• KASI NEWSLETTER
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• s.51
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• pp.1-4
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• 2005

제3대 원장에 박석재 박사 취임/ 조직개편/ 김호일 박사 과학기술우수논문상 수상/ 양홍진 박사 미국 인명사전에 등재/ 취임사/ '보다 넓고 깊고 빠른 한국 대형 망원경을 사용한 천문학 연구' 워크샵 개최/ 2005 하계 교원 천문연수/ 직원화합 다과회 및 원장 설명회/ 인사발령/ 학계동정/ 콜로키움/ KVN 구축사업 진행상황

• The Bulletin of The Korean Astronomical Society
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• v.33 no.2
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• pp.42.1-42.1
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• 2008

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.35.1-35.1
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• 2013

By probing nuclear regions and overall properties of AGN hosts as a function of their environments, we aim to observationally examine how AGN activities are related to their surroundings. We have selected eight representative AGN hosts in the Virgo cluster, including seven cluster members (M49, M60, M84, M87, NGC 4435, NGC 4526, NGC 4636) and one galaxy that is likely to be background (NGC 4261) but still close enough to be studied in high resolution. The selected galaxies are located in a range of density regions showing various morphology in 1.4 GHz continuum. High resolution observations with the KVN allow us to access the inner region of the AGN without suffering from dust extinction and synchrotron self-absorption. Since half of our targets are weak to be detected at K-band within its coherence time, we applied phase referencing (fast antenna position switching) to calibrate fast atmospheric phase fluctuations. We successfully detected relatively bright AGNs, such as M87, M84 and NGC4261, but no detection signature was found to the other members of the sample. In this talk, we will present our first results from our KVN observations, while we will discuss in detail the applied technique and our immediate future plans.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.43.2-43.2
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• 2013

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. In the shortest time scale of AGN activity, unexplored territory still exists: first, the existence of a shortest time scale of AGN activity and secondly the shape of high frequency end of AGN power spectrum. Also the spectral variations at the shortest timescale. Here, we present the preliminary results of AGN fast photometry performed with Korean VLBI Network(KVN). Observations were done in a 'anti-correlated' mode using two antennas, with always either one antenna pointing at the target. This results in an effective time resolution of 3 minutes. We used all four KVN frequencies, 22, 43, 86, and 129 GHz, in order to trace spectral variability. We have been able to derive high-quality light curves for 3C111 and 3C279 at 22 and 86 GHz observed on 31st of May and 30th of Nov. in 2012. We are currently performing detailed statistical analysis in order to assess the levels of variability or the corresponding upper limits.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.71.1-71.1
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• 2014

The black hole occupation fraction in dwarf galaxies can provide an important clue for understanding the black hole seed formation. As a pilot feasibility study, we performed a KVN radio monitoring campaign over 8 months for 4 dwarf galaxies. Two galaxies (IC10 and NGC1569) are detected at 22 GHz, respectively with 39 mJy, 83 mJy. The measured flux (rms) variability is 13% and 8%, respectively for IC10 and NGC1569, while the mean flux uncertainty is 25% and 12%. Thus, the detection of the radio flux variability is at best marginal. Detecting flux variability of faint sources (i.e., 22 GHz flux < 200 mJy) seems challenging with the KVN single dishes. Combining with the 1.4 GHz flux measurements from the NVSS, we find that these two galaxies have a steep spectrum, supporting that the radio sources are AGNs. Instead of a monitoring, single-epoch multi-band observations can be effective for identifying radio AGNs by providing the constraint of the radio continuum slope.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.32.2-32.2
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• 2016

We present the catalog of the KVN Calibrator Survey (KVNCS). This first part of the KVNCS is a single dish radio survey conducted at 22 (K band) and 43 GHz (Q band) simultaneously using the Korean VLBI Network (KVN) from 2009 to 2011. A total 2045 sources selected from the VLBA Calibrator Survey (VCS) with an extrapolated flux limit of 100 mJy at K band. The KVNCS contains 1533 sources in the K band with a flux limit of 70 mJy and 553 sources in the Q band with a flux limit of 120 mJy; it covers the whole sky down to -32. ${\circ}$ 5 in declination. Five hundred thirteen sources were detected in the K and Q bands, simultaneously; ~76% of them are flat-spectrum sources ($-0.5{\leq}{\alpha}{\leq}0.5$). From the flux-flux relationship, we anticipated that the most of the radiation of many of the sources comes from the compact components. Therefore, the sources listed in the KVNCS are strong candidates for high frequency VLBI calibrators.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.41.1-41.1
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• 2016

The Galactic center, Sagittarius A* (Sgr A*), is the closest supermassive black hole and emits synchrotron radiation. It provides great opportunity to study the origin of mm/sub-mm emission. Currently, two competing models have been suggested as a jet base and a radiatively inecient accretion flow (RIAF). To unveil the properties, the extremely high resolution(${\sim}10{\mu}as$) corresponding to the projected Schwarzschild radius of ~0.1AU is necessary. With KVN, a jet model can be tested by multi- frequency simultaneous observations because the optically thick surface in a jet (i.e. radio core) moves toward the center at a higher frequency. We conducted 8 observations with KVN at 43/86GHz in 2015, and found that the measured positional shift to the reference calibrator, J1744-3116, was ~0.3 mas to the south of Sgr A* using the source frequency phase referencing (SFPR) at Q/W bands for the first time. With the result, in the future, we will attempt to measure the variation of source position shifts that can constrain the direction of approaching jets and the variability of black hole activity of Sgr A*.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.66.1-66.1
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• 2012

We have developed superconducting mixer receivers for 129 GHz VLBI observation in Korean VLBI Network(KVN). The developed mixer has a radial waveguide probe with simple transmission line LC transformer as a tuning circuit to its 5 series-connected junctions, which can have 125-165 GHz as operation RF frequency. For IF signal path a high impedance quarter-wavelength line connects the probe to one end of symmetric RF chokes. DSB receiver noise of the mixer was about 40 K over 4-6 GHz IF band whereas we achieved about uncorrected SSB noise temperature of 70 K and better than 10 dB IRR in 2SB configuration with 8-10 GHz IF band. Insert-type receiver cartridges using the mixers have been assembled for all three KVN stations. On-site performance summary in commissioning phase is presented.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.51.1-51.1
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• 2010

We present the interim results of simultaneous observations of SiO and H2O masers toward 401 known stellar SiO and/or H2O maser sources (166 both SiO and H2O maser sources, 83 only SiO maser sources, and 152 only H2O maser sources) using KVN_Yonsei telescope. The results of 166 known SiO/H2O maser sources will be presented by Kim et al. and the results of 83 only SiO maser sources and 152 only H2O maser sources presented here. Both SiO and H2O maser emission were detected from 30 sources giving a detection rate of 36 % toward known 83 only SiO maser sources, while they were detected from 66 sources giving a detection rate of 43 % toward known 152 only H2O maser sources at one epoch observation. Only SiO masers were detected from 42 sources toward 83 only SiO sources, while they were detected from 28 sources toward 152 only H2O sources. Characteristics of these observed sources in the IRAS two-color diagram is investigated including mutual relations between SiO and H2O maser emission. In addition, these results will be useful for statistical study of late-type stars and future VLBI observations.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.44.1-44.1
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• 2010

한국천문연구원에서는 2009년 1단계 한일공동VLBI상관기(Korea-Japan Joint VLBI Correlator, KJJVC)의 개발과 설치를 완료하였다. KJJVC는 한국우주전파관측망(Korean VLBI Network, KVN), KVN과 일본 내의 VLBI 관측망을 포함하는 한일공동VLBI관측망(Korea-Japan Joint VLBI Network, KJJVN), 그리고 한중일의 VLBI관측망인 동아시아 VLBI관측망(East Asian VLBI Network, EAVN)으로 관측한 VLBI 데이터를 상관처리할 수 있도록 설계 제작되었다. 그리고 고속재생기, 데이터 동기재생처리시스템(Raw VLBI Data Buffer, RVDB), VLBI상관서브시스템(VLBI Correlation Subsystem, VCS), 데이터 아카이브 시스템(Peta-scale Epoch Data Archive, PEDA)로 구성된다. 특히 일본국립천문대 VERA에서는 테이프 형태의 DIR2000 고속기록기를 사용하고 있는데, DIR2000의 제품단종으로 인해 이 데이터를 VCS에 재생하기 위해 DIR1000을 개량한 VERA2000 고속재생기를 개발하였다. 즉, VSI 형태의 데이터를 1Gbps 속도로 재생만 하기 위해 기존의 시스템을 개량하였으며, 2009년 10월에 VLBI상관기실에 설치되었다. 본 논문에서는 VERA2000 시스템의 구성 및 개발, 시험운영 등에 대해 간략히 소개하고자 한다.