• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.80.1-80.1
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• 2014

We made simultaneous surveys of 22 GHz water and 44 / 95 GHz methanol masers toward 299 high-mass protostellar objects using the Korea VLBI Network (KVN) 21-m telescope. The sources were selected from the catalog of Red MSX Source (RMS) survey. Initial selection of the sample present high-mass protostellar objects in an evolutionary phase prior to ultra-compact HII regions, which have bolometric luminosities > $10^3L_{\odot}$ but are not associated with any radio continuum emission. After the follow-up work of ongoing RMS survey, final samples contains 56 sources classified as HII regions. We performed a simultaneous survey of 22 GHz water and 44 GHz methanol masers in 2011 and then conducted a simultaneous survey of 22 GHz water and 44 / 95 GHz methanol masers in 2012. The primary scientific goals of these surveys are to investigate the relationship among the three masers and to explore the relationship between each maser and the central star or the parental dense core. The detection rates of two epochs are 42% and 38% for water, 25% and 26% for 44 GHz methanol, and 23% (2012 only) for 95 GHz methanol masers. We performed a statistical analysis on subsample associated with a large data found in literature. In this poster, we will the preliminary data analysis results and discuss the implications.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.57.2-57.2
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• 2017

KVN is a millimeter VLBI array composed of three 21m-diameter radio telescopes at Seoul, Ulsan and Jeju island in Korea. KVN has unique simultaneous multi-frequency receiving systems, which enable us to correct phase fluctuation of troposphere by transferring phase solution of low frequency data to higher frequency data. Although KVN can achieve very high performance up to 130 GHz through multi-frequency technique, imaging capability is highly limited because of lack of the number of baselines. In order to enhance imaging capability and maximizing multi-frequency capability, we plan to extend KVN baselines from 3 to 10 (or more) by constructing new KVN stations. This talk introduce expected performances, science cases, required budgets and periods of the Extended-KVN.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.43.1-43.1
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• 2013

We present here the preliminary results of the fast variability of AGN radio light curves. The shortest time scale of minute in AGN light curves is needed to probe the AGN activity for a few reasons; First, to check if there is any kind of shortest time scale activity. Secondly, to find out what high frequency end of AGN spectra look like. For the last, to see the time delay at several wavelength bands and the change of the spectral index with time. The observation was conducted with three KVN(Korea VLBI Network) antennas with single dish cross scan mode. In order not to lose the target at any given time, whenever one station needs to observe the calibrator, the other station is on the target. Though the detailed data reduction is still going on, there might exist varying feature in the radio light curve. The more fine calibration will be done in near future and another good data set is ready for the reduction.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• v.1
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• pp.529-534
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• 2006

New geocentric geodetic datum has recently been realized in Korea, Korean Geodetic Datum 2002- KGD2002, to overcome problems due to the existing Tokyo datum, which had been used in this country since early 20th century. This transition will support modern surveying techniques, such as Global Navigation Satellite Systems (GNSS) and ensures that spatial data is compatible with other international systems. For this realization, very long baseline interferometry (VLBI) observations were initially carried out in 1995 to determine the coordinates of the origin of KGD2002 based on the International Terrestrial Reference Frame (ITRF). Continuous GPS observations were collected from 14 reference stations across Korea to compute the coordinates of 1st order horizontal geodetic control points. During the campaign, GPS observations were also collected at about 9,000 existing geodetic control points. In 2006, network adjustment with all data obtained using GPS and EDM since 1975 has been performed under the condition of fixing the coordinates of GPS continuous observation stations to compute coordinate measurements of the 2nd and 3rd geodetic control points. This paper describes the GPS campaigns which have been undertaken since 1996 and details of the network adjustment schemes. This is followed

• Journal of Astronomy and Space Sciences
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• v.24 no.3
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• pp.249-260
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• 2007

The crustal deformation due to Ocean Tide Loading (OTL) in the Korean peninsula reaches up to ${\sim}3cm$ in the vertical direction. Considering that the achievable positioning accuracy of current state-of-the-art space geodesy technologies is at the several millimeter level, the centimeter-level OTL effect should be precisely modelled and corrected for. This study begins with comparison of ocean tide models and validation of OTL-prediction softwares. Different ocean tide models caused about ${\sim}6mm$ RMS differences in the vertical deformation in the Kyung-gi Bay area. When we analyzed the OTL displacements in the Seoul, Ulsan, and Seogwipo areas where three VLBI observatories are planned to be installed, the maximum displacement of ${\sim}3.5cm$ was predicted in the Seogwipo area and ${\sim}2cm$ in the Seoul and Ulsan areas. When the OTL corrections were not applied in the GPS data processing, the OTL effect propagates into the Zenith Wet Delay (ZWD) estimates, and the scale factor between ZWD differences and OTL displacements was 3.72.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.61.1-61.1
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• 2014

IC 10 is one of the most well-known irregular starburst dwarf galaxies in the Local Group. Its low metal and oxygen abundance together with proximity make it an excellent laboratory to test star formation models, especially in low-metallicity systems like galaxies in the early Universe as well as many other local dwarfs. Among a number of active star forming regions, we have detected H2O kilo-maser emission in the south-east region of IC 10(IC 10 SE) using the Korean VLBI Network(KVN). This maser line is likely to be associated with a giant molecular cloud identified in IC 10 SE by former CO studies. Using the HI and CO data from the VLA and SMA archive, we probe the atomic and molecular gas properties of IC 10 SE. We discuss how the cool gas morphology and kinematics are related with maser and star formation activity in IC 10 SE.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.60.1-60.1
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• 2014

In the center of an Active Galactic Nuclei(AGN) is a supermassive black hole which accretes matter from its surroundings. The radio-loud AGN launch two relativistic jets perpendicular to the accretion disk which terminates into radio lobes located up to megaparsec away. Blazars form a small subset of radio-loud AGNs with one of two relativistic jets pointing toward the observer's line of sight. Many blazars often show flares at different frequencies. And these flares at different frequencies are known that they often correlate with each other. In 2013 December, there was a gamma-ray flare in 3C 279, one of the brightest blazars, Dec 2013. So we want to reveal that whether this flare correlates with radio flare or not, and where the flare originate. With polarization observation at radio frequencies, we can study the physical properties of the magnetic field in the innermost regions of the relativistic jets. Therefore, we have conducted polarization monitoring of this source from Dec. 2013 to Jun. 2014 with KVN(Korea VLBI Network) radio telescopes at 22, 43 and 86GHz. Here we present the initial results of the monitoring of 3C 279. We prospect that we can reveal the origin of this gamma-ray flare by comparing with our radio data.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.218.1-218.1
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• 2012

2010년 7월부터 한국천문연구원과 일본국립천문대가 공동으로 운영하고 있는 한일상관센터(KJCC)에는 최대 16관측국, 최고 속도 8Gbps, 8192출력채널의 성능을 갖는 한일공동VLBI 상관기(KJJVC)가 설치되어 운용되고 있다. 상관결과는 각 출력 채널별로 관측데이터의 비지빌리티 정보만을 보유하고 있기 때문에, AIPS 등의 천문분석 프로그램에서 활용하기 위해서는 변환작업을 수행해야 한다. KJJVC는 일본국립천문대의 FX 상관기에서 활용하고 있는 CODA(Correlated Output Data Analysis) 파일 시스템을 도입하여, KJJVC의 상관결과 후처리에 적합하도록 수정하였다. 이 CODA 파일 시스템은 관측과 상관처리시 필요한 여러 가지 파라미터 정보를 정렬하고, 상관결과인 비지빌리티 정보를 각 채널별로 정렬하여 파일 시스템을 구축한 것이다. 본 발표에서는 KJJVC에서 개발한 CODA 파일 시스템과 AIPS 등에서 분석에 활용할 수 있는 FITS 형식으로 변환하는 소프트웨어의 개발과 성능에 대해 간략히 소개한다.

• Journal of The Korean Astronomical Society
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• v.56 no.2
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• pp.231-252
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• 2023

We present the analysis results of the simultaneous multifrequency observations of the blazar 4C +28.07. The observations were conducted by the Interferometric Monitoring of Gamma-ray Bright Active Galactic Nuclei (iMOGABA) program, which is a key science program of the Korean Very Long Baseline Interferometry (VLBI) Network (KVN). Observations of the iMOGABA program for 4C +28.07 were conducted from 16 January 2013 (MJD 56308) to 13 March 2020 (MJD 58921). We also used γ-ray data from the Fermi Large Array Telescope (Fermi-LAT) Light Curve Repository, covering the energy range from 100 MeV to 100 GeV. We divided the iMOGABA data and the Fermi-LAT data into five periods from 0 to 4, according to the prosody of the 22 GHz data and the presence or absence of the data. In order to investigate the characteristics of each period, the light curves were plotted and compared. However, a peak that formed a hill was observed earlier than the period of a strong γ-ray flare at 43-86 GHz in period 3 (MJD 57400-58100). Therefore, we assumed that the minimum total CLEANed flux density for each frequency was quiescent flux (S_q) in which the core of 4C +28.07 emitted the minimum, with the variable flux (S_var) obtained by subtracting S_q from the values of the total CLEANed flux density. We then compared the variability of the spectral indices (α) between adjacent frequencies through a spectral analysis. Most notably, α_22-43 showed optically thick spectra in the absence of a strong γ-ray flare, and when the flare appeared, α_22-43 became optically thinner. In order to find out the characteristics of the magnetic field in the variable region, the magnetic field strength in the synchrotron self-absorption (B_SSA) and the equipartition magnetic field strength (B_eq) were obtained. We found that B_SSA is largely consistent with B_eq within the uncertainty, implying that the SSA region in the source is not significantly deviated from the equipartition condition in the γ-ray quiescent periods.

• Journal of The Korean Astronomical Society
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• v.48 no.5
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• pp.313-323
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• 2015

Active galactic nuclei (AGN) are known for irregular variability on all time scales, down to intra-day variability with relative variations of a few percent within minutes to hours. On such short timescales, unexplored territory, such as the possible existence of a shortest characteristic time scale of activity and the shape of the high frequency end of AGN power spectra, still exists. We present the results of AGN single-dish fast photometry performed with the Korean VLBI Network (KVN). Observations were done in a “anti-correlated” mode using two antennas, with always at least one antenna pointing at the target. This results in an effective time resolution of less than three minutes. We used all four KVN frequencies, 22, 43, 86, and 129 GHz, in order to trace spectral variability, if any. We were able to derive high-quality light curves for 3C 111, 3C 454.3, and BL Lacertae at 22 and 43 GHz, and for 3C 279 at 86 GHz, between May 2012 and April 2013. We performed a detailed statistical analysis in order to assess the levels of variability and the corresponding upper limits. We found upper limits on flux variability ranging from ~1.6% to ~7.6%. The upper limits on the derived brightness temperatures exceed the inverse Compton limit by three to six orders of magnitude. From our results, plus comparison with data obtained by the University of Michigan Radio Astronomy Observatory, we conclude that we have not detected source-intrinsic variability which would have to occur at sub-per cent levels.