• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.29.1-29.1
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• 2016

I introduce the field of gravitational microlensing that I have worked on for more than 2 decades. I describe how microlensing can be applied to various fields in astrophysics including dark matter, Galactic structure, binary objects, and extrasolar planets and present my scientific achievements in the individual fields. I start with a description of basic microlensing physics and state how microlensing can be applied to various fields. Finally, I briefly describe ongoing efforts and future projects in microlensing.

• Journal of The Korean Astronomical Society
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• v.34 no.2
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• pp.81-97
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• 2001

Since the first proposal by Paczynski, great efforts to detect Galactic dark matter by detecting light variations of stars located in the Magellanic Clouds and Galactic bulge caused by gravitational microlensing have been and are being carried out and more than 1,000 events have been successfully detected. In this paper, we review the progress in the theoretical and experimental progresses in microlensing. We begin with basics of microlensing and summarize the results obtained from the last 8 year observations along with the implications of the results. We also discuss the usefulness of microlensing in other fields of astronomy such as the stellar atmosphere, Galactic binary systems, and extra-solar planets. We finally discuss the problems of the current experiments and the new types of observations that can overcome these problems.

• Journal of The Korean Astronomical Society
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• v.31 no.1
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• pp.27-37
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• 1998

We have made semi-analytical studies to investigate the configurations of caustics and the probability distribution of the flux factor K for the binary microlensing including external shears. A parametric equation of critical curve is derived in a 4th order complex polynomial. We present the topological dependencies of the caustics for selected gamma parameters (0, 0.3, 0.6, 1.3, 2.0, and 2.5) and convergence terms (0., 0.4, 0.8, 1.2, 1.6, and 2.0). For the purpose of analyzing the efficiency of High Amplification Event (HAE) on each caustics, we examine the probability distribution of the flux factor by a Monte Carlo method. Changing the separation of the binary system from 0.8 to 1.3 (in normalied unit), we examine the probability distribution of the K-values in various gamma parameters. The relationship between gamma parameters, seperations and their probabilties of the flux factor K have been studied. Our results show that the relatively higher K values (K>1.5) are increased as increasing the separation of the binary system. We therfore conclude that, in the N-body microlensing, the probabilities of higher HAEs are inversely proportional to the star density as well. We also point out that the present research might be used as a preliminary step toward investigating heavy N-body microlensing simulations.

• Journal of Astronomy and Space Sciences
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• v.29 no.2
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• pp.191-194
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• 2012

The observed distribution of a blending-corrected sample of Einstein ring crossing times, $t_E$, for microlensing events toward the galactic bulge/bar are analyzed. An inspection of the distribution of crossing times suggests that it may be bimodal, indicating that two populations of lenses could be responsible for observed microlensing events. Given the possibility that microlensing in this direction can be due to the two most common classes of stars, main-sequence and white dwarf, we analyze and show via Monte Carlo simulations that the observed bimodality of $t_E$ can be derived from their accepted mass functions, and the density distributions of both stellar populations in the galactic disk and bulge/bar, with a transverse velocity distribution that is consistent with the density distribution. Kolmogorov-Smirnov (KS) one sample tests shows that a white dwarf population of about 25% of all stars in the galaxy agrees well with the observed bimodality with a KS significance level greater than 97%. This is an expanded and updated version of a previous investigation (Wickramasinghe, Neusima, & Struble, in Mao 2008). A power-point version of the talk, with introductory figures, is found at: https://sites.google.com/site/rhkochconference/agenda-1/program.

• Journal of The Korean Astronomical Society
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• v.47 no.4
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• pp.153-158
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• 2014

I show that the standard microlensing technique to measure the angular radius of a star using color/surface-brightness relations can be inverted, via late-time proper motion measurements, to calibrate these relations. The method is especially useful for very metal-rich stars because such stars are in short supply in the solar neighborhood where other methods are most effective, but very abundant in Galactic bulge microlensing fields. I provide a list of eight spectroscopically identified high-metallicity bulge stars with the requisite finite-source effects, seven of which will be suitable calibrators when the Giant Magellan Telescope comes on line. Many more such sources can be extracted from current and future microlensing surveys.

• Journal of The Korean Astronomical Society
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• v.49 no.4
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• pp.123-126
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• 2016

Microlensing is generally thought to probe planetary systems only out to a few Einstein radii. Microlensing events generated by bound planets beyond about 10 Einstein radii generally do not yield any trace of their hosts, and so would be classified as free floating planets (FFPs). I show that it is already possible, using adaptive optics (AO), to constrain the presence of potential hosts to FFP candidates at separations comparable to the Oort Cloud. With next-generation telescopes, planets at Kuiper-Belt separations can be probed. Next generation telescopes will also permit routine vetting for all FFP candidates, simply by obtaining second epochs 4-8 years after the event. At present, the search for such hosts is restricted to within the "confusion limit" of θ_confus ∼ 0.25′′, but future WFIRST (Wide Field Infrared Survey Telescope) observations will allow one to probe beyond this confusion limit as well.

• Journal of The Korean Astronomical Society
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• v.53 no.5
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• pp.99-102
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• 2020

I show that when the observables (π_E, t_E, θ_E, π_s, µ_s) are well measured up to a discrete degeneracy in the microlensing parallax vector π_E, the relative likelihood of the different solutions can be written in closed form P_i = KH_iB_i, where H_i is the number of stars (potential lenses) having the mass and kinematics of the inferred parameters of solution i and B_i is an additional factor that is formally derived from the Jacobian of the transformation from Galactic to microlensing parameters. Here t_E is the Einstein timescale, θ_E is the angular Einstein radius, and (π_s, µ_s) are the (parallax, proper motion) of the microlensed source. The Jacobian term B_i constitutes an explicit evaluation of the "Rich Argument", i.e., that there is an extra geometric factor disfavoring large-parallax solutions in addition to the reduced frequency of lenses given by H_i. I also discuss how this analytic expression degrades in the presence of finite errors in the measured observables.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.40.4-41
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• 2018

We introduce a coordinate catalog for photometry of the KMTNet Galactic bulge observation program and how to find the microlensing event candidates in the photometry result. Basically, the KMTNet bulge program is monitoring a total of 27 target fields (108 deg2) with four different cadences of 0.5, 1.0, 2.5, and 5.0 hours. In order to measure the stellar flux of each target, we made a 'observation coordinate catalog' by using the 'OGLE-III catalog' and 'DoPhot package'. The catalog contains approximately 0.3 billion stars in the bulge fields. We also search for a microlensing event candidates by means of the 'Event Finder algorithm' which calculates the restricted single-lens fitting (t0, teff, u0; u0 = 0 or 1) model. As a result, we found more than 2,000 microlensing event candidates per each year including about 700 events from the other survey groups such as OGLE and MOA. In this year, we will improve our current pipeline system, e.g. upgrading the catalog and applying real-time photometry.

• Journal of The Korean Astronomical Society
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• v.47 no.6
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• pp.279-291
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• 2014

I show that the WFIRST microlensing survey will enable detection and precision orbit determination of Kuiper Belt Objects (KBOs) down to $H_{vega}=28.2$ over an effective area of ${\sim}17deg^2$. Typical fractional period errors will be ${\sim}1.5%{\times}10^{0.4(H-28.2)}$ with similar errors in other parameters for roughly 5000 KBOs. Binary companions to detected KBOs can be detected to even fainter limits, $H_{vega}=29$, corresponding to R~30.5 and effective diameters D~7 km. For KBOs H~23, binary companions can be found with separations down to 10 mas. This will provide an unprecedented probe of orbital resonance and KBO mass measurements. More than a thousand stellar occultations by KBOs can be combined to determine the mean size as a function of KBO magnitude down to H~25. Current ground-based microlensing surveys can make a significant start on finding and characterizing KBOs using existing and soon-to-be-acquired data.

• Publications of The Korean Astronomical Society
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• v.26 no.1
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• pp.37-44
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• 2011

We constructed a photometric database system which is optimally designed for microlensing events from KMTNet (Korea Microlensing Telescope Network) observation. We developed a framework software for the convenience of archiving, uploading, searching, and downloading of processed photometric data. From various tests for optimal data archiving engines, we found that the MyISAM storage engine shows the best performance. For the high performance of database system, data types of each field are carefully suggested from various combinations of tests especially to correct round-off errors. The developed framework provides the convenience of access to the database server using query forms via web pages, and displays the light curve of selected target for a quick view.