Changsu Choi
A5075641217- Gamma-ray bursts and supernovae
- Astronomy and Astrophysical Research
- Astronomical Observations and Instrumentation
- Stellar, planetary, and galactic studies
- Galaxies: Formation, Evolution, Phenomena
- Astrophysics and Cosmic Phenomena
- Astrophysical Phenomena and Observations
- Adaptive optics and wavefront sensing
- Pulsars and Gravitational Waves Research
- Geophysics and Gravity Measurements
- Optical Systems and Laser Technology
- CCD and CMOS Imaging Sensors
- Radio Astronomy Observations and Technology
- Infrared Target Detection Methodologies
- Astrophysics and Star Formation Studies
- SAS software applications and methods
- Inertial Sensor and Navigation
- Advanced Optical Sensing Technologies
- Image Processing and 3D Reconstruction
- Particle Detector Development and Performance
- Advanced Data Processing Techniques
- Seismology and Earthquake Studies
- Neutrino Physics Research
- Medical Imaging Techniques and Applications
- Optical measurement and interference techniques
Seoul National University
2014-2024
Korea Astronomy and Space Science Institute
2022-2024
LSIS (South Korea)
2010
After the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered a significant brightening of inner region NGC 2617, we began ~70 day photometric and spectroscopic monitoring campaign from X-ray through near-infrared (NIR) wavelengths. We report that 2617 went dramatic outburst, during which its flux increased by over an order magnitude followed increase optical/ultraviolet (UV) continuum almost magnitude. classified as Seyfert 1.8 galaxy in 2003, is now 1 due to appearance broad...
The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. survey region is stripe 2.5° wide centered the celestial equator in Southern Galactic Cap that been imaged numerous times earlier years, enabling construction deep reference image discovery new objects. Supernova observations are being...
The double explosion of SN 2009ip in 2012 raises questions about our understanding the late stages massive star evolution. Here we present a comprehensive study during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from GeV to radio band obtained variety ground-based space facilities (including Very Large Array, Swift, Fermi, Hubble Space Telescope, XMM) constrain be low energy (E ∼ 1050 erg for an ejecta mass ∼0.5 M☉) asymmetric complex medium shaped...
The Sloan Digital Sky Survey-II Supernova Survey has identified a large number of new transient sources in 300 deg2 region along the celestial equator during its first two seasons three-season campaign. Multi-band (ugriz) light curves were measured for most sources, which include solar system objects, galactic variable stars, active nuclei, supernovae (SNe), and other astronomical transients. imaging survey is augmented by an extensive spectroscopic follow-up program to identify SNe, measure...
We present a measurement of the rate type Ia supernovae (SNe Ia) from first three seasons data SDSS-II Supernova Survey. For this measurement, we include 17 SNe at redshift $z\le0.12$. Assuming flat cosmology with $\Omega_m = 0.3=1-\Omega_\Lambda$, find volumetric SN $[2.93^{+0.17}_{-0.04}({\rm systematic})^{+0.90}_{-0.71}({\rm statistical})] \times 10^{-5} {\rm SNe} Mpc}^{-3} h_{70}^3 year}^{-1}$, volume-weighted mean 0.09. This result is consistent previous measurements in similar range....
Recently, the optical counterpart of a gravitational wave source GW170817 has been identified in NGC 4993 galaxy. Together with evidence from observations electromagnetic waves, event suggested as result merger two neutron stars. We analyze multi-wavelength data to characterize host galaxy property and its distance examine if properties are consistent this picture. Our analysis shows that is bulge-dominated reff ~ 2-3 kpc Sersic index n = 3-4 for bulge component. The spectral energy...
Abstract One of the keys to success multimessenger astronomy is rapid identification electromagnetic wave counterpart, kilonova (KN), gravitational-wave (GW) event. Despite its importance, it hard find a KN associated with GW event, due poorly constrained localization map and numerous signals that could be confused as KN. Here, we present Gravitational-wave Electromagnetic Counterpart Korean Observatory (GECKO) project, GECKO observation GW190425, prospects in fourth observing run (O4)...
Context . Gamma-ray bursts (GRBs) are ideal probes of the Universe at high redshift (ɀ), pinpointing locations earliest star-forming galaxies and providing bright backlights with simple featureless power-law spectra that can be used to spectrally fingerprint intergalactic medium host galaxy during period reionization. Future missions such as Gamow Explorer (hereafter Gamow) being proposed unlock this potential by increasing rate identification high-ɀ (ɀ > 5) GRBs in order rapidly trigger...
Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of universe. Confirmation this scenario requires spectroscopic identification quasars (M1450 > −24 mag) at z ≳ 6, but only a very small number such have been spectroscopically identified so far. Here, we report discovery quasar IMS J220417.92+011144.8 ∼ 6 in 12.5 deg2 region SA22 field Infrared Medium-deep Survey (IMS). The spectrum shows sharp break ∼8443 Å, with emission lines redshifted to =...
This paper presents spectroscopy of supernovae discovered in the first season Sloan Digital Sky Survey-II Supernova Survey. program searches for and measures multi-band light curves redshift range z = 0.05 - 0.4, complementing existing surveys at lower higher redshifts. Our goal is to better characterize supernova population, with a particular focus on SNe Ia, improving their utility as cosmological distance indicators probes dark energy. features rapid-response observations using telescopes...
Faint $z\sim5$ quasars with $M_{1450}\sim-23$ mag are known to be the potentially important contributors ultraviolet ionizing background in post-reionization era. However, their number density has not been well determined, making it difficult assess role early ionization of intergalactic medium (IGM). In this work, we present updated results our quasar survey using Infrared Medium-deep Survey (IMS), a near-infrared imaging covering an area 85 deg$^{2}$. From spectroscopic observations Gemini...
We present the characteristics of Seoul National University 4k Camera (SNUCAM) and report its performance on 1.5m telescope at Maidanak observatory in Uzbekistan. SNUCAM is a CCD camera with pixel scale 0.266" <TEX>$4096{\times}4096$</TEX> format, covering <TEX>$18.1'{\times}18.1'$</TEX> field view 1.5m. The currently equipped Bessell UBVRI, <TEX>$H{\alpha}$</TEX>, SDSS ugriz, Y-band filters, allowing us to carry out variety scientific programs ranging from exoplanet studies survey quasars...
The main progenitor candidates of Type Ia supernovae (SNe Ia) are white dwarfs in binary systems where the companion star is another dwarf (double degenerate (DD) system) or a less-evolved, non-degenerate with R* ≳ 0.1 R⊙ (single system). However, no direct observational evidence exists to tell us which system more common. Recent studies suggest that light curve supernova shortly after its explosion can be used set limit on size, R*. Here, we report high-cadence monitoring observations SN...
Abstract The progenitor system of Type Ia supernovae (SNe Ia) is expected to be a close binary consisting carbon/oxygen white dwarf (WD) and nondegenerate star or another WD. Here, we present results from high-cadence monitoring observations SN 2021hpr in spiral galaxy, NGC 3147, constraints on the based its early multicolor light-curve data. First, classify as normal long-term photometric spectroscopic More interestingly, found significant “early excess” light curve over simple power-law ∼...
We describe the overall characteristics and performance of an optical CCD camera system, Camera for QUasars in EArly uNiverse (CQUEAN), which is being used at 2.1 m Otto Struve Telescope McDonald Observatory since 2010 August. CQUEAN was developed follow-up imaging observations red sources such as high redshift quasar candidates (z >= 5), Gamma Ray Bursts, brown dwarfs, young stellar objects. For efficient objects, has a science with deep depletion chip boasts higher quantum efficiency 0.7 -...
Abstract The faint quasars with M 1450 > −24 mag are known to hold the key determination of ultraviolet emissivity for cosmic reionization. But only a few have been identified so far because limitations on survey data. Here we present first results z ∼ 5 quasar Infrared Medium-deep Survey (IMS), which covers ∼100 deg 2 areas in J band depths AB 23 mag. To improve selection methods, medium-band follow-up imaging has carried out using SED camera QUasars Early uNiverse (SQUEAN) Otto Struve...
ABSTRACT The ΛCDM cosmological model successfully reproduces many aspects of the galaxy and structure formation universe. However, growth large-scale structures (LSSs) in early universe is not well tested yet with observational data. Here, we have utilized wide deep optical–near-infrared data order to search for distant clusters superclusters (0.8 < z 1.2). From spectroscopic observation Inamori Magellan Areal Camera Spectrograph (IMACS) on telescope, three massive at ∼ 0.91 are confirmed...
Abstract The intergalactic medium (IGM) at z ∼ 5 to 6 is largely ionized, and yet the main source for IGM ionization in early universe uncertain. Of possible contributors are faint quasars with <?CDATA $-26\lesssim {M}_{1450}\lesssim -23$?> , but their number density poorly constrained 5. In this paper, we present our survey of European Large-Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field over a area 6.51 deg 2 examine if such can be dominant ionization. We use deep...
We report the discovery of 40 bright quasars and active galactic nuclei (AGNs) at low Galactic latitude (b<20deg). The region has been considered a place to avoid when searching for extragalactic sources, because high extinction, as well large number stars contaminating sample selection. Bright (R
To date, most of the luminous quasars known at $z\sim6$ have been found to be in maximal accretion with Eddington ratios, $\lambda_{\rm{Edd}}\sim1$, suggesting enhanced nuclear activities early universe. However, this may not whole picture supermassive black hole (SMBH) growth since previous studies reached on faint that are more likely harbor SMBHs low $\lambda_{\rm{Edd}}$. gain a better understanding universe, we obtained deep near-infrared (NIR) spectrum quasar, IMS J220417.92+011144.8...
자연어 처리에서 벤치마크 데이터셋은 거대 언어 모델의 평가에 중요한 역할을 한다. 그러나 비용과 자원의 제한으로 인해 대부분의 처리 벤치마크는 주로 영어와 중국어로 개발되어 왔다. 이 연구에서는 한국어 능력 평가를 위해 다양한 도메인을 포괄하는 데이터셋을 구축하고, 이를 바탕으로 6개의 및 다국어 모델(LLM)의 성능을 평가하였다. 여섯 가지 평가 도메인과 일곱 지표를 사용하여 종합적인 능력을 분석하였다. 과정은 정량적 평가와 더불어 명의 평가자가 수행한 인간 포함하며, GPT-4를 활용한 자동 방법을 적용하여 평가와의 상관관계를 검토하였다. 연구 결과는 제한된 자원으로도 방식을 통해 LLM의 종합적으로 진단할 수 있음을 보여주었고, 평가의 자동화 가능성을 검증하였다. 연구에서 제안한 벤치마크가 사례로 활용되기를 기대한다.
We present our first results of the survey for high redshift quasars at $5 \lesssim {\rm z} 5.7$. The search in this range has been known to be challenging due limitations filter sets used previous studies. conducted a quasar two specific ranges, 4.60 $\leq$ z 5.40 and 5.50 6.05, using multi-wavelength data that include observations custom-designed filters, $is$ $iz$. Using these filters new selection technique, we were able reduce fraction interlopers. Through optical spectroscopy,...