ABBL-iTHEMS Joint Astro Seminar

Testing Astrophysical Models from the Shadow of the Galactic Center Black Hole

August 26 (Fri) at 14:00 - 15:00, 2022

Yosuke Mizuno (T.D. Lee Fellow / Associate Professor, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China)

We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength 1.3 mm. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of ~50 micro-arcsecond. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 million solar mass, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50 deg), as well as non-spinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way. In this talk, I will focus on more theoretical interpretation and model comparison to understand the accretion flow properties nearby Sgr A*.

Venue: via Zoom

Event Official Language: English

Long-term evolution of a supernova remnant hosting a double neutron star binary

July 1 (Fri) at 14:00 - 15:00, 2022

Tomoki Matsuoka (Ph.D. Student, Graduate School of Science, Kyoto University)

Stellar mass loss is one of the crucial elements which determine the fate of progenitors of core-collapse supernovae (SNe). Since the material released from the progenitor will be distributed as circumstellar medium (CSM), it can also have an influence on the subsequent evolution of the SN or supernova remnant (SNR). Despite its importance, mass loss histories predicted by stellar evolution models have not been incorporated with modeling for SNRs. As a first step, we investigate the dynamical evolution of an ultra-stripped supernova remnant (USSNR), originated from a type of core-collapse SN explosion proposed to be a candidate formation site of a double neutron star binary. By accounting for the mass-loss history of the progenitor binary using a model developed by a previous study, we construct the large-scale structure of the CSM up to a radius ∼100 pc, and simulate the explosion and subsequent evolution of a USSN surrounded by such a CSM environment. We find that the CSM encompasses an extended region characterized by a hot plasma with a temperature ∼10^8 K located around the termination shock of the wind from the progenitor binary (∼10 pc), and the USSNR blast wave is drastically weakened while penetrating through this hot plasma. Radio continuum emission from a young USSNR is sufficiently bright to be detectable if it inhabits our galaxy but faint compared to the observed Galactic SNRs. In this seminar I will talk about the background of the connection between the models for stellar evolution and SNRs, the details of our methods, and future prospects.

Venue: via Zoom

Event Official Language: English

Core-collapse Supernova Models with Heavy Axion-like Particles

June 3 (Fri) at 14:00 - 15:00, 2022

Kanji Mori (Research Institute of Stellar Explosive Phenomena (REISEP), Fukuoka University)

Axion-like particles (ALPs) are a class of hypothetical bosons which feebly interact with ordinary matter. The hot plasma of stars and core-collapse supernovae is a possible laboratory to explore physics beyond the standard model including ALPs. Once produced in a supernova, some of the ALPs can be absorbed by the supernova matter and affect energy transfer. We recently calculated the ALP emission in core-collapse supernovae and the backreaction on supernova dynamics consistently. It is found that the stalled bounce shock can be revived if the coupling between ALPs and photons is as high as $g_{a\gamma}\sim 10^{-9}$ GeV$^{-1}$ and the ALP mass is 40-400 MeV.

Venue: via Zoom

Event Official Language: English

The Hunt for Extraterrestrial Neutrino Counterparts

May 20 (Fri) at 16:00 - 17:00, 2022

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

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

Venue: via Zoom

Event Official Language: English

Coherent emission from 3D relativistic shocks

April 22 (Fri) at 14:00 - 15:00, 2022

Masanori Iwamoto (Kyushu University)

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

Venue: via Zoom

Event Official Language: English

Toward modeling complete supernova neutrino emissions

March 11 (Fri) at 16:00 - 17:00, 2022

Yudai Suwa (Associate Professor, Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo / Affiliate Associate Professor, Yukawa Institute for Theoretical Physics, Kyoto University)

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

Venue: via Zoom

Event Official Language: English

Spin transport in ultracold atomic gases

February 18 (Fri) at 14:00 - 15:00, 2022

Yuta Sekino (Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

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

Venue: via Zoom

Event Official Language: English

Galactic archaeology with r-process elements

January 28 (Fri) at 10:00 - 11:30, 2022

Yutaka Hirai (JSPS Research Fellow, Department of Astronomy, Graduate School of Science, Tohoku University / JSPS Research Fellow (Visiting Scholar), Department of Physics, University of Notre Dame, USA)

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

Venue: via Zoom

Event Official Language: English

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

December 10 (Fri) at 14:00 - 15:00, 2021

Jin Matsumoto (Assistant Professor, Keio Institute of Pure and Applied Sciences (KiPAS), Graduate School of Science and Technology, Keio University)

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

Venue: via Zoom

Event Official Language: English

Axions around rotating black holes

November 12 (Fri) at 14:00 - 16:00, 2021

Hirotaka Yoshino (Institute of Cosmophysics, Department of Physics, Graduate School of Science, Kobe University)

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

Venue: via Zoom

Event Official Language: English

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

October 15 (Fri) at 16:00 - 18:00, 2021

Nobuya Nishimura (Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

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

Venue: via Zoom

Event Official Language: English

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 Fellow, 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, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (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, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (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, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (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, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (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