2021/2/11 (木)

No seminar (National Holiday)

2021/2/4 (木)


Speaker: Koh Ueno

2021/1/28 (木)


Speaker: Naoto Kuriyama

2021/1/21 (木)


Speaker 1: Minori Shikauchi (n.b. slides and oral presentation will be in English)

Title: On the use of CHIME to Detect Long-Duration Radio Transients from Neutron Star Mergers

A search for afterglows with radio telescope Canadian Hydrogen Intensity Mapping Experiment (CHIME) will provide an alternative measurement of the BNS merger rate relying on entirely different physical process and so will be an important confirmation of the GW-measured rate. Using a more specific detection algorithm based on likelihood ratio statistics, we estimated 155 afterglows per year can be detected with CHIME and 95 % of them are off-axis afterglow. Our result leads to the prediction of a slightly smaller event rate than previous work (Feng+2014), but confirms the essential conclusion of the earlier analysis, namely that using CHIME to search for synchrotron afterglows will be effective at constraining the astrophysical merger rate as well as orphan afterglow search.

Speaker 2: Fumio Uchida (n.b. slides and oral presentation will be in Japanese)

Title: Generation of baryon isocurvature perturbations from primordial magnetic fields and its cosmological consequence (初期磁場によるバリオン等曲率揺らぎの生成とその宇宙論的帰結)

Magnetic fields are ubiquitous in the universe, and even in the voids, where few astrophysical objects that could amplify seed fields through e.g., the galactic dynamo exist, lower bounds of the strength of the so-called intergalactic magnetic fields are observationally suggested. A possibility is that such cosmological magnetic fields originate in the very early universe. In the talk, I assume primordial magnetic fields before the electroweak symmetry breaking and argue that such fields unavoidably generate baryon isocurvature perturbations through a conversion from magnetic helicity to baryon number through the chiral anomaly in the Standard Model. For a successful big-bang nucleosynthesis is a constraint put on the baryon isocurvature perturbations, and hence we can constrain the properties of the primordial magnetic fields. The condition is so strong that the intergalactic magnetic fields cannot be purely primordial and that some mechanisms of amplification or generation of magnetic fields after the electroweak symmetry breaking are needed.

2021/1/14 (木)


Speaker 1: Yuting Lu

Title: Measuring Stellar Rotation Periods and Stellar Inclinations of Kepler Solar-type Stars

Spin-orbit angle, the angle between the stellar rotation spin and planetary orbital axis, has been an important indicator of the formation and evolution of exoplanetary systems. Unlike our solar system, large spin-orbit angles are observed in some hot Jupiter systems. To understand the origin of such large misalignment, more observations are required for different type of systems. Kepler telescope provides high quality photometric data for a large number of stars, which allows us to put constraints on spin-orbit angles for more systems by measuring the stellar inclination of their host star. In this presentation, I will first introduce current problem encountered in the estimation of stellar inclinations using asteroseismology. Then, I will discuss about several possible attempts to improve the measurement.

Speaker 2: Koki Tokeshi (n.b. slides and oral presentation will be in Japanese)

Title: Towards the resolution of uncertainty and refinement of the primordial black hole mass function

As my mock master defence, I will report two results around the peak counting theory. One is on the completely, locally and spherically symmetric perturbations in the peak counting theory, in which the predicted PBH abundance becomes larger compared to that of the conventional formulation. The other is on the refined version of the peak counting theory, in which the one-to-one correspondence among the mass, formation time, and the smoothing scale of PBHs is implemented, with the investigation of the choice of the window function, leading to the result that almost no correction is observed around the typical scale. The first topic was ongoing in my first talk on 19/11/2020 but this time the result is included, while there is almost no update on the second subject.

2020/12/17 (木)

This is the final Thursday seminar this year.
The seminar starts at 12pm (The summary talks start at 11:30).

Speaker: Heather Fong

2020/12/10 (木)


Speaker: Minori Shikauchi

Title: On the use of CHIME to Detect Long-Duration Radio Transients from Neutron Star Mergers (電波望遠鏡CHIMEによるショートガンマ線バーストからの残光観測)

Based on the assumption that BNS merger may cause short gamma-ray bursts (Pazcynski1986, Goodman1986, Shemi&Piran1990, Paczysnki1990), a search for afterglows with radio telescope Canadian Hydrogen Intensity Mapping Experiment (CHIME) will provide an alternative measurement of the BNS merger rate relying on entirely different physical process and so will be an important confirmation of the GW-measured rate.

A previous research (Feng+2014) estimated the detectability of afterglows from SGRBs with respect to the circum medium density. They employed a fixed threshold in Janski to define "detectable". They concluded that up to 10^4 yr^{-1} afterglows can be detected with CHIME Pathfinder, a prototype of CHIME Full.

Here, we introduce a specific detection algorithm based on likelihood ratio statistics and simulate its response to signals embedded in noises. From the result, we also estimate a false alarm rate caused by the search over sky location and the choice of light curve templates. Taking all of that into consideration, we set a detection threshold based on target sample purity.

Assuming thermal noise is dominant, we estimated 50, 10 at least, and 154 at most afterglows per year can be detected with CHIME Pathfinder in one year observation. Our more detailed analysis leads to the prediction of a slightly smaller rate of detections, but it confirms the essential conclusion of the earlier analysis, namely that using CHIME to search for synchrotron afterglows will be effective.

2020/12/3 (木)


Speaker1: Leo Tsukada

Title: Modeling and Searching for Stochastic Gravitational-Wave Backgrounds from Ultra-light Boson Particles

Ultralight bosons, which are predicted in a variety of beyond-Standard-Model scenarios as dark- matter candidates, can trigger the superradiant instability around spinning black holes. This in- stability gives rise to oscillating boson condensates which then dissipate through the emission of nearly monochromatic gravitational waves. Such systems are promising sources for current and future gravitational-wave detectors. In this work, we consider minimally-coupled, massive scalar and vector bosons, each of which could emit a population of signals that incoherently superpose with one another, called stochastic gravitational-wave background. We adopt recently obtained numerical results for the gravitational-wave flux, and astrophysical models of black hole populations that include both isolated black holes and binary merger remnants, to compute and study in detail the background spectrum produced by these sources. Using a Bayesian framework, we search for such a background signal using data from the first and second observing runs of Advanced LIGO. We find no evidence for such a signal, which allows us to place constraints on those boson masses. We find that our results disfavor the scalar (vector) boson mass in the range [2.0 - 6.0] * 10^-13 eV ([0.8 - 7.0] * 10^-13 eV ) at 95% credibility, assuming the optimistic spin distribution for the isolated black hole population.

Speaker2: Fumio Uchida

Title: Generation of baryon isocurvature perturbations from primordial magnetic fields and its cosmological consequence. (初期磁場によるバリオン等曲率揺らぎの生成とその宇宙論的帰結)

As a possible origin of the baryon asymmetry of the universe, helical magnetic fields before the electroweak symmetry breaking are studied. In the scenario, the chiral anomaly in the Standard Model converts the hypermagnetic helicity of the magnetic field into baryon (and lepton) number.
Conditions on the strength and the coherence length of the magnetic fields are imposed to reproduce the observed baryon-to-entropy ratio, and the generation of baryon isocurvature perturbations gives another constraint because it affects the big-bang nucleosynthesis. I will show the constraints in terms of those on the present intergalactic magnetic fields, and discuss their cosmological implications.

2020/11/26 (木)


Speaker : Yuting Lu

Title: Measuring stellar rotation periods and stellar inclinations of Kepler solar-type stars

The determination of stellar rotation period benefits studies of a wide range of topics such as stellar evolution, magnetic activities and stellar dynamo. An immediate application of rotation period estimation is to derive the stellar inclination relative to our line of sight. Such an angle is an important parameter to determine the angle between the stellar rotation spin and planetary orbital spin, i.e., the spin-orbit angle, which is believed to offer important clues to the formation and evolution of planetary systems. The advent of space mission like Kepler provides a large amount of high-quality photometric data for stars, and various techniques have been developed to measure stellar rotation periods based on Kepler photometric data. In this project, we derive the rotation periods for 91 solar type stars using three mostly-used photometric analysis techniques, including the Lomb-Scargle Periodogram(LS), auto-correlation function(ACF), and wavelet method. We then compare period estimation results using photometric analysis with those using asteroseismic analysis, with the latter method being an independent approach of measuring stellar parameters by studying the stellar oscillation. In the presentation, I will present our results of photometric analysis and comparison with asteroseismic estimations.

2020/11/19 (木)


Speaker 1: Koki Tokeshi

Title: Towards the resolution of uncertainty and refinement of the primordial black hole mass function

All of my researches on primordial black holes are based on the peak counting theory and its refinement version. The first subject is the window function uncertainty of the refined mass function: we have clarified that around the typical scales the additional condition for precise estimation of the PBH mass, at least under the top-hat power spectrum, has almost no effect on the abundance. Various shapes of the power spectra should also be studied as well and this is currently under consideration.
The second topic is on the spherically symmetric perturbations and the peak counting theory: the conventional formalism could not be applied for the case. This is also an ongoing subject so I will report the current status with some obtained results, but maybe without the final consequences.

Speaker 2: Naoto Kuriyama

Title: The features of mass eruptions from progenitors of Type IIn/Ibn supernovae

Some massive stars experience episodic and intense mass eruptions accompanied by fluctuations in the luminosity in the late phase of stellar evolution. Mass loss rates of such eruptions are too high to be explained by stellar wind mass loss model and thus dynamical eruptions seem to contribute to the mass loss and fluctuations in the luminosity. These mass eruptions form circumstellar matter (CSM) around the star and they could play an important role in the subsequent core-collapse supernova because the interaction between SN ejecta and CSM could be the thermal source of the SN. This kind of SNe are classified as Type IIn/Ibn SNe. However, the mechanism and energy sources which trigger these pre-SN eruptions have not been fully explained. In Kuriyama & Shigeyama (2020a), we investigate the properties of a mass eruption conducting a sudden energy injection into the stellar envelope using 1-D radiation hydrodynamical simulation code. Additionally, we recently find that the first and second mass eruptions show different features if the eruption repeats twice with an appropriate time separation, because the density structure of the stellar envelope is altered by the first eruption (Kuriyama & Shigeyama 2020b). This fact would be useful when we interpret observations which show multiple eruption events. In this presentation, we introduce the features of pre-SN mass eruptions from the progenitors of SNe IIn/Ibn.

2020/11/12 (木)


Speaker: Kojiro Kawana

Title: Observational signatures from tidal disruption events of white dwarfs

A star passing close to a black hole (BH) can be disrupted by the tidal force. In the tidal disruption event (TDE), the disrupted star leaves debris bound to the BH causing bright emission, which is observed as a multi-wavelength transient. In TDEs of a white dwarf (WD) by a black hole (BH), the WD is not only tidally disrupted but possibly ignites nuclear burning with the help of tidal compression. The emission from the thermonuclear explosion can be distinguishable signatures of WD TDEs, and detecting the events will help us to study properties of BHs. However, the observational signatures are not well understood, especially on their variety depending on BH/WD mass and orbital parameters. We study a variety of emission emerging from thermonuclear explosion in WD TDEs by performing a suite of hydrodynamic simulations coupled with nuclear reactions and post-process radiative transfer calculations. We thus derive synthetic multi-band light curves and spectra. I will compare the observational signatures of our model with those of observed transients.

2020/11/5 (木)


Speaker: Mihoko Nojiri

Title: Morphology for Jet physics

The Minkowski functional(MF) is commonly used technique in astrophysics. In this paper, I introduce a new application of MF to jet physics. Tagging boosted objects is important to identify new physics at HL-LHC. The techniques of deep learning have been actively applied to the problem. In this paper we introduce new NN(neural network) model using a morphological information with the Minkowski functionals (MFs). The MFs provide geometric information of the jet constituents, and they are independent of the IRC(Infrared collinear) safe observables commonly used in jet physics. We emphasize a potential relationship between the morphology and the convolutional neural network.The new classifier is computationally cheap because geometrical information of jet image is efficiently compressed in a few numbers, and the tagging performance is comparable to the existing jet image classifiers. The result suggests that the our model gives efficient decomposition of the feature space for jet classification.

2020/10/29 (木)


Speaker: Conor Omand

Title: Multiwavelength Signals From Pulsar-Driven Supernovae

The pulsar-driven supernova model, where the initial radiation is powered by the nebula of a rapidly rotating, highly magnetized neutron star, is commonly used to explain superluminous supernovae (SLSNe) and some gamma-ray bursts (GRBs), but has yet to be confirmed. Quasi-thermal optical supernova emission can not differentiate between different engines (ie. pulsar wind nebulae (PWNe) or fallback onto a black hole), but there may be late-phase non-thermal emission unique to a pulsar engine. I'll overview our theoretical predictions for direct detection of PWN emission in the radio band and indirect detection via dust re-radiation in the infrared, as well as our radio observations that have tested and constrained the model.

2020/10/22 (木)


Speaker: Tilman Hartwig

Title: Be careful what you wish for: Reward Modelling in AI

A computer program will do what you say and not what you mean. In the
everyday life of an astronomer, this can for example lead to poor
fitting results, which can be spotted and corrected with human
intuition. However, this problem becomes more serious for artificial
intelligence with explicit, human-designed reward functions. I will
present examples from various scientific domains where an AI naively
exploits the reward function, which leads to undesired behaviour.
Finally, I will present Reward Modelling as a novel solution to this
problem of specification learning.

2020/10/15 (木)

No seminar

2020/10/8 (木)


Speaker: Takuma Suda

Title: A massive star with a black hole companion as a candidate for gravitational wave emission

Binary systems are important probes of the origin of stars in terms of interactions between two stars. We have studied the evolution of supernova binaries after a blast wave hits a low-mass companion to identify the survivors of the first stars in the universe. Theoretical models are developed using hydrodynamic simulations of the system consisting of supernova ejecta and companion stars. In combination with the theoretical research, we have searched for the progenitor binary systems based on radial velocity monitoring. As a byproduct of the search for binaries consisting of a massive star and a low-mass star, we may have discovered a binary system with a black hole companion. If it is the case, it will be an interesting object as a possible source of gravitational wave emission. We will discuss the possibility whether the observed O-type star has a black hole companion.

2020/7/16 (木)


Speaker: Yusuke Yamada

Title: Quantum tunneling and related phenomena

Quantum tunneling is a hallmark of nonclassical physics. Such a phenomenon is relevant to microscopic physics to cosmology. In this talk, I will briefly review examples of tunneling phenomena mostly in cosmology. In tunneling phenomena, the notion of decay rate of a state is one of the most important quantities. I will review how the decay rate is precisely defined and computed in a modern way. For simplicity, I will discuss it in quantum mechanics, but the formulation is straightforwardly applied to quantum field theory as well.

I will discuss several topics, but the topic discussed in detail is based on the following paper : A. Andreassen, D. Farhi, W. Frost, M. D. Schwartz, Phys.Rev.D 95 (2017) 8, 085011

※Notice: This is scheduled to be the last seminar in this semester.

2020/7/9 (木)


Speaker: Kotaro Fujisawa

Title: How to use a quantum computer

Quantum computing is one of the most exciting fields in computer science and physics. Quantum computers must be powerful tools in cosmology and astrophysics in the future. Of course, I and you are not experts on quantum computing, but we can enjoy the quantum computer and quantum computing easily. In this talk, I will introduce basic quantum programmings and how to use IBM quantum computers. I will also talk about recent hot topics of quantum computers, such as quantum machine learning if I can.

Web Reference
IBM Quantum Experience
Quantum Native dojo (Japanese)
TensorFlow Quantum

2020/7/2 (木)


Speaker: Kazuhiro Kanagawa

Title: Theory and observation of planet formation in protoplanetary disks

A planet is born in a protoplanetary disk that is formed around a young star. The planet is formed from the dust grains which is only typically 1% of the mass of gas. The mass and orbital radius of the planet are changed due to interaction with the surrounding gas. After disk gas dissipation, a planetary system like our solar system is formed. At the present, over 4000 exoplanets have been confirmed. Moreover, a planet-forming region in protoplanetary disks has been directly observed by e.g., Atacama Large Millimeter/submillimeter Array (ALMA). In this talk, I will review theory and recent observations of planets formation in protoplanetary disks.

2020/6/25 (木)


Speaker: Kohei Kamada

Title: An Introduction to Chiral Kinetic Theory

It has recently been recognized that the chiral anomaly in the quantum field theory
can cause a macroscopic and non-negligible phenomena such as the chiral magnetic effect.
In this talk, I will introduce the chiral kinetic theory, which implements the effect of chiral anomaly.
In this theory, the kinetic equations or the Boltzmann equations for fermions are modified
with the so-called Berry curvature, and it reconstructs the anomaly equation.
I will also explain that this leads to the chiral magnetic effect, where the electric current
parallel to the magnetic field in the presence of the chiral asymmetry in the system,
at the hydrodynamic limit.
This talk is done with the whiteboard on the ipad.

Note available here

トップ   編集 凍結 差分 バックアップ 添付 複製 名前変更 リロード   新規 一覧 単語検索 最終更新   ヘルプ   最終更新のRSS
Last-modified: 2013-11-20 (水) 11:11:54 (2619d)