). This information should reflect the initial ejecta structure of SNe Ia, so it is important for understanding the Type Ia explosion itself. Our preliminary investigations into constraining the structure of SN Ia remnants using Fourier and wavelet-transform analyses did not turn out to be sufficiently powerful at discriminating the two hydro models and the observed Tycho image from each other. This led us to investigate an approach that would be more sensitive to patterns in the distribution of clumps and holes in the images, such as the "genus statistic.” In this study, for the first time, the genus statistics have been applied to a famous type Ia remnant, Tycho (SN 1572) to understand the formation of the clumps by comparing with hydrodynamical models (Sato et al. 2019, arXiv: 1903.00764). We found the genus curve from Tycho's supernova remnant strongly indicates a skewed non-Gaussian distribution of the ejecta clumps, which is similar to that of a hydrodynamical model for the clumpy ejecta model. In contrast, a hydrodynamical model for the smooth ejecta model has a genus curve that is similar to that of a random Gaussian distribution. Thus, our results support the initial clumpiness in the Type Ia ejecta is more reasonable for the origin of the clumps and demonstrate usefulness of the genus statistics for this field. In addition, we will also discuss the origin of “Fe-rich” ejecta clumps in Type Ia SNRs in this seminar.

会場: 研究本館 433号室

イベント公式言語: 英語

Cosmic-ray air showers: new arrays for searching origins and link to the collider physics

2018年12月7日(金) 14:00 - 15:00

﨏 隆志 (東京大学 宇宙線研究所 (ICRR))

会場: セミナー室 (160号室)

イベント公式言語: 英語