ABBL-iTHEMS Joint Astro Seminar

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

September 3 (Fri) at 14:00 - 16:00, 2021

Yosuke Mizuno (T.D. Lee Fellow / Associate Professor, 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.

Venue: via Zoom

Event Official Language: English

Fallback Accretion in Binary Neutron Star Mergers

July 9 (Fri) at 16:00 - 17:30, 2021

Wataru Ishizaki (Postdoctoral Fellows, Yukawa Institute for Theoretical Physics, Kyoto University)

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.

Venue: via Zoom

Event Official Language: English

Theory of Core-Collapse Supernovae

June 25 (Fri) at 16:00 - 17:00, 2021

Akira Harada (Special Postdoctoral Researcher, iTHEMS)

Venue: via Zoom

Event Official Language: English

Magnetorotational Instability: Current Understanding and Perspective

May 28 (Fri) at 16:00 - 17:00, 2021

Takashi Minoshima (Researcher, Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

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.

Venue: via Zoom

Event Official Language: English

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

February 22 (Mon) at 10:00 - 11:30, 2021

Gordon Baym (Senior Visiting Scientist, iTHEMS / 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.

Venue: via Online

Event Official Language: English

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

January 31 (Fri) at 14:00 - 15:00, 2020

Eiji Kido (Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Venue: Seminar Room #132

Event Official Language: English

ABBL/iTHEMS seminar - talk on neutron stars

January 24 (Fri) at 14:00 - 15:00, 2020

Hajime Sotani (Research Scientist, iTHEMS / Research Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Venue: Seminar Room #132

Event Official Language: English

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

December 20 (Fri) at 14:00 - 15:00, 2019

Yana Zhezher (Institute for Cosmic Ray Research (ICRR), The University of Tokyo)

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 ?, 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.

Venue: Seminar Room #132

Event Official Language: English

Seminar talk on GRB190114C

December 11 (Wed) at 13:45 - 15:30, 2019

Susumu Inoue (Research Scientist, iTHEMS)

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.

Venue: Seminar Room #132

Event Official Language: English

A multiscale study of turbulent heating in hot accretion flows

November 18 (Mon) at 14:00 - 15:00, 2019

Yohei Kawazura (Assistant Professor, Tohoku University)

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.

Venue: Seminar Room #132

Event Official Language: English

Dark matter search in extended dwarf spheroidal galaxies with CTA

October 11 (Fri) at 14:00 - 15:00, 2019

Nagisa Hiroshima (Postdoctoral Researcher, iTHEMS)

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.

Venue: Seminar Room #132

Event Official Language: English

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

September 18 (Wed) at 14:00 - 15:00, 2019

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.

Venue: #160, 1F, Main Research Building

Event Official Language: English

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

June 28 (Fri) at 14:00 - 15:00, 2019

Haozhao Liang (Senior Research Scientist, Quantum Hadron Physics Laboratory, RIKEN Nishina Center for Accelerator-Based Science (RNC))

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.

Venue: 224-226, Main Research Building

Event Official Language: English

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

June 17 (Mon) at 14:00 - 15:00, 2019

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.

Venue: Seminar Room #132

Event Official Language: English

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

May 31 (Fri) at 15:15 - 17:35, 2019

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"

Venue: #435-437, Main Research Building

Event Official Language: English

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

April 12 (Fri) at 15:00 - 16:00, 2019

Kanji Mori

Venue: Seminar Room #132

Event Official Language: English

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

April 3 (Wed) at 14:00 - 15:00, 2019

Toshiki Sato (Special Postdoctoral Researcher, High Energy Astrophysics Laboratory, RIKEN Cluster for Pioneering Research (CPR))

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.

Venue: #433, Main Research Building

Event Official Language: English

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

December 7 (Fri) at 14:00 - 15:00, 2018

Takashi Sako (Institute for Cosmic Ray Research (ICRR), The University of Tokyo)

Venue: Seminar Room #160

Event Official Language: English