Search Event
69 results
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Seminar
From Galaxies to Cosmological Structures: The Multi-Scale Influence of Cosmic Rays
June 13 (Fri) at 14:00 - 15:15, 2025
Ellis Owen (Special Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))
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
Insights on Issues in the Cold Dark Matter Hypothesis
April 25 (Fri) at 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) at 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) at 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) at 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-α process, the αp 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) at 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) at 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) at 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) at 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) at 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) at 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) at 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
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Seminar
Dynamics of the very early universe: towards decoding its signature through primordial black hole abundance, dark matter, and gravitational waves.
July 5 (Fri) at 14:00 - 15:15, 2024
Riajul Haque (Postdoctoral Researcher, Department of Physics, Indian Institute of Technology, India)
I will start my talk with a brief overview of the standard reheating scenario. Then, I will discuss reheating through the evaporation of primordial black holes (PBHs) if one assumes PBHs are formed during the phase of reheating. Depending on their initial mass, abundance, and inflaton coupling with the radiation, I discuss two physically distinct possibilities of reheating the universe. In one possibility, the thermal bath is solely obtained from the decay of PBHs, while inflaton plays the role of the dominant energy component in the entire process. In the other possibility, PBHs dominate the total energy budget of the universe during evolution, and then their subsequent evaporation leads to a radiation-dominated universe. Furthermore, I will discuss the impact of both monochromatic and extended PBH mass functions and estimate the detailed parameter ranges for which those distinct reheating histories are realized. The evaporation of PBHs is also responsible for the production of DM. I will show its parameters in the background of reheating obtained from two chief systems in the early universe: the inflaton and the primordial black holes (PBHs). Then, I will move my discussion towards stable PBHs and discuss the effects of the parameters describing the epoch of reheating on the abundance of PBHs and the fraction of cold dark matter that can be composed of PBHs. If PBHs are produced due to the enhancement of the primordial scalar power spectrum on small scales, such primordial spectra also inevitably lead to strong amplification of the scalar-induced secondary gravitational waves (GWs) at higher frequencies. I will show how the recent detection of the stochastic gravitational wave background (SGWB) by the pulsar timing arrays (PTAs) has opened up the possibility of directly probing the very early universe through the scalar-induced secondary gravitational waves. Finally, I will conclude my talk by elaborating on the effect of quantum correction on the Hawking radiation for ultra-light PBHs and its observational signature through dark matter and gravitational waves.
Venue: via Zoom
Event Official Language: English
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Seminar
Challenging conventional wisdom in binary evolution
June 28 (Fri) at 14:00 - 15:15, 2024
Ryosuke Hirai (Special Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))
The majority of massive stars, stars with more than 8 times the mass of the Sun, are known to be born in binary or higher-order multiple systems. During the course of their evolution, the stars can interact in many different ways and cause interesting astrophysical phenomena such as eruptions and explosions or create objects like X-ray binaries, gravitational wave sources, etc. Many studies have been conducted over the last few decades to tie our latest models to these observables in order to refine our understanding of massive binary evolution. However, in some cases "refining" a model is not enough and a paradigm shift is required to explain all the observables in a coherent way. In this talk, I will introduce some topics from my past work where I challenge conventional wisdom to resolve long-standing problems. The topics are as follows: 1. impact of supernova ejecta on companion star evolution, 2. wind accretion onto black holes, 3. common-envelope evolution, 4. neutron star kicks. I will also discuss how these new views impact the overall landscape of binary evolution theory.
Venue: via Zoom
Event Official Language: English
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Seminar
Surrogate Modeling for Supernova Feedback toward Star-by-Star Simulations of Milky-Way-sized Galaxies
May 10 (Fri) at 14:00 - 15:15, 2024
Keiya Hirashima (Ph.D. Student, Department of Astronomy, Graduate School of Science, The University of Tokyo)
Galaxy simulations have found the interdependence of multiscale gas physics, such as star formation, stellar feedback, inflow/outflow, and so on, by improving the physical models and resolution. The mass resolution remains capped at around 1,000 solar masses (e.g., Applebaum et al. 2021). To overcome the limitations, we are developing a new N-body/SPH code, ASURA-FDPS, to leverage exascale computing (e.g., Fugaku), handle approximately one billion particles, and simulate individual stars and stellar feedback within the galaxy. However, the emergence of communication costs hinders scalability beyond one thousand CPU cores. One of the causes is short timescale events localized in tiny regions, such as supernova explosions. In response, we have developed a surrogate model using machine learning to duplicate supernova feedback quickly (Hirashima et al., 2023a,b). In the presentation, I report the fidelity and progress of the simulations with our new machine-learning technique.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
How Stars End Their Lives
April 26 (Fri) at 14:00 - 15:15, 2024
Philipp Podsiadlowski (Professor, University of Oxford, UK)
While the basic evolution of stars has been understood for many decades, there are still major uncertainties in our overall understanding of how stars end their lives, both in the context of low- and intermediate-mass stars (including the Sun) and massive stars. I will first review some of key principles that govern the structure and evolution of stars and then present recent progress that has been made for both groups of stars. I will argue and present numerical simulations that show that all stars become dynamically unstable when they become large giant stars, which leads to sporadic, dynamical mass ejections. Low- and intermediate-mass stars may lose all of their envelopes as a consequence, leaving white-dwarf remnants. More massive stars experience core collapse, leaving a neutron-star or black-hole remnant, possibly associated with a supernova explosion. I will show how the dramatic recent progress on understanding the core-collapse process, for the first time, allows us to connect the late evolution of massive stars with the resulting supernova explosions and the final remnants and discuss how observations with current gravitational-wave detectors (such as LIGO) will allow us to test this theoretical connection.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Do plants have bones? Silica phytoliths and their role and fate in the development of terrestrial plants and human civilizations
March 1 (Fri) at 14:00 - 15:15, 2024
Mikhail Blinnikov (Professor, St. Cloud State University, USA)
Silicon is the second most common element in the Earth’s crust. Some families of higher plants evolved mechanisms for soluble silica to be carried by xylem from groundwater and deposited as plant opal in or around plant cells as phytoliths thought to play a role in the structural support and defense against herbivores. While known since the early 19th century, phytoliths remain an intriguing class of microfossils whose formation and role in plants and their preservation in soils and sediments are a subject for a lot of active research. I outline some emerging themes in phytolith analysis including phytoliths’ role in global biogeochemical cycles, plant-herbivore interactions, and their tracing of evolution of cultural plants, especially cereals such as rice (Oryza), wild rice (Zizania), maize (Zea), wheat (Triticum) and millet (Panicum), all relevant to global archaeology. Some emerging research on phytoliths connects their changes in shapes to plant taxonomy of some families such as grasses and opens up avenues for further investigation of their active construction in the cells of some taxa by yet undiscovered genetically mediated mechanisms. New image analysis techniques and some advanced microscopy methods will allow us to further the field of phytolith study using deep machine learning algorithms and true 3D analysis of their shapes, something where contribution from other branches of science are most welcome.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Relativistic Jet Simulations and Modeling on Horizon Scale
February 8 (Thu) at 13:00 - 14:30, 2024
Yosuke Mizuno (T.D. Lee Fellow / Associate Professor, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China)
Relativistic jets are launched in the vicinity of the central black holes and emit powerful radiation across the electromagnetic spectrum. According to our current understanding, relativistic jets are launched by directly tapping the rotational energy of spinning black holes via the so-called Blandford-Znajek process. In addition to the spin of the black hole, numerical simulations showed the amount of accreted magnetized flux has a major impact on the formation of relativistic jets. We have investigated the radiative signatures of self-consistently launched relativistic jets using 3D general relativistic magneto-hydrodynamical simulations and general relativistic radiative transfer calculations in horizon scale to the connection with large-scale structure. We discuss our findings and comparison with observations.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Probing structure of neutron stars through X-ray bursters
January 12 (Fri) at 14:00 - 15:15, 2024
Akira Dohi (Special Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))
Type-I X-ray bursts are rapidly brightening phenomena triggered by the nuclear burning of light elements near the surface of accreting neutron stars. Most of the X-ray bursters show irregular behavior of light curves. However, some X-ray bursters are somehow quite regular, i.e., constant recurrence time and constant shaper of light curves, and are often called Clocked bursters, which are powerful sites to probe uncertainties of many model parameters such as accretion rate, the composition of accreted matter, reaction rates, neutron star structure, and temperature. In this study, we focus on the uncertainties of the equation of states, which determines the latter two properties. Based on our numerical models covering whole areas of neutron stars, we will present their impact on X-ray burst light curves. Furthermore, we will discuss the possibility of constraining the equation of states from Clocked bursters such as GS 1826-24 and 1RXS J180408.9-342058.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Exploring material strengths of dust aggregates in planet formation by numerical simulations
December 8 (Fri) at 14:00 - 15:15, 2023
Misako Tatsuuma (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
The planet formation process is the growth from sub-micrometer-sized cosmic dust grains to thousand-kilometer-sized planets. This growth process has broadly two phases: the growth from dust grains to kilometer-sized planetesimals, mainly driven by intermolecular forces like van der Waals forces and hydrogen bonds, and the subsequent growth from planetesimals to planets, governed by gravitational forces. However, the planetesimal formation process encounters various challenges, including fragmentation and bouncing resulting from collisions among dust aggregates. To gain insights into the planetesimal formation process and how to avoid these obstacles, I have been focused on measuring and formulating the material strengths of dust aggregates using grain simulations. In this talk, I will introduce my works on the material strengths of dust aggregates and their applications to kilometer-sized bodies in the solar system, such as comets and asteroids.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English