Gravitational waves (GWs) emitted from the hyperbolic encounter of two black holes provide valuable insights into highly dynamical spacetime interactions. We investigate the characteristics of these GWs and address the challenges associated with nonlinear drift arising from their integration. By employing the subtract-by-fitting method, we successfully calculated the $\dot{h}$ and calculated the radiated energy. The results demonstrated improved agreement with the energy computed using the mass quadrupole compared to previous integration method. Additionally, we analyzed the temporal behavior of trajectory-driven (TD) and ringdown (RD) waveforms at different extraction radii, identifying variations through the ratio of their time delays. To further interpret the dynamics, Psi4 was visualized in the xy-plane, offering a clear depiction of the waveforms. These findings provide a deeper understanding of the complex nature of GWs in strong-field regimes.

The Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) detectors have successfully observed gravitational wave (GW) signals since the first detection in 2015. Despite this success, the capability of the detectors to observe GWs from various astronomical sources is limited by coating thermal noise (CTN), especially in the most sensitive frequency range in 100 Hz ~ 300 Hz. To enhance sensitivity, it is essential to replace the high refractive index material in the high reflective mirror coating of the test mass with the lower mechanical loss property material. In order to investigate efficiently, characterization for candidate coating materials is done using Transmission Electron Microscope (TEM) experiment. This talk will present the methods and preliminary results of characterization on the atomic structure of coating material and interfaces of actual multi-layer coating.

For more information, the following papers are recommended: https://doi.org/10.1103/PhysRevLett.123.045501, https://doi.org/10.3390/coatings6040061.

The Penrose process (PP) presents a theoretical mechanism for extracting energy from rotating black holes (Kerr black holes). However, its astrophysical applicability is limited due to the condition of a relative velocity between decayed fragments. In contrast, the magnetic Penrose process (MPP) overcomes this limitation and achieves maximum efficiency when a neutral particle decays near a rotating black hole. We apply this model to Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Galaxy, which likely hosts an advection-dominated accretion flow (ADAF). ADAF is hot enough for neutron production via thermonuclear reactions and supplies neutrons to polar regions of Sgr A*. Using the Wald solution for the electromagnetic field, we calculate the production rate of accelerated protons by MPP and estimate the gamma-ray flux from proton-proton interactions with the hydrogen gas at the Galactic center. Our model suggests that this gamma-ray emission may account for a meaningful fraction of the observed gamma-ray flux for energies above 10 TeV. Additionally, we estimate that at PeV energies, the accelerated protons transported to Earth contribute roughly 1-4% of the observed cosmic ray flux, assuming stationary diffusion and magnetic field strength of 100 G near Sgr A*.

Gravitational waves emitted by binary neutron stars (BNS) provide information about the internal structure of neutron stars (NSs), helping to verify dense matter equations of state. We investigate how the measurement accuracy of NS's tidal deformability can be improved by incorporating the higher-order post-Newtonian (pN) tidal corrections. We assume an aligned-spin BNS system and adopt TaylorF2 which is one of the pN waveform models. To compute the measurement errors, we use the Fisher Matrix method which is much faster than performing Monte Carlo simulations. We find that the higher-order corrections beyond 6 post-Newtonian order in adiabatic tides do not contribute largely, while the dynamical tides, describing Love number varying with binary's frequency, significantly reduce the measurement errors.

The cold dark matter (CDM) model used in standard cosmology fails to explain some structures on galactic scales (~kpc). Recently, as an alternative to CDM, there is growing interest in the ultra-light dark matter (ULDM) model, which consists of a pseudoscalar particle motivated by ultra-light axions. This model exhibits different dynamical behavior from CDM, allowing it to explain various phenomena that remain unresolved in our universe. In this presentation, I'm going to discuss the dynamical properties of ULDM, and how it can address specific galactic-scale issues in CDM by offering alternative explanations. Additionally, I also 'very lightly' introduce some research topics of ULDM from a general relativistic perspective.

Radio interferometry, particularly with very long baselines, enables observations of the universe at the highest angular resolutions. This talk will review key applications of radio interferometry in studies of compact astrophysical objects, especially supermassive black holes. I will begin with an overview of the basic principles of radio interferometry and high-resolution imaging. We will then examine the Event Horizon Telescope (EHT) imaging of black hole shadows in M87* and, particularly, Sagittarius A*, which allows for precise tests of general relativity and constraints on alternative compact objects. I will also briefly review the basics of black hole accretion and outflow physics, followed by broader applications of radio interferometry to related research areas, including high-resolution imaging of nearby AGNs with the EHT, the search for compact binary supermassive black holes with VLBI, and the role of radio monitoring and follow-ups in studying neutrino-emitting jetted AGNs.

Nuclear astrophysics (NA) has been a longstanding inter-disciplinary research subject bridging nuclear physics and astronomy. For example, NA provides fundamental knowledge for the stellar evolution. In particular, understanding the lifetime of the massive star can hint the formation of the compact binary system which eventually emits gravitational wave (GW) when they merge. This lecture introduces some basics of NA and presents some exemplary topics which show how NA is related to GW and numerical relativity.

Supermassive black holes (SMBHs) are believed to reside at the centers of most galaxies. The inflow of matter toward SMBHs often results in strong radiation and relativistic jet outflows, which can play crucial roles in the evolution of galaxies and galaxy clusters. The region where this energy is generated is too small to observe directly, and the detailed physical processes of mass accretion and jet outflows have long remained elusive. The recent advent of global Very Long Baseline Interferometry, including the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA) in conjunction with the phased ALMA, has enabled unprecedented ultra-high resolution, allowing us to directly image the horizon-scale view of SMBHs. In this talk, I present recent breakthroughs in supermassive black hole research facilitated by the EHT and GMVA+ALMA, with a primary focus on studies of M87*. I will also discuss how numerical relativity provides a more robust test of black hole accretion and jet formation models based on the observed black hole images of M87.