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Seminar
Contribution of star-forming galaxies to the cosmic gamma-ray background
November 14 (Fri) 14:00 - 15:15, 2025
Junling Chen (Ph.D. Student, Graduate School of Mathematical Sciences, The University of Tokyo)
Fermi Gamma-Ray Space Telescope has measured the diffuse extragalactic gamma-ray background (EGB) radiation in the energy range of 100 MeV to 820 GeV. Several candidate γ -ray sources have been proposed as the candidate components of the unresolved EGB, including active galactic nuclei (AGNs), millisecond pulsars, dark matter annihilation, and star-forming galaxies (SFGs), but their quantitative contribution has not yet been precisely determined. In this talk, I will introduce our latest physical model describing the gamma-ray emission mechanism from SFGs, and our estimate of the contribution of SFGs based on careful calibration with gamma-ray luminosities of nearby galaxies and physical quantities (star formation rate, stellar mass, and size) of galaxies observed by high-redshift galaxy surveys.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Pairing in Bose-Fermi and Fermi-Fermi systems
November 6 (Thu) 15:00 - 16:30, 2025
Pierbiagio Pieri (Associate Professor, Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Italy)
This seminar is co-hosted by GWX-EOS Working Group and iTHEMS-ABBL Joint Astro Study Group. Abstract: In the first part of my talk, I will review recent work on Bose-Fermi mixtures with an attractive interaction inducing pairing between bosons and fermions. After discussing a recent experiment on this system [1], which has confirmed predictions obtained by us some time ago within a many-body diagrammatic approach [2], I will present novel results for the compressibility [3] that suggest a metastable nature for the many-body phase observed in [1]. Then, I will discuss the extension of our calculations to two-dimensional Bose-Fermi mixtures [4,5]. The results obtained in 2D challenge previous beliefs formulated for 3D systems. In the second part, I will discuss attractive polarized Fermi systems, for which the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase was proposed many years ago as a possible superfluid phase. I will discuss how significant precursor FFLO fluctuation effects appear already in the normal phase of polarized Fermi gases at finite temperature [6], and how they could be experimentally detected with ultracold gases. At zero temperature [7], I will discuss how the quasi-particle parameters of the normal Fermi gas change when approaching an FFLO quantum critical point, with a complete breakdown of the quasi-particle picture analogous to what found in heavy-fermion materials at an antiferromagnetic quantum critical point. Finally, I will discuss a recent joint experimental-theoretical work on the motion of a vortex orbiting a pinned anti-vortex in a strongly interacting Fermi gas [8], highlighting the interplay between Andreev bound states in the vortex core and delocalized thermal excitations in shaping the vortex dynamics.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Particle-in-Cell Simulations on Collisionless Shocks and Particle Acceleration in Black Hole Coronae
October 16 (Thu) 14:00 - 15:15, 2025
Nhat-Minh Ly (Ph.D. Student, Department of Physics, Osaka University)
Multiple nearby Active Galactic Nuclei have been reported as sources of high-energy neutrinos detected by the IceCube observatory. These results strongly suggest efficient proton acceleration to (sub-)PeV energies, likely within Black Hole (BH) coronae, given the lack of γ-ray counterparts. The acceleration mechanisms remain unconfirmed due to limited understanding of coronal environments and challenges in modeling hot, relativistic plasmas. Although diffusive shock acceleration (DSA) has been proposed, a self-consistent treatment incorporating detailed kinetic plasma effects has been lacking. In this study, we present the particle-in-cell (PIC) method as a first-principles approach to investigate particle acceleration by collisionless shocks under conditions inferred from multi-wavelength observations of BH coronae. Using large-scale 1D3V simulations, we surveyed shock parameters, focusing on underexplored effects such as initial ion–electron temperature ratios and trans-relativistic shock velocities, and found that collisionless shocks can form even in hot, low-Mach plasmas. These shocks accelerate protons up to ~100 TeV, consistent with the energies required for IceCube neutrino detections, across a wide range of coronal conditions. The shocks partition ~10% of dissipated energy into nonthermal protons and <1% into electrons, providing critical, observationally testable constraints on the plasma state of BH coronae.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
A Continuous Galactic Line Source of Axions: The Remarkable Case of 23Na
September 30 (Tue) 14:00 - 15:00, 2025
Wick C. Haxton (Professor, Department of Physics, University of California, Berkeley, USA)
While it is unusual for odd-A nuclear species to be abundant in massive stars, 23Na is an interesting exception. Typically 0.1 solar masses of 23Na is synthesized during the carbon burning phase of supernova and ONeMg white dwarf progenitors, then maintained at approximately 10^9 K for periods ranging up to 60,000 years. Under these conditions, 23Na can pump the thermal energy of the star into escaping axions: the mechanism is the Boltzmann occupation of and subsequent axion emission from the 440 keV level. We develop a galactic model to show that the resulting flux of line axions is continuous, arising from hundreds of contributing sources. As they travel through the intra-galactic magnetic field, some of these axions convert to detectable gamma rays. Consequently, future all-sky detectors like COSI will be able to set new limits on light axion-like particles. Other interesting aspects of these axions will be discussed.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Confined Circumstellar Material as a Dust Formation Site in Type II Supernovae
September 26 (Fri) 14:00 - 15:15, 2025
Yuki Takei (Program-Specific Researcher, Yukawa Institute for Theoretical Physics, Kyoto University)
Some massive stars undergo episodic mass loss shortly before core-collapse, producing dense circumstellar material (CSM) in their immediate surroundings. If the supernova (SN) ejecta strongly interacts with such CSM, narrow emission lines appear in the spectrum, classifying the event as Type IIn. In these cases, efficient radiative cooling forms a cold, dense shell (CDS), providing ideal conditions for dust condensation. Infrared observations of several SNe IIn have indeed confirmed newly formed dust. Recent time-domain surveys, however, suggest that compact and dense CSM, often termed “confined CSM”, is also present around a broader class of Type II SN progenitors with hydrogen-rich envelopes, beyond the traditional Type IIn subclass. This raises the possibility that dust formation in dense CSM is more common among core-collapse SNe than previously thought. In this talk, I will demonstrate that CDS formation occurs robustly across a wide parameter space for confined CSM using numerical simulations based on the open-source code CHIPS. I will also discuss the resulting dust mass and infrared emission, as well as the potential contribution of this process to the galactic dust budget.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Dust formation in the universe: reconstructing of theoretical models
August 29 (Fri) 14:00 - 15:15, 2025
Kyoko Tanaka (Appointed Associate Professor, Tokyo Woman's Christian University)
The formation process of cosmic dust is the starting point of solid matter and is important for understanding the evolution of cosmic material and planet formation processes. The nucleation process at the initial stage of the phase transition is a key to how cosmic dust is formed and evolves. Recent studies of the nucleation have shown possibilities that are very different from the theoretical models that have been considered so far. We have investigated the nucleation process using molecular dynamics simulations, which allow us to observe the nucleation process at the molecular level and obtain new information. We also present our attempt to develop and reconstruct a new theoretical model to elucidate a comprehensive picture of cosmic dust formation in collaboration with experiments.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Gyromagnetic Angular Momentum Interconversion in Neutron Stars
August 28 (Thu) 10:00 - 12:00, 2025
Hiroshi Funaki (Specially Appointed Assistant Professor, Faculty of Science and Technology, Keio University)
[NOTE] This informal seminar mainly organized by ABBL will be held in Japanese and is a joint event for GWX-EOS Working Group and iTHEMS-ABBL Joint Astro Study Group. Abstract: We propose a novel mechanism for angular momentum (AM) exchange between the crust and core of a neutron star (NS) via the gyromagnetic effect. Using extended hydrodynamics, we model the star by incorporating macroscopic AM and microscopic AM originating from neutron orbital and spin AM. We reveal that macroscopic dynamics in the crust can inform microscopic AM in the core leading to neutron spin polarization, and offer alternative scenario of (anti-)glitches. This work highlights the overlooked multi-scale AM interconversions in NS physics, paving the way for gyromagnetic astrophysics.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: Japanese
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Seminar
From Galaxies to Cosmological Structures: The Multi-Scale Influence of Cosmic Rays
June 13 (Fri) 14:00 - 15:15, 2025
Ellis Owen (Special Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Pioneering Research Institute (PRI))
Cosmic rays interact with astrophysical systems over a broad range of scales. They go hand-in-hand with violent, energetic astrophysical environments, and are an active agent able to regulate the evolution and physical conditions of galactic and circum-galactic ecosystems. Depending on their energy, cosmic rays can also escape from their galactic environments of origin, and propagate into larger-scale cosmological structures. In this talk, I will discuss the impacts of cosmic rays retained in galaxies. I will show they can deposit energy and momentum to alter the initial conditions of star-formation, modify the circulation of baryons around galaxies, and have the potential to regulate long-term galaxy evolution. I will highlight some of the astrophysical consequences of contained hadronic and leptonic cosmic rays in and around galaxies, and how their influence can be probed using signatures including X-rays, gamma-rays and neutrinos. I will also discuss what happens to the cosmic rays that escape from galaxies, including their interactions with the magnetized large-scale structures of our Universe, and the fate of distant high-energy cosmic rays that do not reach us on Earth.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
SISSI: Supernovae in a Shearing, Stratified Interstellar Medium
May 23 (Fri) 14:00 - 15:15, 2025
Leonard Romano (Ph.D. Student, Ludwig-Maximilians-Universität München, Germany)
Supernovae (SNe) are an important driver of the multiphase structure in the Interstellar Medium (ISM) and play an important role for regulating star formation. SNe inflate large bubbles of hot gas dubbed Supernova Remnants (SNRs) that can remain hot for several 10⁵-10⁶ years, contributing substantially to the volume filling hot phase, galactic outflows and the driving of turbulence in the ISM. In this talk, I am presenting the results of zoom-in simulations of SNRs embedded in a simulated isolated Milky-Way analogue, in order to investigate how environmental effects like shear, vertical stratification and a self-consistently generated ISM can affect various properties of SNRs. I find that initially microscopic SNRs, whose dynamics are dominated by local shock physics, after a few Myr enter a mesoscopic regime, where their dynamics are increasingly dominated by galactic scale processes. Based on these findings, I make predictions about SN-driven large-scale structure, such as galactic outflows and the geometry of large superbubbles in disk galaxies.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Insights on Issues in the Cold Dark Matter Hypothesis
April 25 (Fri) 14:00 - 15:15, 2025
Yuka Kaneda (Ph.D. Student, University of Tsukuba)
Dark matter accounts for 85% of the matter component of our universe, but its true nature is still unclear. The Lambda-Cold Dark Matter (CDM) model, which thought to be the standard model, reproduces well the statistical properties of the large-scale structure of our universe. However, at the scale of galaxies and dwarf galaxies, serious discrepancies between the predictions of the CDM model and observations have been pointed out. In this study, we tackle on the “cusp-core problem” and the “missing satellite problem,” which are typical examples of such discrepancies, using N-body simulations. In the talk, the physical trigger of cusp-to-core transition and the novel method to find missed satellites are presented.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Fast radio bursts as precursor radio emission from monster shocks
March 21 (Fri) 16:00 - 17:15, 2025
Arno Vanthieghem (Assistant Professor, Observatoire de Paris and Sorbonne Université, France)
It has been proposed recently that the breaking of MHD waves in the inner magnetosphere of strongly magnetized neutron stars can power different types of high-energy transients. Motivated by these considerations, we study the steepening and dissipation of a strongly magnetized fast magnetosonic wave propagating in a declining background magnetic field, by means of particle-in-cell simulations that encompass MHD scales. Our analysis confirms the formation of a monster shock, that dissipates about half of the fast magnetosonic wave energy. It also reveals, for the first time, the generation of a high-frequency precursor wave by a synchrotron maser instability at the monster shock front, carrying a fraction of 0.1% of the total energy dissipated at the shock. The spectrum of the precursor wave exhibits several sharp harmonic peaks, with frequencies in the GHz band under conditions anticipated in magnetars. Such signals may appear as fast radio bursts.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Investigating the Crust Urca Process in Accretion Neutron Stars: Implications for Superburst Ignition and Hot Cooling Curve of MAXI J0556-332
February 7 (Fri) 16:00 - 17:15, 2025
Hao Huang (Ph.D. Student, Institute of Modern Physics, China)
This seminar investigates the Urca cooling strength of the 63Fe-63Mn pair, which varies due to uncertainties in the spin-parity of 63Fe, relevant to the Island of Inversion at N = 40. We present simulations that analyze the impact of this cooling mechanism on the thermal evolution of neutron star crusts, focusing on superburst ignition and anomalous hot quiescent phase cooling of MAXI J0556-332. Additionally, we explore the potential crust Urca process through the anomalous cooling curve of MAXI J0556-332, fitting observational data to determine neutron star mass and radius preferences. Preliminary results suggest that neutron stars with a crust Urca process tend to have smaller masses and larger radii, highlighting the need for precise β-decay measurements to further understand these phenomena.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English
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Seminar
Fireworks in the cosmos: The Hidden Power of Nuclear Reactions
February 7 (Fri) 14:00 - 15:15, 2025
Irin Sultana (Ph.D. Student, Department of Physics, Central Michigan University, USA)
Neutron stars in low-mass X-ray binaries, accreting hydrogen- or helium-rich material from a companion star, frequently exhibit thermonuclear runaways on their surfaces known as Type-I X-ray bursts (XRBs). These bursts are powered by nuclear processes, such as the triple-$\alpha$ process, the $\alpha p$ process, and the rapid proton capture process, which play a critical role in model-observation comparisons. In this study, we investigate the impact of nuclear reaction uncertainties on XRBs using the ONEZONE model (Cyburt et al., 2016), considering different accreted compositions and accretion rates for the binary systems that are within the range of observed burst sources. The study is carried out in two stages. First, we determine the burst ignition conditions by simulating the settling of the accreted material with a full reaction network and a semi-analytical model. Second, we perform a sensitivity analysis by varying proton- and alpha-induced reaction rates in JINA REACLIBV2.2 within their estimated uncertainties. We explore the influence of these reactions on the XRBs light curve and the final abundances. The findings highlight key nuclear reactions that significantly affect XRB observables and the final abundances produced, offering guidance for future experimental efforts to improve our understanding of the uncertainties in the reaction rates involved in XRBs.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Search for BSM particles from high energy supernova neutrinos
January 10 (Fri) 14:00 - 15:15, 2025
Kensuke Akita (JSPS Research Fellow, Graduate School of Science, The University of Tokyo)
Light hypothetical particles with masses up to O(100) MeV can be produced in the core of supernovae. Their subsequent decays to neutrinos can produce a flux component with higher energies than the standard flux. We study the impact of heavy neutral leptons, Z′ bosons, in particular U(1)Lμ−Lτ and U(1)B−L gauge bosons, and majorons coupled to neutrinos flavor-dependently. We obtain new strong limits on these particles from no events of high-energy SN 1987A neutrinos and their future sensitivities from observations of galactic supernova neutrinos.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
EOS Dependence on Cooling of Isolated Neutron Stars
December 20 (Fri) 14:00 - 15:15, 2024
Stavros Fakiolas (Ph.D. Student, University of Oxford, UK)
Neutron stars - the densest stars in the Universe - cool down mainly by loss of neutrinos, emitted from the stars' interior due to particle reactions. By comparing cooling models with observed surface temperature or luminosity, one can probe the properties of high-density matter, such as what kind of particles and states exist inside neutron stars. In this presentation, I will first review cooling theory, focusing on the neutrino cooling processes. In particular, we focus on the equation of state (EOS) uncertainties, which significantly affect cooling curves. We discuss aspects such as the effect of including hyperons in our EOS. Using the updated cooling code, C-HERES, we calculate cooling curves with different EOS. Finally, we present the future prospects for this study.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Particle acceleration in relativistic astrophysical plasmas
December 13 (Fri) 14:00 - 15:15, 2024
Camilia Demidem (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Relativistic astrophysical objects often display evidence of very efficient particle acceleration, such as X-ray and gamma-ray nonthermal emission and are widely recognized as potential sources of cosmic rays. Elucidating the physical mechanisms that turn these environments into such formidable particle accelerators is a longstanding problem of high-energy astrophysics. In this talk, I will briefly explain why shocks and turbulence, naturally expected to occur in these environments, could play an essential role in the acceleration of particles. I will then discuss some of the challenges that poses the description of these nonlinear processes, especially in the context of high-energy astrophysical sources, which involve astronomical ranges of scales and physical conditions much more extreme than we can probe in our laboratories or in the Solar system. Finally, I will share some recent results from my simulations.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
White dwarf binary stars as physics laboratories
November 29 (Fri) 14:00 - 15:15, 2024
Lucy McNeill (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
White dwarfs are the most common remnant of stellar evolution, and most often orbit a binary companion. Orbital decay from gravitational radiation and binary stellar evolution can proceed to mass transfer onto the white dwarf, which may result in a Type Ia supernova. While these reliable thermonuclear explosions are essential tools for observational cosmology, the nature of the progenitor binary (double white dwarf, or white dwarf + evolved star) is still not clear. Surprisingly, recent galaxy surveys revealed that most Type Ia supernova come from exploding white dwarfs below the Chandrasekhar limit of 1.4 solar mass. Plus, observations of Milky Way white dwarf binaries suggest unexpectedly hot temperatures in double white dwarf merger progenitors. I will summarise our recent developments on the stellar structure and orbital evolution of finite temperature, partially degenerate white dwarfs in binary systems. Tidal heating can explain how candidate white dwarf merger progenitors are generically hot, which places more restrictive conditions required for a double white dwarf merger.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Asymmetries in Stripped Envelope Supernovae
October 25 (Fri) 14:00 - 15:15, 2024
Thomas Maunder (Ph.D. Student, Department of Astronomy, Monash University, Australia)
The explosion mechanism of supernovae is not yet fully understood. In order to better understand the inner-workings of the explosion we need to be able to test our models with observations. Current hydrodynamic simulations of stellar explosions often do not provide photometry or spectroscopy as this requires a treatment of the radiation transport of the ejecta. This project takes hydrodynamic simulations of Type Ib/c (stripped-envelope) supernovae and then performs Monte Carlo Radiative Transport simulations on the ejecta to obtain results we can compare with observations. We choose stripped-envelope supernovae because the lack of Hydrogen shell provides a more direct view into the core and the asymmetries of the explosion mechanism. Through these comparisons between models and observations we can improve our understanding of the explosion mechanism in core-collapse supernovae.
Venue: via Zoom
Event Official Language: English
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Seminar
Dense Matter Physics and Exotic States in Neutron Stars
October 7 (Mon) 14:00 - 15:15, 2024
Vivek Thapa (Assistant Professor, Bhawanipur Anchalik College, India)
In the presentation, I will discuss the exploration of neutron star matter using phenomenological models, focusing on how exotic particles like antikaons, hyperons as well as Delta-resonances influence the neutron star equation of state (EoS). The discussion will cover how antikaon optical potentials and kaon condensation affect the stability and structure of neutron stars, as well as the potential for hadron-quark phase transitions leading to quark matter cores in massive stars. I will also highlight the study of non-radial oscillation modes which provide insights into the internal structure and composition of neutron stars. These oscillation modes are essential for understanding neutron star asteroseismology and interpreting gravitational wave signals from neutron star mergers. By comparing theoretical predictions with observational data, including mass, radius, cooling rates, and gravitational wave detections, the presentation aims to refine constraints on the EoS and enhance our understanding of dense matter in compact stars.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Supernovae as Tracers of Massive-Star Evolution
July 17 (Wed) 14:00 - 15:15, 2024
Daichi Hiramatsu (Post-Doctoral fellow, Harvard University, USA)
Supernovae are the terminal explosions of massive stars with influences on every astrophysical scale. Advanced wide-field and high-cadence transient surveys routinely discover supernovae near the moment of explosion. Coupled with prompt and continuous follow-up facilities, these observations have revealed unprecedented features of dense circumstellar medium in various spatial scales as traced by the expanding supernova ejecta. Such circumstellar medium is thought to originate from mass-loss activities in the final years to decades of stellar evolution; however, their inferred densities exceed the expectations from standard theory by many orders of magnitude. In this talk, I will first introduce standard stellar evolution and supernova explosion mechanisms, and then describe novel observational probes and modeling techniques of supernovae interacting with circumstellar medium to reconstruct their explosion properties and progenitor mass-loss histories. Finally, I will discuss our on-going largest sample study of interacting supernovae and emerging pictures of dramatic dying breaths of massive stars.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English