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REFERENCE LINKING PLATFORM OF KOREA S&T JOURNALS
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Journal of The Korean Astronomical Society
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The Korean Astronomical Society
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Volume & Issues
Volume 48, Issue 6 - Dec 2015
Volume 48, Issue 5 - Oct 2015
Volume 48, Issue 4 - Aug 2015
Volume 48, Issue 3 - Jun 2015
Volume 48, Issue 2 - Apr 2015
Volume 48, Issue 1 - Feb 2015
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AMPLITUDE CORRECTION FACTORS OF KOREAN VLBI NETWORK OBSERVATIONS
LEE, SANG-SUNG ; BYUN, DO-YOUNG ; OH, CHUNG SIK ; KIM, HYO RYOUNG ; KIM, JONGSOO ; JUNG, TAEHYUN ; OH, SE-JIN ; ROH, DUK-GYOO ; JUNG, DONG-KYU ; YEOM, JAE-HWAN ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 229~236
DOI : 10.5303/JKAS.2015.48.5.229
We report results of investigation of amplitude calibration for very long baseline interferometry (VLBI) observations with Korean VLBI Network (KVN). Amplitude correction factors are estimated based on comparison of KVN observations at 22 GHz correlated by Daejeon hardware correlator and DiFX software correlator in Korea Astronomy and Space Science Institute (KASI) with Very Long Baseline Array (VLBA) observations at 22 GHz by DiFX software correlator in National Radio Astronomy Observatory (NRAO). We used the observations for compact radio sources, 3C 454.3, NRAO 512, OJ 287, BL Lac, 3C 279, 1633+382, and 1510–089, which are almost unresolved for baselines in a range of 350-477 km. Visibility data of the sources obtained with similar baselines at KVN and VLBA are selected, fringe-fitted, calibrated, and compared for their amplitudes. We find that visibility amplitudes of KVN observations should be corrected by factors of 1.10 and 1.35 when correlated by DiFX and Daejeon correlators, respectively. These correction factors are attributed to the combination of two steps of 2-bit quantization in KVN observing systems and characteristics of Daejeon correlator.
INTERFEROMETRIC MONITORING OF GAMMA–RAY BRIGHT ACTIVE GALACTIC NUCLEI II: FREQUENCY PHASE TRANSFER
ALGABA, JUAN-CARLOS ; ZHAO, GUANG-YAO ; LEE, SANG-SUNG ; BYUN, DO-YOUNG ; KANG, SIN-CHEOL ; KIM, DAE-WON ; KIM, JAE-YOUNG ; KIM, JEONG-SOOK ; KIM, SOON-WOOK ; KINO, MOTOKI ; MIYAZAKI, ATSUSHI ; PARK, JONG-HO ; TRIPPE, SASCHA ; WAJIMA, KIYOAKI ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 237~255
DOI : 10.5303/JKAS.2015.48.5.237
The Interferometric Monitoring of Gamma–ray Bright Active galactic nuclei (iMOGABA) program provides not only simultaneous multifrequency observations of bright gamma–ray detected active galactic nuclei (AGN), but also covers the highest Very Large Baseline Interferometry (VLBI) frequencies ever being systematically monitored, up to 129 GHz. However, observation and imaging of weak sources at the highest observed frequencies is very challenging. In the second paper in this series, we evaluate the viability of the frequency phase transfer technique to iMOGABA in order to obtain larger coherence time at the higher frequencies of this program (86 and 129 GHz) and image additional sources that were not detected using standard techniques. We find that this method is applicable to the iMOGABA program even under non–optimal weather conditions.
MONITORING OF GAMMA-RAY BRIGHT AGN: THE MULTI-FREQUENCY POLARIZATION OF THE FLARING BLAZAR 3C 279
KANG, SINCHEOL ; LEE, SANG-SUNG ; BYUN, DO-YOUNG ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 257~265
DOI : 10.5303/JKAS.2015.48.5.257
We present results of long-term multi-wavelength polarization observations of the powerful blazar 3C 279 after its γ-ray flare on 2013 December 20. We followed up this flare with single-dish polarization observations using two 21-m telescopes of the Korean VLBI Network. Observations carried out weekly from 2013 December 25 to 2015 January 11, at 22 GHz, 43 GHz, 86 GHz simultaneously, as part of the Monitoring Of GAmma-ray Bright AGN (MOGABA) program. We measured 3C 279 total flux densities of 22–34 Jy at 22 GHz, 15–28 Jy (43 GHz), and 10–21 Jy (86 GHz), showing mild variability of ≤ 50 % over the period of our observations. The spectral index between 22 GHz and 86 GHz ranged from −0.13 to −0.36. Linear polarization angles were 27°–38°, 30°–42°, and 33°–50° at 22 GHz, 43 GHz, and 86 GHz, respectively. The degree of linear polarization was in the range of 6–12 %, and slightly decreased with time at all frequencies. We investigated Faraday rotation and depolarization of the polarized emission at 22–86 GHz, and found Faraday rotation measures (RM) of −300 to −1200 rad m
between 22 GHz and 43 GHz, and −800 to −5100 rad m
between 43 GHz and 86 GHz. The RM values follow a power law with a mean power law index a of 2.2, implying that the polarized emission at these frequencies travels through a Faraday screen in or near the jet. We conclude that the regions emitting polarized radio emission may be different from the region responsible for the 2013 December γ-ray flare and are maintained by the dominant magnetic field perpendicular to the direction of the radio jet at milliarcsecond scales.
KVN MONITORING OBSERVATIONS TOWARD THE RECENT OUTBURST SYMBIOTIC STAR V407 CYGNI
CHO, SE-HYUNG ; KIM, JAEHEON ; YUN, YOUNGJOO ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 267~275
DOI : 10.5303/JKAS.2015.48.5.267
Simultaneous time monitoring observations of H
O and SiO maser lines were performed toward the D-type symbiotic binary system V407 Cyg with the Korean VLBI Network single dish radio telescope. These monitoring observations were carried out from March 2, 2010 (optical phase ϕ = 0.0), 8 days before the nova outburst on March 10, 2010 to June 5, 2014 (ϕ = 2.13). Eight days before the nova outburst, we detected the SiO v = 1, 2, J = 1–0 maser lines which exhibited values of 0.51 K (∼ 6.70 Jy) and 0.71 K (∼ 9.30 Jy), respectively, while after the outburst we could not detect them on April 2 (ϕ = 0.04), May 5 (ϕ = 0.09), May 8 (ϕ = 0.09), or on June 5, 2010 (ϕ = 0.13) within the upper limits of our KVN observations. After restarting our monitoring observations, we detected SiO v = 2, J = 1–0 masers starting on October 20, 2011 (ϕ = 0.83) and detected SiO v = 1, J = 1–0 masers starting on December 22, 2011 (ϕ = 0.92). These results provide clear evidence of the interaction between the shock from the nova outburst and the SiO maser regions of the Mira envelope. The peak emission of SiO v = 1, 2, J = 1–0 masers always occurred at blueshifted velocities with respect to the stellar velocity except for that of SiO v = 1 at one epoch. These phenomena may be related to the redistribution of SiO maser regions after the outburst. The peak velocity variations of SiO masers associated with stellar pulsation phases show an increasing blueshifted trend during our monitoring interval after the outburst.
MEASURING THE CORE SHIFT EFFECT IN AGN JETS WITH THE EXTENDED KOREAN VLBI NETWORK
JUNG, TAEHYUN ; DODSON, RICHARD ; HAN, SEOG-TAE ; RIOJA, MARIA J. ; BYUN, DO-YOUNG ; HONMA, MAREKI ; STEVENS, JAMIE ; VICENTE, PABLO DE ; SOHN, BONG WON ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 277~284
DOI : 10.5303/JKAS.2015.48.5.277
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.
PAGAN I: MULTI-FREQUENCY POLARIMETRY OF AGN JETS WITH KVN
KIM, JAE-YOUNG ; TRIPPE, SASCHA ; SOHN, BONG WON ; OH, JUNGHWAN ; PARK, JONG-HO ; LEE, SANG-SUNG ; LEE, TAESEOK ; KIM, DAEWON ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 285~298
DOI : 10.5303/JKAS.2015.48.5.285
Active Galactic Nuclei (AGN) with bright radio jets offer the opportunity to study the structure of and physical conditions in relativistic outflows. For such studies, multi-frequency polarimetric very long baseline interferometric (VLBI) observations are important as they directly probe particle densities, magnetic field geometries, and several other parameters. We present results from first-epoch data obtained by the Korean VLBI Network (KVN) within the frame of the Plasma Physics of Active Galactic Nuclei (PAGaN) project. We observed seven radio-bright nearby AGN at frequencies of 22, 43, 86, and 129 GHz in dual polarization mode. Our observations constrain apparent brightness temperatures of jet components and radio cores in our sample to > 10
K and > 10
K, respectively. Degrees of linear polarization m
are relatively low overall: less than 10%. This indicates suppression of polarization by strong turbulence in the jets. We found an exceptionally high degree of polarization in a jet component of BL Lac at 43 GHz, with m
~ 40%. Assuming a transverse shock front propagating downstream along the jet, the shock front being almost parallel to the line of sight can explain the high degree of polarization.
PAGAN II: THE EVOLUTION OF AGN JETS ON SUB-PARSEC SCALES
OH, JUNGHWAN ; TRIPPE, SASCHA ; KANG, SINCHEOL ; KIM, JAE-YOUNG ; PARK, JONG-HO ; LEE, TAESEOK ; KIM, DAEWON ; KINO, MOTOKI ; LEE, SANG-SUNG ; SOHN, BONG WON ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 299~311
DOI : 10.5303/JKAS.2015.48.5.299
We report first results from KVN and VERA Array (KaVA) VLBI observations obtained in the frame of our Plasma-physics of Active Galactic Nuclei (PAGaN) project. We observed eight selected AGN at 22 and 43 GHz in single polarization (LCP) between March 2014 and April 2015. Each source was observed for 6 to 8 hours per observing run to maximize the uv coverage. We obtained a total of 15 deep high-resolution images permitting the identification of individual circular Gaussian jet components and three spectral index maps of BL Lac, 3C 111 and 3C 345 from simultaneous dual-frequency observations. The spectral index maps show trends in agreement with general expectations – flat core and steep jets – while the actual value of the spectral index for jets shows indications for a dependence on AGN type. We analyzed the kinematics of jet components of BL Lac and 3C 111, detecting superluminal proper motions with maximum apparent speeds of about 5c. This constrains the lower limits of the intrinsic component velocities to ~ 0.98c and the upper limits of the angle between jet and line of sight to ~20°. In agreement with global jet expansion, jet components show systematically larger diameters d at larger core distances r, following the global relation d ≈ 0.2r, albeit within substantial scatter.
A SEARCH FOR AGN INTRA-DAY VARIABILITY WITH KVN
LEE, TAESEOK ; TRIPPE, SASCHA ; OH, JUNGHWAN ; BYUN, DO-YOUNG ; SOHN, BONG-WON ; LEE, SANG-SUNG ;
Journal of The Korean Astronomical Society, volume 48, issue 5, 2015, Pages 313~323
DOI : 10.5303/JKAS.2015.48.5.313
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.