ABBL-iTHEMSジョイントアストロセミナー

The Hunt for Extraterrestrial Neutrino Counterparts

2022年5月20日16:00 - 17:00

Dr. Yannis Liodakis (Postdoctoral Researcher, University of Turku, Finland)

The origin of high-energy neutrinos is fundamental to our understanding of the Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater. The sources are unknown, unpredictable, and we lack clear signatures. Neutrino astronomy therefore represents the greatest challenge faced by the astronomy and physics communities thus far. The possible neutrino sources range from accretion disks and tidal disruption events, through relativistic jets to galaxy clusters with blazar TXS 0506+056 the most compelling association thus far. Since then, immense effort has been put into associating AGN-jets with high-energy neutrinos, but to no avail. I will discuss our current efforts in understanding the multimessenger processes in the Universe, and once and for all proving or disproving if AGN-jets are neutrino emitters.

会場: via Zoom

イベント公式言語: 英語

Coherent emission from 3D relativistic shocks

2022年4月22日14:00 - 15:00

岩本 昌倫 (九州大学)

The origin of fast radio bursts (FRBs; Lorimer et al. 2007) is one of the unsolved problems in astrophysics. Many observations of FRBs indicate that FRBs must be coherent emission in the sense that coherently moving electrons radiate electromagnetic waves. In relativistic shocks, it is well known that coherent electromagnetic waves are excited by synchrotron maser instability (SMI) in the shock transition (Hoshino & Arons 1991). The SMI is also known as the emission mechanism of coherent radio sources such as auroral kilometric radiation at Earth and Jovian decametric radiation. Recently, some models of fast radio burst based on the coherent emission from relativistic shock via the SMI have been proposed (e.g., Lyubarsky 2014; Beloborodov 2017; Plotnikov & Sironi 2019; Metzger et al. 2019) and the SMI in the context of relativistic shocks attracts more attention from astrophysics. In this study, by performing the world’s first three-dimensional (3D) particle-in-cell (PIC) simulation of relativistic shocks, we will demonstrate that large-amplitude electromagnetic waves are indeed excited by the SMI even in 3D and that the wave amplitude is significantly amplified and comparable to that in pair plasmas due to a positive feedback process associated with ion-electron coupling. Based on the simulation results, we will discuss the applicability of the SMI for FRBs in this talk.

会場: via Zoom

イベント公式言語: 英語

Toward modeling complete supernova neutrino emissions

2022年3月11日16:00 - 17:00

諏訪 雄大 (東京大学 大学院総合文化研究科 宇宙地球部会 准教授 / 京都大学 基礎物理学研究所 基研特任准教授)

Neutrinos are guaranteed observable from the next Galactic supernova (SN). Optical lights and gravitational waves are also observable but can be difficult to observe if SN location in the galaxy and the explosion details are unsuitable. The key to the next coming SN observation will be understanding various physical quantities using neutrinos first and then connecting them to other signals. In particular, understanding neutrinos in the late time (> 1 sec after the onset of explosion) is essential, since physics in this time scale has much smaller uncertainties than that of the early time. We should construct a simple and understandable neutrino model based on the late-time emissions. It allows us to tackle the physics in the early phase like the explosion mechanism. In this talk, I will discuss the following topics: 1) how to model the complete neutrino emissions from the very early phase up to the last observable event. 2) what physical quantities (e.g., mass and radius of neutron stars) can be extracted from observations using large statistical neutrinos as physics probes. 3) how to use these extracted physical quantities to link with the explosion mechanism of SN and multi-messenger observations.

会場: via Zoom

イベント公式言語: 英語

Spin transport in ultracold atomic gases

2022年2月18日14:00 - 15:00

関野 裕太 (理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室 特別研究員)

In condensed matter physics, transport measurement has played crucial roles in understanding fascinating phenomena such as superconductivity and quantum Hall and Kondo effects. In this talk, we discuss the usefulness of spin transport as a probe for many-body properties in ultracold atoms. In the first part, we focus on the conductivity of alternating spin current, which includes information on superfluid gap, pseudogap, and topological phase transition. In the latter part, we consider mesoscopic spin transport between two Fermi gases weakly connected with each other. Our analysis suggests that the spin current is sensitive to whether the gases have pseudogaps, which are gap-like structures in densities of states just above the superfluid transition temperature. In this talk, we also mention similarities of ultracold atoms to neutron star matter.

会場: via Zoom

イベント公式言語: 英語

Galactic archaeology with r-process elements

2022年1月28日10:00 - 11:30

平居 悠 (東北大学 大学院理学研究科 天文学専攻 / JSPS Research Fellow (Visiting Scholar), Department of Physics, University of Notre Dame, USA)

Galactic archaeology studies the evolutionary histories of galaxies using information preserved in stars. Abundances of elements in stars are keys to understanding how the galaxies were evolved. It is, therefore, crucial to making it clear the origin of elements and the cycle of materials in galaxies. This talk will show the enrichment of heavy elements, including r-process elements, in dwarf galaxies and the Milky Way. Our high-resolution simulations of galaxies suggest that binary neutron star mergers play an important role in enriching r-process elements in dwarf galaxies and the Milky Way. I will also show that r-process enhanced stars in the Milky Way tend to form in dwarf galaxies previously accreted to the Milky Way. I will demonstrate that the abundance of r-process elements in stars can be used as an indicator for the early evolution of the Milky Way.

会場: via Zoom

イベント公式言語: 英語

Magnetic field dependence of neutrino-driven core-collapse supernova models

2021年12月10日14:00 - 15:00

松本 仁 (慶應義塾大学 理工学研究科 慶應義塾基礎科学・基盤工学インスティテュート 助教)

Massive stars can explode and release huge energy (typically 10^51 erg) at the end of their life. It is one of the most energetic explosions in the Universe and is called a core-collapse supernova. The impact of the magnetic field on the explosion mechanisms of the core-collapse supernova is a long-standing mystery. Recently, we have updated our neutrino-radiation-hydrodynamics supernova code (3DnSNe, Takiwaki et al. 2016) to include magnetohydrodynamics (MHD). Using this code, we have performed three-dimensional MHD simulations for the evolution of non-rotating stellar cores focusing on the difference in the magnetic field of the progenitors. Initially, 20 and 27 solar mass pre-supernova progenitors are threaded by only the poloidal component of the magnetic field, which strength is 10^10 (weak) or 10^12 (strong) G. We find that the neutrino-driven explosion occurs in both the weak and strong magnetic field models. The neutrino heating is the main driver for the explosion in our models, whereas the strong magnetic field slightly supports the explosion. In my talk, I will introduce the details of this mechanism.

会場: via Zoom

イベント公式言語: 英語

Axions around rotating black holes

2021年11月12日14:00 - 16:00

吉野 裕高 (神戸大学 大学院理学研究科 物理学専攻 宇宙論研究室)

String theories indicate the existence of many axionlike scalar fields with light masses in addition to the QCD axion. If this is the case, an axion field around a rotating black hole extracts the energy of the black hole by the mechanism called the “superradiant instability”. Then, every astrophysical black hole is expected to wear a cloud of the axion. In this talk, I would like to give an overview on this topic, and introduce our numerical studies on the phenomena caused by the axion cloud at the last stage of the superradiant instability where the self-interaction of axions becomes important.

会場: via Zoom

イベント公式言語: 英語

Recent progress on the r-process in the era of gravitational-wave astronomy

2021年10月15日16:00 - 18:00

西村 信哉 (理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室)

The r-process, the rapid neutron-capture process, is a major origin of heavy nuclei beyond iron in the universe, occurring in explosive astrophysical phenomena with very neutron-rich environments. In the studies of r-process nucleosynthesis, there are several unsolved problems in nuclear physics and astrophysics. In this talk, I will briefly summarize recent progress on the studies of the r-process, mainly focusing on neutron star mergers. We will see that the scenario of neutron star mergers is consistent with several observations, e.g., GW170817 with a kilonova, chemical evolution of r-process elements. In addition, nevertheless, there are several remaining (or newly realized) problems on the origin of r-process elements in the universe. Focusing on our own research, I will introduce attempts to address these issues.

会場: via Zoom

イベント公式言語: 英語

The Polarised ring of the Supermassive Black Hole in M87: EHT observations and theoretical modeling

2021年9月3日14:00 - 16:00

水野 陽介 (Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China)

The Event Horizon Telescope has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. These images show a prominent ring with a diameter of ~40 micro-arcsecond, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. Recently EHT has provided new images of the polarised emission around the central black hole in M87 on event-horizon scale. This polarised synchrotron emission probes the structure of magnetic fields and the plasma properties near the black hole. We found that the net azimuthal linear polarisation pattern may result from organised, poloidal magnetic fields in the emission region. In a quantitative comparison with a large simulated polarimetric image library, we found that magnetically arrested accretion disks are favoured to explain polarimetric EHT observations. In this talk, I also briefly discuss about a new modelling study of M87 jets in the collimation and acceleration region.

会場: via Zoom

イベント公式言語: 英語

Fallback Accretion in Binary Neutron Star Mergers

2021年7月9日16:00 - 17:30

石﨑 渉 (京都大学 基礎物理学研究所 ポスドク研究員)

The gravitational wave event GW170817 with a kilonova shows that a merger of two neutron stars ejects matter with radioactivity including r-process nucleosynthesis. A part of the ejecta inevitably falls back to the central object, possibly powering long-lasting activities of a short gamma-ray burst (sGRB), such as extended and plateau emissions. We investigate the fallback accretion with the r-process heating by performing one-dimensional hydrodynamic simulations and developing a semi-analytical model. We show that the usual fallback rate dM/dt \propto t^{-5/3} is halted by the heating. The characteristic halting timescale is $\sim 10^4$--$10^8$ sec for the GW170817-like r-process heating, which is long enough to continue the long-lasting emission of sGRBs. Furthermore, we propose a new interpretation of the recently reported re-brightening in the annual-scale X-ray light curve of GW170817. We model the fallback of the merger ejecta and construct a simple light curve model from the accreting ejecta. We find that the X-ray flux excess can be well explained by the fallback of the post-merger ejecta such as the disk wind from the accretion disk of the merger remnant rather than by the fallback of the dynamical ejecta. The duration of the constant luminosity phase conveys the initial fallback timescale t_0 in the past. Future observations in the next decades will probe the timescale of t_0 \sim 10--10^4 sec, around the time of extended emission in short gamma-ray bursts.

会場: via Zoom

イベント公式言語: 英語

Theory of Core-Collapse Supernovae

2021年6月25日16:00 - 17:00

原田 了 (数理創造プログラム 基礎科学特別研究員)

会場: via Zoom

イベント公式言語: 英語

Magnetorotational Instability: Current Understanding and Perspective

2021年5月28日16:00 - 17:00

簑島 敬 (海洋研究開発機構 (JAMSTEC) 副主任研究員)

The differentially rotating flow can be destabilized in the presence of a weak magnetic field through the magnetorotational instability (MRI). The MRI is considered as a possible mechanism for outward angular momentum transport and subsequent mass accretion in accretion disks. Numerous studies have been devoted to understand its nature and judge whether it can supply the power sufficient for observed transport efficiency. For example, the MHD simulation studies have attempted to reveal the scaling of the MRI on numerical (e.g., resolution and domain size) as well as physical parameters (e.g., magnetic field intensity and configuration). In this talk, I would like to discuss current understanding and perspective of the MRI through theoretical and numerical studies. I will especially focus on the impact of transport coefficients (viscosity, resistivity, and their ratio) on the evolution of the MRI and disk.

会場: via Zoom

イベント公式言語: 英語

The Evolution of Primordial Neutrino Helicities under Gravitational and Magnetic Fields and Implications for their Detection

2021年2月22日10:00 - 11:30

ゴードン・ベイム (数理創造プログラム 客員主管研究員 / Professor Emeritus, University of Illinois, USA)

Feb.22 (Mon) 10:00am-11:30am (JST) Primordial neutrinos decoupled in the early universe in helicity eigenstates. As I will discuss, two effects -- dependent on neutrinos having a non-zero mass -- can modify their helicities as they propagate through the cosmos. First, finite mass neutrinos have a magnetic moment and thus their spins, but not their momenta, precess in cosmic and galactic magnetic fields. The second is the propagation of neutrinos past cosmic matter density fluctuations, which bend their momenta, and bend their spins by a smaller amount. (The latter is a general relativistic effect.) Both effects turn a fraction of left-handed neutrinos into right-handed neutrinos, and right-handed antineutrinos into left-handed. If neutrino magnetic moments approach that suggested by the XENON1T experiment as a possible explanation of their excess of low energy electron events -- a value well beyond the moment predicted by the standard model -- helicities of relic Dirac (but not Majorana) neutrinos could be considerably randomized. I finally will discuss the implications of neutrino helicity rotation, as well as their Dirac vs. Majorana nature, on their detection rates via the Inverse Tritium Beta Decay reaction.

会場: via Online

イベント公式言語: 英語

ABBL/iTHEMS seminar - talk on ultra-high energy cosmic rays

2020年1月31日14:00 - 15:00

木戸 英治 (理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室)

会場: セミナー室 (132号室)

イベント公式言語: 英語

ABBL/iTHEMS seminar - talk on neutron stars

2020年1月24日14:00 - 15:00

祖谷 元 (数理創造プログラム 研究員 / 理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室 研究員)

会場: セミナー室 (132号室)

イベント公式言語: 英語

Constraining superheavy dark matter with the multi-messenger observations of accompanying radiation

2019年12月20日14:00 - 15:00

Dr. Yana Zhezher (東京大学 宇宙線研究所 (ICRR))

One of the main alternatives for the weakly interacting massive particles (WIMP) dark matter scenario are the super-heavy X particles with masses larger than the weak scale by orders of magnitude. We assume the experimentally more plausible scenario of decaying superheavy dark matter (SHDM), which leads to the production secondary particles: electrons, positrons, gamma rays and neutrinos. The hypothetical X-particle has two main parameters: it’s mass MX and lifetime 𝜏, which can be indirectly constrained by comparisons of predicted flux of secondary particles with the astrophysical observations. We present the limits on the SHDM parameters derived with the multi-messenger data from the Fermi-LAT, IceCube and other experiments.

会場: セミナー室 (132号室)

イベント公式言語: 英語

Seminar talk on GRB190114C

2019年12月11日13:45 - 15:30

井上 進 (数理創造プログラム 研究員)

Detection of very high energy gamma-rays (~TeV) from GRB190114C by MAGIC telescope is reported in the latest nature issue. Dr. Susumu Inoue (iThems) who is one of MAGIC team members will give a seminar talk on this exciting event.

会場: セミナー室 (132号室)

イベント公式言語: 英語

A multiscale study of turbulent heating in hot accretion flows

2019年11月18日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: 研究本館 1階 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14:00 - 15:00

廣島 渚 (数理創造プログラム 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: 研究本館 1階 132号室

イベント公式言語: 英語