• 천문학회지
• /
• 제48권5호
• /
• pp.277-284
• /
• 2015

We present our efforts for extending the simultaneous multi-frequency receiver system of the Korean Very Long Baseline Interferometry (VLBI) Network (KVN) to global baselines in order to measure the frequency-dependent position shifts in Active Galactic Nuclei (AGN) jets, the so called core shift effect, with an unprecedented accuracy (a few micro-arcseconds). Millimeter VLBI observations with simultaneous multi-frequency receiver systems, like those of the KVN, enable us to explore the innermost regions of AGN and high precision astrometry. Such a system is capable of locating the frequency dependent opacity changes accurately. We have conducted the feasibility test-observations with the interested partners by implementing the KVN-compatible systems. Here we describe the science case for measuring the core shift effect in the AGN jet and report progress and future plans on extending the simultaneous multi-frequency system to global baselines.

• 천문학회보
• /
• 제37권2호
• /
• pp.240.2-240.2
• /
• 2012

AGN(Active Galactic Nucleus) consists of a supermassive black hole located at its center, an accretion disk around the black hole, and bipolar jets. Since May 2011, we have performed the MOGABA(Monitoring Of GAmma-ray Bright AGN) project for observing gamma-ray bright AGN once a week at multifrequencies using KVN(Korean VLBI Network) 21m radio telescopes. The MOGABA project is the observations for measuring the degree of polarization, polarization angle, and total flux of about 20 AGN at 22, 43 and 86GHz. By this project, we are able to investigate polarization characteristics, spectral index, and variation of rotation measure at radio wavelengths of gamma-ray bright AGN and to study possible relation between gamma-ray flares and magnetic field structure change in AGN. According to previous research, gamma-ray flares of some AGN are coincident with large changes in angle of linear polarization. In this paper we report the preliminary results of linear polarization and total flux at 22, 43, 86GHz of gamma-ray bright quasars 3C279 and 1510-089 showing noticeable variation of total flux at 22GHz in late 2011, and discuss possible correlation with gamma ray light curves.

• 융합신호처리학회논문지
• /
• 제10권4호
• /
• pp.280-288
• /
• 2009

본 논문에서는 한국우주전파관측망(Korean VLBI Network; KVN)을 통하여 천체의 미약한 우주전파를 고속으로 기록하는 Mark5B 고속기록기 시스템의 제어운용 소프트웨어를 분석하고 VSI(VLBI Standard Interface) 형식의 시각정보를 기록할 수 있는 소프트웨어를 개발하였다. 고속기록기 시스템의 제어운용 소프트웨어는 대용량 하드디스크를 제어할 수 있는 RAID 보드를 구동하는 소프트웨어와 네트워크를 통한 제어운용을 할 수 있다. 핵심 제어 소프트웨어가 시스템에서 구동하고 있으면 네트워크로 입력된 명령을 해석하고 실행 후 결과를 다시 네트워크를 통해 출력하도록 구성되어 있다. 이러한 소프트웨어 분석을 통하여 기록된 관측데이터를 상관처리하기 위해 필요한 시각정보를 함께 기록할 수 있는 소프트웨어를 본 논문에서 개발하였다. 개발된 소프트웨어는 데이터를 기록할 때 입력되는 1PPS 신호의 인터럽트를 확인한 후 데이터 기록과 함께 기록시각정보도 함께 데이터 기록시간동안 기록한다. 개발한 소프트웨어의 정상동작을 확인하기 위해 실제 관측을 수행하였으며, 기록한 데이터의 분석을 통하여 그 유효성을 확인하였다.

• 천문학회보
• /
• 제41권2호
• /
• pp.60.2-60.2
• /
• 2016

We present recent results of very long baseline interferometry (VLBI) observations of gamma-ray bright active galactic nuclei (AGNs) using Korean VLBI Network (KVN) at 22, 43, 86, and 129~GHz bands, which are part of a KVN key science program; Interferometric Monitoring of Gamma-ray Bright AGNs (iMOGABA). We selected a total of 34 radio-loud AGNs of which 30 sources are gamma-ray bright AGNs, including 24 sources monitored by the Fermi Gamma-ray Space Telescope using the Large Area Telescope on board. The selected sources consist of 24 quasars, 7 BL Lacs, and 3 radio galaxies. In this talk, we summarize recent results of the iMOGABA, including results of single-epoch multi-frequency VLBI observations of the target sources, conducted during a 24-hr session on 2013 November 19 and 20. All observed sources were detected and imaged at all frequency bands with or without a frequency phase transfer technique which enabled to detect and image 12 faint sources at 129 GHz, except for 0218+357 which was detected for only one baseline at all frequency bands.

• 천문학회보
• /
• 제44권1호
• /
• pp.76.1-76.1
• /
• 2019

CTA 102, one of gamma-ray bright active galactic nuclei (AGN) has been observed with Korean very long baseline interferometry (VLBI) network (KVN) during the period of 2012 December-2018 May as part of interferometric Monitoring Of Gamma-ray Bright AGN (iMOGABA). Multi-frequency VLBI observations enable us to compare the milliarcsecond(mas)-scale iMOGABA images of relativistic jets with those from the Monitoring Of Jets in AGN with Very long baseline array (VLBA) Experiments (MOJAVE) and the VLBA-Boston University(BU)-BLAZAR programs which use VLBA with its angular resolutions of 0.2-1.3 mas. In spite of the relative larger beam sizes of KVN (1-10 mas), we are able to identify jet components of CTA 102 using the KVN multi-frequency VLBI observations with those resolved with VLBA. Considering an instrumental beam blending effect on the jet component identification, we were able to obtain a blending shift of the core position based on a convolution analysis using the VLBA data. When we apply the core position shift to the KVN images of CTA 102, we find that the identified jet components of CTA 102 from the KVN observations are well matched with those from the VLBA observations. Based on the results of the analysis, we may be able to study the jet kinematics and its correlation with gamma-ray flare activity.

• 한국전자통신학회논문지
• /
• 제12권6호
• /
• pp.1009-1018
• /
• 2017

우리는 한국우주전파관측망(: Korea VLBI Network: KVN)에 적용 가능한 새로운 디지털 샘플러를 개발하였다. 이 샘플러는 1024MHz 샘플링으로 2bits/sample의 성능을 가지고 있다. 입력 기준 주파수 Clock은 1PPS(: Pulse per second)와 10MHz를 사용하며, 1PPS 신호에 동기되어 UTC(: Universal Time Coordinated) 시각정보가 출력된다. 샘플링된 데이터 출력은 시각정보를 포함한 VSI(: VLBI Standard Interface)규격을 채택하고 있다. 개발된 샘플러의 성능을 검증하기 위하여, KVN 울산전파천문대에 설치하여 우주 전파 관측 시험을 수행하였다. 관측시험 결과, 안정적인 성능을 보여 주었다. 본 논문에서는 새로 개발된 샘플러와 관측 시험 결과를 발표하고자 한다.

• 천문학논총
• /
• 제37권1호
• /
• pp.1-11
• /
• 2022

During the course of analysing both single-dish and very long baseline interferometry (VLBI) data obtained from the Korean VLBI Network (KVN), we found a systematic offset between flux density measurements from different antennas. We were able to attribute a majority of the systematic offsets to changes in the "a priori" antenna gains, which were found to have varied up to 10 percent at 22 GHz and up to 30 percent at 43 GHz. Using historical calibrator observations, we present a revised set of gains that may be applied to KVN data taken from 2015 August to 2019 January. Application of the revised gains to the KVN results in a consistency of correlated flux density measurements between the three baselines of approximately five percent. We found that images from the recalibrated data typically have a 50 percent higher dynamic range, with some cases showing an increase of dynamic range of up to a factor of three.

• 천문학회지
• /
• 제55권6호
• /
• pp.207-213
• /
• 2022

The Seoul Radio Astronomy Observatory (SRAO) operates a 6.1-meter radio telescope on the Gwanak campus of Seoul National University. We present the efforts to reform SRAO to a Very Long Baseline Interferometry (VLBI) station, motivated by recent achievements by millimeter interferometer networks such as Event Horizon Telescope, East Asia VLBI Network, and Korean VLBI Network (KVN). For this goal, we installed a receiver that had been used in the Combined Array for Research in Millimeter-wave Astronomy and a digital backend, including an H-maser clock. The existing hardware and software were also revised, which had been dedicated only to single-dish operations. After several years of preparations and test observations in 1 and 3-millimeter bands, a fringe was successfully detected toward 3C 84 in 86 GHz in June 2022 for a baseline between SRAO and KVN Ulsan station separated by 300 km. Thanks to the dual frequency operation of the receiver, the VLBI observations will soon be extended to the 1 mm band and verify the frequency phase referencing technique between 1 and 3-millimeter bands.

• 천문학회보
• /
• 제40권1호
• /
• pp.42.2-42.2
• /
• 2015

High frequency peakers (HFPs) are promising candidates for young active galactic nuclei (AGNs). Their small physical scale (< 1 kpc) and radio spectrum peaked at high frequency (> 5 GHz) are suggestive that it has been only about $10^2-10^3$ years since a central massive black hole in their host galaxies was launched. Until recently however, long-term monitoring radio observations at frequencies which are high enough to cover the true peak of HFP candidates were rare. Therefore, previous HFP samples are often contaminated by blazars, which are highly variable, hence may show a similar radio spectrum as HFPs depending on the observational epoch. In this work, we challenge to identify genuine young AGNs by monitoring HFP candidates at high radio frequencies. We performed single-dish monitoring of 19 candidates in 18 epochs over 2.5 years at 22 and 43 GHz using the Korean VLBI Network (KVN). Also, using KaVA, a combined array of the KVN and the VERA in Japan, we carried out 22 GHz VLBI observations of two HFPs and one blazar selected from our sample in order to compare their parsec scale (milli-arcsecond scale) morphology. HFPs are expected to have double/triple features, so called compact symmetric objects, which are scaled-down versions of extended radio galaxies, while blazars typically show core-jet morphology. We discuss the properties of AGNs at their very early evolutionary stage based on the results of the KVN and KaVA observations.

• 융합신호처리학회논문지
• /
• 제12권4호
• /
• pp.333-340
• /
• 2011

한국천문연구원은 한일상관센터 (Korea-Japan Correlation Center, KJCC)에 2009년부터 한일공동VLBI상관기(Korea-Japan Joint VLBI Correlator, KJJVC)를 설치하여 운영하고 있다. 한일공동VLBI상관기는 한국우주전파관측망(Korean VLBI Network, KVN), VERA(VLBI Exploration of Radio Astrometry), JVN(Japanese VLBI Network) 및 공동 관측망 등에서 관측한 VLBI(Very Long Baseline Interferometer) 데이터를 상관처리하고, 과학적 목적을 위해 관측데이터를 처리하는 전용 계산기로 사용된다. KJJVC는 각 구성시스템 사이의 데이터 입출력 규격으로 VLBI 국제표준인 VSI(VLBI Standard Interface)를 따르고 있다. 특히 관측된 데이터를 상관처리하기 위해 고속재생기인 Mark5B시스템과 동기재생처리장치(Raw VLBI Data Buffer, RVDB) 사이에는 1024 Mbps급으로 데이터가 전송된다. 고속 데이터 전송에 있어 발생하는 전자기 방해 (Electromagnetic Interference, EMI)는 관측데이터의 손실을 야기 시키며, 전송 케이블의 길이가 길수록 손실 발생빈도가 많고, 디지털 데이터 신호의 전압레벨을 감소시켜 데이터 인식 오류를 초래한다. 따라서 본 논문에서는 VSI 규격의 통신에서 발생하는 EMI 잡음 정도를 측정하고, 데이터 손실을 최소화할 수 있는 방법으로 1) RC 필터를 사용한 방법, 2) Microstrip 라인을 이용한 임피던스 매칭, 3) Differential line driver를 이용한 신호 복원 방법 등을 제안하였다. 각 제안방법들의 유효성을 확인하기 위해 제안방법들은 시뮬레이션과 실험적인 구현을 통하여 성능시험을 수행하였으며, 각각의 제안 방법이 VSI 규격의 고속 데이터 전송에 유효함을 확인하였다.