コロキウム要旨 / Colloquium ABSTRACTS

Prof. Rennan Barkana (Tel Aviv University)




Prof. Daisuke Kawata (MSSL, University College London)




Prof. Andrzej A. Zdziarski (Nicolaus Copernicus Astronomical Center)

Jets in accreting black-hole binaries


The most spectacular jets are observed from active galactic nuclei, in particular from quasars. However, highly interesting jets are also launched by accretion flows in stellar binaries containing a normal star accreting onto a stellar-mass black hole. Such systems are analogs of quasars on a much smaller scale, and are called microquasars. There are two distinctly different types of jets in microquasars. Jets of the first type are steady, and are launched during accretion states characterized by hard X-ray emission. They are launched over weeks to months, but are observed only up to maximum distances of about a 1/1000 of a parsec. Those of the other type are launched on time scales of only a day during transitions of the accretion flow from the hard to soft spectral states, but are observed as moving blobs up to a parsec scale, i.e., up to ~1000 times larger distances. I will discuss possible causes of this difference, the jet emission mechanisms, collimation, the presence of electron-positron pairs, magnetic fields, bulk Lorentz factors and the jet power.

Dr. Grégoire Marcel (Nicolaus Copernicus Astronomical Center)

A unified accretion-ejection paradigm for X-ray binaries


X-ray binaries are binary systems composed of a star and a compact object, black hole or neutron star. These system are amazing laboratories to understand accretion-ejection phenomena. However, the complexity of their behavior limits us since their first discovery in the 60's. These systems spend most of their lives in a barely detectable state, sometimes for years, before experiencing sudden outbursts in luminosities that last up to few months. During a typical outburst the entire spectra changes, but what is the most striking is what happens in X-rays. These X-rays are emitted by the inner-most regions of the system, very close to the compact object, where the evolution timescales are smaller than a millisecond. This truly astronomical difference might well be the sole reason the behavior of those object is so hard to grasp entirely. During my talk, I will first present both the observational and theoretical state of the art of the field. I will then detail my work on the JED—SAD paradigm: a dynamical view based on accretion-ejection phenomena that has been tested on many different observables; from radio jets to X-ray variability.

Dr. Yves Revaz (EPFL)

Challenges in reproducing the faintest galaxies of our Universe: failure of galaxy formation models or questioning of the ΛCDM?


Dwarf galaxies are the faintest but most abundant galaxies known in our Universe. Being so small, they represent a unique tool to constrain cosmological models down to the smallest scales, out of reach by direct observations. While the properties of dwarf spheroidal galaxies are well captured by the current numerical simulations, I will show how those same models fail to reproduce some observed features of ultra-faint dwarf galaxies (UFDs), like their metallicity and size. As these difficulties are directly interconnected to the complex build-up history imposed by the LCDM they question either its validity at the smallest scales or reveal the limitation of current state-of-the-art-numerical models.

Mr. Kosuke Mizutani (Osaka University)

3D HD simulation of the onset of the Common Envelope phase in a binary system


Binaries are related to almost all phenomena that occur in the universe. The key difference between binary evolution and single-star evolution is the presence of mass transfer. Mass transfer alters not only the properties of each star but also orbital separation. If this mass transfer becomes unstable, the binary system enters the Common Envelope phase, during which the companion star orbits inside the envelope of the primary star. The changes of energy and angular momentum in this phase are important to understand the whole binary evolution, but they are still not well understood. To find out the effect of the nature of the primary star’s envelope on this phase, we present and analyze the results of a three-dimensional hydrodynamic model focusing on the onset of the Common Envelope phase. We found a difference in the efficiency of orbital decay depending on whether the envelope is radiative or convective. This result implies that when studying the Common Envelope phase in binary evolution, it is essential to consider the timing of mass transfer and the properties of the primary star.

Mr. Tatsuki Fujiwara (Osaka University)

The Lunar MeV Gamma-Ray Spectrum: New Window for MeV Cosmic Rays


The Moon is the closest celestial object and shines in the gamma-ray energy band. Detection of lunar gamma-rays by Fermi-LAT has allowed the GeV Galactic cosmic-ray spectrum to be measured. However, gamma-ray investigation of the Moon also has potential for the study of MeV Galactic cosmic rays with upcoming MeV missions. Nuclear spallation and excitation interactions on the lunar surface would produce various gamma-ray lines, while inelastic hadronic collisions and bremsstrahlung could cause an intense MeV continuum. These signatures would help us measure the MeV Galactic cosmic-ray spectrum. Taking into account the composition of the lunar surface, we performed spectral simulations using the latest Geant4 Monte Carlo code. Our simulations are consistent with the observed Fermi-LAT lunar spectrum. They also show that the MeV cosmic-ray spectrum will be detectable with lunar measurements by upcoming MeV gamma-ray instruments, such as the Compton Spectrometer and Imager (COSI). Based on our results, we report the future scientific prospects for lunar observations.

Dr. Yuh Tsunetoe (BHI, Harvard University)

Investigating Black Hole-Disk-Jet System through Polarization Images


The realization of Very Long Baseline Interferometry (VLBI) observations on a global scale has opened a new horizon in the study of the magnetic driving mechanisms of black hole jets. At this opportune moment, what is needed are concrete indicators that link observational images with theoretical models. In light of this, we performed general relativistic radiative transfer calculations based on magnetohydrodynamics models to theoretically predict the polarization images of black holes. As a first result, we found that the linear and circular polarization components are distributed downstream and upstream along the jet, respectively. This linear-circular polarization separation depends on the electron temperature distribution in the accretion disk and jet, as well as the mass accretion rate onto the black hole, enabling the exploration of the black hole-disk-jet structure through polarization observations. Secondly, we showed that the 90-degree flip of the linear polarization vector, characteristic of optically thick non-thermal plasmas, can be detected at the base of the jet and on the photon ring. These serve as strong indicators of the electron energy distribution near the black hole. Lastly, as a recent result, we demonstrated that fluctuations in the magnetic flux penetrating the black hole, which drives the jet, are strongly reflected in the variations in the jet width observed up to several hundred Schwarzschild radii. This result implies that the dynamics on the horizon scale can be estimated from jet observations, and conversely, changes in the jet shape can be predicted from fluctuations on the horizon scale.

Prof. Shun Saito (Missouri University of Science and Technology)

Hunting Dark Energy through the Baryon Acoustic Oscillations


More than one decade has passed since Nobel Prize Physics 2011 was featured for the discovery of the cosmic acceleration in 1998. The fact that dark energy, attributed to explain the cosmic acceleration, exists and is the most dominant energy budget of the current Universe leads to one of the most mysterious and fundamental questions in physics. What is dark energy and why does it become important only now? Given the lack of our solid theoretical prediction, there is a whole bunch of observational campaigns to reveal the nature of dark energy. In this talk, I will overview the Baryon Acoustic Oscillations (BAOs) as an extremely robust probe of dark energy. In particular, I would like to give my personal view on the recent (somewhat confusing) claim of the evidence of dynamical dark energy by the Dark Energy Spectroscopic Instrument (DESI) collaboration. Then I will discuss how we are trying to improve upon the BAO result in the galaxy redshift surveys I am involved in, introducing the Hobby-Eberly Telescope Dark Energy Experiment, Subaru Prime Focus Spectrograph, and the Nancy Grace Roman Space Telescope.

Mr. Nobuyuki Sakai (Osaka University)

Disentangling the Contribution from Disk Winds to the GeV Gamma-Ray Emission of the Seyfert Galaxy GRS 1734-292


The origin of gamma-ray emission from radio-quiet active galactic nuclei (AGN) detected by Fermi-LAT remains unresolved. Several mechanisms have been proposed, including starburst activities, weak jets and disk winds. Recent stacked observations indicate that disk winds could be a plausible mechanism for producing their observed gamma-ray flux. However, the complexity of individual systems makes it challenging to definitively identify the emission processes. Our study focuses on the Seyfert galaxy GRS 1734-292, where we find that neither starburst nor jet activities expected from infrared and radio observations can explain the observed GeV gamma-ray flux. We employ a lepto-hadronic emission model that incorporates the dynamical evolution of interactions between the interstellar medium (ISM) and AGN disk winds, building on our previous study (Yamada, Sakai, Inoue, & Michiyama, 2024, ApJ). We consider both shocked ISM and disk wind regions as potential emission sites. Our findings suggest that a disk wind, extending over 20 pc, can account for the observed GeV gamma-ray emission. These results position GRS 1734-292 as a potential example of gamma-ray emission driven primarily by disk winds and underscore the need for further multi-messenger studies to confirm this mechanism across other AGNs.

Mr. Kenji Kihara (Osaka University)

Formation processes of dark matter deficient galaxies: Insights from tidal scenarios and phase space behavior


A dark matter deficient galaxy (DMDG) is a type of galaxy where the stellar mass exceeds the dark matter mass, unlike galaxies typically seen in the ΛCDM model. Since its discovery, the formation process of these galaxies has been under debate. In this presentation, I will focus on the tidal scenario. My research, based on Mr. Katayama's master thesis, introduces a novel emphasis on the behavior in phase space, which is thought to provide insights into the formation of DMDGs. Additionally, I will explore whether different dark matter models result in varying outcomes in future work.

Ms. Anastasiia Omeliukh (Ruhr University Bochum)

Modeling neutrino emission from blazars


The discovery of astrophysical high-energy neutrino flux by IceCube South Pole Neutrino Observatory opened a new era of neutrino astronomy. Neutrinos, weakly interacting neutral particles with little mass, can travel long distances without being deviated in magnetic fields or absorbed and, therefore, are perfect messengers of the signals from far Universe. The origin of astrophysical neutrino flux remains unknown. Prospective candidates for neutrino emitters are blazars. Blazars are a sub-class of active galactic nuclei with relativistic jet pointing close to the observer's line of sight. The relativistic Doppler boosting makes them one of the most powerful sources in the Universe with possible effective particle acceleration and, thus, also subsequent neutrino emission. When the detection of high-energy neutrino coincides in space and time with the increased blazar activity in multiple energy bands, the blazar becomes a candidate for a neutrino emitting source. We collect observational data at multiple frequencies to build a spectral energy distribution of a blazar. We perform state-of-the-art numerical modeling of the radiation processes in the source to explain the observed photon and neutrino fluxes. The recent findings have shown that the contribution of hadronic processes to the observed photon fluxes cannot be constrained based on the available data which impact the predictions of the expected neutrino emission. Possible ways to overcome this challenge are also discussed in this talk.

Dr. Yongming Liang (ICRR, The University of Tokyo)

Cosmic Himalayas: The Highest Quasar Density Peak at Cosmic Noon in a 10,000 deg^2 Sky


Discovered by the MAMMOTH-Subaru survey in BOSSJ0210, the Cosmic Himalayas is an extraordinary structure hosting 11 SDSS/eBOSS luminous (L_bol≳10^45.5 erg s^-1) Type-1 quasars at z=2.2, within a (40 cMpc)^3 region. This cluster represents the most significant quasar density peak at z>2, being 30 times the average with a remarkable 17σ significance. Mapped by Subaru/HSC NB387 for z=2.2 Lyman-alpha emitters (LAEs), the quasar overdensity intriguingly does not match LAE overdensities but lies perpendicular to a ~100 cMpc large-scale filament. This filament’s nodes showcase distinct galaxy properties, such as varied star formation rates, AGN hosting probabilities, and extended Lyman-alpha emission detection, with quasars positioned at intermediate points. Initial insights from 3D intergalactic medium (IGM) tomography, created using SDSS/eBOSS background quasars, indicate a significant ionizing stage difference between the filament’s nodes, suggesting quasars play a central role in shaping the ionizing topology. These findings highlight the Cosmic Himalayas as a unique structure for advancing our understanding of the interplay between quasars, galaxies, and the IGM.

Dr. Hiddo Algera (Hiroshima University/NAOJ)

The dust properties of high-redshift galaxies


To date, sensitive ALMA observations have revealed dust emission in dozens of “normal” galaxies at z > 6.5. This not only demonstrates that dust can build up rapidly in the early Universe, but also suggests that obscured star formation is likely to be significant even at these early cosmic epochs. In this talk, I will discuss our current understanding of dust build-up in high-redshift galaxies from an observational perspective. First, I will use multi-band ALMA observations to accurately measure the dust masses of several galaxies at z > 6.5. These measurements suggest either efficient dust production through supernovae or — more likely — additional mechanisms of dust build-up. Following this, I will introduce new JWST/NIRSpec IFU observations of z ~ 7 dusty galaxies, and discuss what we can learn about early dust production pathways from a combined study of dust and metals.

Dr. Jennifer Y. H. Chan (Canadian Institute for Theoretical Astrophysics)

Decoding the Universe's First Billion Years with Cosmological 21-cm Line Radiative Transfer


21-cm line of neutral hydrogen serves as a fundamental probe of the Universe's first billion years, illuminating the early phases of cosmic structure formation during the Dark Ages, Cosmic Dawn, and the Epoch of Reionization (EoR). Precise modeling of this line is crucial for accurately extracting information from observations, yet traditional methods often inadequately address the complex interactions between local processes and global cosmic expansion & structural evolution. To fill this gap, we have developed a novel covariant formulation of 21-cm line radiative transfer, which corrects previous oversimplifications. This seminar focuses on two primary research questions: How does this covariant formulation enhance model accuracy, and what impact do ionized cavities have on the redshifted 21-cm spectrum? I will present the key findings from our recent studies. Specifically, utilizing the cosmological 21-cm radiative transfer (C21LRT) code, our study quantifies traditional 21-cm spectra modeling errors, which amount to 5% for redshifts 12 ≲ z ≲ 35 and exceed 10% for z ≲ 8 under a smoothly varying global reionization model. We also reveal how ionized cavities introduce distinct spectral features in the 21-cm line not captured by traditional optical depth parametrization methods. These insights are crucial for refining theoretical predictions of the redshifted 21-cm signals and optimizing scientific gains from various EoR experiments, such as EDGES, LEDA, LOFAR, HERA, and the SKA.

Dr. Alvina Y. L. On (NCTS)

Listening to the radio symphony of cosmic magnetic fields


Magnetic fields are everywhere, permeating across all scales from interstellar space to cosmic voids. Yet their origins and evolution remain poorly understood. On galactic scales and beyond, Faraday rotation measure (RM) at radio wavelengths is commonly used to diagnose large-scale magnetic fields. The magnetic field length scales are often inferred from correlations in the observed RM. However, RM is a quantity derived from the polarised radiative transfer (PRT) equations under restrictive conditions. In this talk, I will assess the use of rotation measure fluctuation (RMF) analyses for magnetic field diagnostics in the framework of PRT. I will demonstrate how density fluctuations can affect the correlation length of magnetic fields inferred from the conventional RMF analyses. For complex astrophysical situations, a covariant PRT calculation is essential to properly track the radiative and transport processes, otherwise, the interpretations of magnetism in galaxy clusters and larger-scale structures would be ambiguous. Lastly, I will discuss the implications of our work on radio observations, particularly with BURSTT and the SKA.

Dr. Enrico Garaldi (IFPU/Osaka University)

Towards a complete picture of early structure formation using radiation-hydrodynamical simulations


Within a decade, an array of observational facilities will deliver the first comprehensive picture of the reionizing Universe, from galaxies to the diffuse inter-galactic medium (IGM). This will enable the detailed investigation of the intricate connection between the first collapsed structures and cosmic reionization. I will present my ongoing efforts to build a solid theoretical framework to study this phenomenon from the largest IGM scales down to pc-scale structures in galaxies. The starting point of my talk will be a suite of large-volume radiation-hydrodynamical simulations, that I will introduce and use to provide new insights on the distribution of bubble sizes in the early Universe, their impact on the nearby Lyman-alpha forest and the role of galaxy properties in shaping their properties. I will then show how results from such computation-limited simulations can be extended to the larger volumes required for 21cm science. Then, I will move on to introduce a suite of high-resolution radiation-hydrodynamical simulations that for the first time capture simultaneously the complex physics of the interstellar medium within the first galaxies and the O(100 Mpc)-scale radiation field. Through a tiered approach, these new simulations will enable to simultaneously study the first galaxies and the coeval IGM over an unprecedented range of scales. In the final part of the talk, I will discuss the often-forgotten helium reionization, showing how current observations of the quasar luminosity function are in tension with measurements of the IGM ionization state at z~3 and discussing the implications of the abundant population of high-z quasars recently observed by JWST.

Prof. Shinsuke Takasao (Osaka University)

Global 3D MHD simulation of accretion onto a protostar from a magnetized, turbulent disk


Protostars grow through gas accretion from the surrounding disk. In the early protostellar evolution, the disk extends down to the protostellar surface, forming a narrow layer in which the angular velocity of the accreting gas adjusts to the rotational speed of the protostar. This accretion mode is called boundary layer accretion. The mass and angular momentum transfer in boundary layer accretion, an expected accretion mode in the early protostellar evolution, has been poorly understood. To reveal the process, we present and analyze results of a global, three-dimensional magnetohydrodynamic model of boundary layer accretion around a strongly magnetized, convective protostar. The simulation found a new accretion mode which is involved with stellar magnetic fields. Our findings could provide a solution to a long-standing problem about the angular momentum transfer mechanism in the boundary layer. We also discuss some observational implications such as the protostellar luminosity problem.

Prof. emeritus Hideaki Takabe (Osaka University)

Laboratory Astrophysics and Fusion Physics with Intense Lasers


Since the invention of the laser in 1960, lasers have been applied as a tool for research in our surroundings and in advanced technology and basic science. Today, I will give talks on the following two physics topics in our research field. Finally, I would like to describe a project for which an open call has been launched. 1. Study of astrophysical plasmas by intense laser 2. Research on laser fusion using the world's largest NIF facility 3. Formation of an interdisciplinary research group for the fusion energy project. (Full text here)

Dr. Ryota Tomaru (Osaka University)

Accretion disk winds from X-ray binaries: Tests for those driving mechanisms and motivation of a physical model for AGN feedback


Accretion disk winds in compact objects in X-ray binaries, suggested by the blue-shifted absorption lines in spectroscopic data, are important phenomena to address the understanding of the physics of AGN feedback. Those objects show strong X-ray radiation from the accretion flows and clear absorption/emission lines of winds without contamination from stellar components and dust extinction of interstellar medium. However, the driving mechanisms of the winds are unclear. To tackle this, we build a comprehensive model for X-ray spectra. The detail line profiles are simulated by Monte-Carlo radiation transfer, whose input density/velocity fields are calculated by the radiation hydrodynamics code. Our models show that the observed spectra are well described by the winds launched at a larger radius driven by radiative heating/acceleration, where the mass/momentum/energy transfer to outer systems is calculable by the observed SED, luminosity, and the disk size. This motivates us to make quantitative models of AGN feedback for galaxy formation by radiative (accelerated/heated) winds determined by fundamental parameters related to black holes, such as mass and accretion rate.

Dr. Akihiro Inoue (Osaka University)

General relativistic radiation MHD simulations of super-Eddington accretion flows onto the magnetized neutron star


Ultraluminous X-ray sources (ULXs) are bright, point-like, off-nuclear X-ray sources. Their X-ray luminosity exceeds the Eddington luminosity for the stellar mass black hole, and the energy production mechanism remains a great mystery. We have performed general relativistic radiation magnetohydrodynamics simulations of super-Eddington accretion flows onto neutron stars with dipole and quadrupole magnetic fields, as modeling the neutron-star-powered ULXs. In our simulations, accretion disk, accretion flows along the neutron star’s magnetic field, and optically thick outflows driven by the strong radiation force appear. Such outflows can explain the thermal emission with a temperature of 1E+7 K and a blackbody radius of ~100 km detected in ULX. In this talk, the magnetic field structure (strength and configuration) of the neutron star in the ULXs is also discussed.