• Publications of The Korean Astronomical Society
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• v.7 no.1
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• pp.97-105
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• 1992

The most favourable possibilities to observe the phenomena of gravitational lensing are the high amplification events and the time delay between the images. These effects provide us the information to determine the Hubble parameter and the matter distribution in the universe. The image properties due to micro-lensing also is of an importance to find out the size and the structure of the source.

• Journal of The Korean Astronomical Society
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• v.24 no.1
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• pp.1-12
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• 1991

The mathematical properties of gravitational lens equations are examined in the frame work of gravitational micro-lensing effects. The caustics of the gravitational lens may be defined in terms of "cusp" and "folding" in general. In some cases for overfocussing, however, the critical curves (caustics) have no cusp and no folding. If the observer is in the overfocussed region, he may not see any lensed image.

• Journal of The Korean Astronomical Society
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• v.21 no.2
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• pp.97-104
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• 1988

This study presents the specific rule governing the image configurations of an extended source for micro lensing of the two body gravitational lens system developed by Chang and Refsdal (1979). Various topological situations of a source are considered in relation to the regions bounded by the so-called critical curves.

• Journal of The Korean Astronomical Society
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• v.25 no.2
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• pp.79-90
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• 1992

The physical properties of the flux factor K and its application are discussed in connection to the high amplification events. The effects due to random motions of stars in a foreground galaxy are examined in the frame work of moving caustics of gravitational microlenses.

• Journal of The Korean Astronomical Society
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• v.44 no.4
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• pp.109-113
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• 2011

In current microlensing planet searches that are being carried out in a survey/follow-up mode, the most important targets for follow-up observations are lensing events with high magnifications resulting from the very close approach of background source stars to the lens. In this paper, we investigate the dependence of the sensitivity to planets on detailed properties of high-magnification events. From this, it is found that the sensitivity does not monotonically increase as the impact parameter between the lens and the source trajectory decreases. Instead, it is roughly the same for events with impact parameters less than a certain threshold value. It is also found that events involving main-sequence source stars are sensitive to planets in a much wider range of separation and mass ratio, than those events involved with giant source stars. Based on these results, we propose observational strategies for maximal planet detections considering the types of telescopes available for follow-up observations.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.235-236
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• 2015

Microlensing can be seen as a version of strong gravitation lensing where the separation angle of the image formed by light deflection by a massive object is too small to be seen by a ground based optical telescope. As a result, what can be observed is the change in light intensity as function of time; the light curve. Conventionally, the intensity of the source is expressed in magnitudes, which uses a logarithmic function of the apparent flux, known as the Pogson formulae. In this work, we compare the magnitudes from the Pogson formulae with magnitudes from the Asinh formulae (Lupton et al. 1999). We found for small fluxes, Asinh magnitudes give smaller deviations, about 0.01 magnitudes smalller than Pogson magnitudes. This result is expected to give significant improvement in detection level of microlensing light curves.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.221.1-221.1
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• 2012

The Korea Astronomy and Space Science Institute (KASI) is developing three 1.6m optical telescopes with $18k{\times}18k$ mosaic CCD cameras. These telescopes will be installed and operated at Chile, South Africa, and Australia for Korea Micro-lensing Telescope Network (KMTNet) project. The main scientific goal of the project is to discover earth-like extra-solar planets using the gravitational micro-lensing technique. To achieve the goal, each telescope at three sites will continuously monitor the specific region of Galactic bulge with 2.5 minute cadence for five years. Assuming 12 hour observation in maximum for a night, the amount of 200 GB file storage is required for one night observation at one observatory. If we consider the whole project period and the data processing procedure, a few PB class data storage, high-speed network, and high performance computers are essential. In this presentation, we introduce the KMTNet data management plan that handles gigantic data; raw image collecting, image processing, photometry pipeline, database archiving, and backup.

• Journal of The Korean Astronomical Society
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• v.51 no.6
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• pp.197-206
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• 2018

We present the analysis of KMT-2016-BLG-0212, a low flux-variation ($I_{flux-var}{\sim}20mag$) microlensing event, which is in a high-cadence (${\Gamma}=4hr^{-1}$) field of the three-telescope Korea Microlensing Telescope Network (KMTNet) survey. The event shows a short anomaly that is incompletely covered due to the brief visibility intervals that characterize the early microlensing season when the anomaly occurred. We show that the data are consistent with two classes of solutions, characterized respectively by low-mass brown-dwarf (q = 0.037) and sub-Neptune (q < $10^{-4}$) companions. Future high-resolution imaging should easily distinguish between these solutions.

• Journal of The Korean Astronomical Society
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• v.53 no.6
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• pp.161-168
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• 2020

We report the discovery of a giant exoplanet in the microlensing event OGLE-2017-BLG-1049, with a planet-host star mass ratio of q = 9.53 ± 0.39 × 10-3 and a caustic crossing feature in Korea Microlensing Telescope Network (KMTNet) observations. The caustic crossing feature yields an angular Einstein radius of θE = 0.52 ± 0.11 mas. However, the microlens parallax is not measured because the time scale of the event, tE ≃ 29 days, is too short. Thus, we perform a Bayesian analysis to estimate physical quantities of the lens system. We find that the lens system has a star with mass Mh = 0.55+0.36-0.29 M⊙ hosting a giant planet with Mp = 5.53+3.62-2.87 MJup, at a distance of DL = 5.67+1.11-1.52 kpc. The projected star-planet separation is a⊥ = 3.92+1.10-1.32 au. This means that the planet is located beyond the snow line of the host. The relative lens-source proper motion is μrel ~ 7 mas yr-1, thus the lens and source will be separated from each other within 10 years. After this, it will be possible to measure the flux of the host star with 30 meter class telescopes and to determine its mass.

• Journal of The Korean Astronomical Society
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• v.53 no.1
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• pp.9-26
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• 2020

At q = 1.81 ± 0.20 × 10^-5, KMT-2018-BLG-0029Lb has the lowest planet-host mass ratio q of any microlensing planet to date by more than a factor of two. Hence, it is the first planet that probes below the apparent "pile-up" at q = 5-10 ×10^-5. The event was observed by Spitzer, yielding a microlens-parallax π_E measurement. Combined with a measurement of the Einstein radius θ_E from finite-source effects during the caustic crossings, these measurements imply masses of the host M_host = 1.14^+0.10_-0.12 M_⊙ and planet M_planet = 7.59^+0.75_-0.69 M_⊕, system distance D_L = 3.38^+0.22_-0.26 kpc and projected separation a_⊥ = 4.27^+0.21_-0.23 AU. The blended light, which is substantially brighter than the microlensed source, is plausibly due to the lens and could be observed at high resolution immediately.