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Algaba, Juan-Carlos,Lee, Sang-Sung,Kim, Dae-Won,Rani, Bindu,Hodgson, Jeffrey,Kino, Motoki,Trippe, Sascha,Park, Jong-Ho,Zhao, Guang-Yao,Byun, Do-Young,Gurwell, Mark,Kang, Sin-Cheol,Kim, Jae-Young,Kim, American Astronomical Society 2018 The Astrophysical Journal Vol.852 No.1
<P>We present multi-frequency simultaneous VLBI radio observations of the flat spectrum radio quasar 1633+382 (4C 38.41) as part of the interferometric monitoring of gamma-ray-bright active galactic nuclei (iMOGABA) program combined with additional observations in the radio, optical, X-rays, and gamma-rays carried out during the period 2012 March-2015 August. The monitoring of this source reveals a significant long-lived increase in its activity for approximately two years in the radio bands, which correlates with a similar increase in all other bands from submillimeter to gamma-rays. A significant correlation is also found between radio fluxes and simultaneous spectral indices during this period. The study of the discrete correlation function indicates time lags smaller than the uncertainties of similar to 40 days among both radio bands and high-energy bands, and a time lag of similar to 70 days, with gamma-rays leading radio emission. We interpret this as showing that the high-energy and radio fluxes arise from different emitting regions, located at 1 +/- 13 and 40 +/- 13 pc from the central engine respectively.</P>
Core-Jet Blending Effects in AGN under the KVN View at 43 GHz
Juan-Carlos Algaba,Sincheol Kang,Dae-Won Kim,Jae-Young Kim,Jee Won Lee,Sang-Sung Lee,Sascha Trippe 한국천문학회 2019 Journal of The Korean Astronomical Society Vol.52 No.2
A long standing problem in the study of Active Galactic Nuclei (AGNs) is that the observed VLBI core is in fact a blending of the actual AGN core (classically defined by the tau=1 surface) and the upstream regions of the jet or optically thin flows. This blending may cause some biases in the observables of the core, such as its flux density, size or brightness temperature, which may lead to misleading interpretation of the derived quantities and physics. We study the effects of such blending under the view of the Korean VLBI Network (KVN) for a sample of AGNs at 43 GHz by comparing their observed properties with observations obtained using the Very Large Baseline Array (VLBA). Our results suggest that the observed core sizes are a factor 11 larger than these of VLBA, which is similar to the factor expected by considering the different resolutions of the two facilities. We suggest the use of this factor to consider blending effects in KVN measurements. Other parameters, such as flux density or brightness temperature, seem to possess a more complicated dependence.
Exploring the Variability of the Flat-spectrum Radio Source 1633+382. II. Physical Properties
Algaba, Juan-Carlos,Lee, Sang-Sung,Rani, Bindu,Kim, Dae-Won,Kino, Motoki,Hodgson, Jeffrey,Zhao, Guang-Yao,Byun, Do-Young,Gurwell, Mark,Kang, Sin-Cheol,Kim, Jae-Young,Kim, Jeong-Sook,Kim, Soon-Wook,Par American Astronomical Society 2018 The Astrophysical journal Vol.859 No.2
<P>The flat-spectrum radio quasar 1633+382 (4C 38.41) showed a significant increase of its radio flux density during the period 2012 March-2015 August, which correlates with gamma-ray flaring activity. Multi-frequency simultaneous very long baseline interferometry (VLBI) observations were conducted as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program and supplemented with additional radio monitoring observations with the OVRO 40. m telescope, the Boston University VLBI program, and the Submillimeter Array. The epochs of the maxima for the two largest gamma-ray flares coincide with the ejection of two respective new VLBI components. Analysis of the spectral energy distribution indicates a higher turnover frequency after the flaring events. The evolution of the flare in the turnover frequency-turnover flux density plane probes the adiabatic losses in agreement with the shock-in-jet model. The derived synchrotron self-absorption magnetic fields, of the order of 0.1. mG, do not seem to change dramatically during the flares, and are much weaker, by a factor 10(4), than the estimated equipartition magnetic fields, indicating that the source of the flare may be associated with a particle-dominated emitting region.</P>
INTERFEROMETRIC MONITORING OF GAMMA–RAY BRIGHT ACTIVE GALACTIC NUCLEI II: FREQUENCY PHASE TRANSFER
Juan-Carlos Algaba,Guang-Yao Zhao,이상성,변도영,강신철,김대원,김재영,김정숙,김순욱,KINOMOTOKI,Atsushi Miyazaki,박종호,Sascha Trippe,Kiyoaki Wajima 한국천문학회 2015 Journal of The Korean Astronomical Society Vol.48 No.5
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.
The Power of Simultaneous Multi-frequency Observations for mm-VLBI: Beyond Frequency Phase Transfer
Zhao, Guang-Yao,Algaba, Juan Carlos,Lee, Sang Sung,Jung, Taehyun,Dodson, Richard,Rioja, Marí,a,Byun, Do-Young,Hodgson, Jeffrey,Kang, Sincheol,Kim, Dae-Won,Kim, Jae-Young,Kim, Jeong-Sook,Kim, Soo American Institute of Physics 2018 The Astronomical journal Vol.155 No.1
<P>Atmospheric propagation effects at millimeter wavelengths can significantly alter the phases of radio signals and reduce the coherence time, putting tight constraints on high-frequency Very Long Baseline Interferometry (VLBI) observations. In previous works, it has been shown that non-dispersive (e.g., tropospheric) effects can be calibrated with the frequency phase transfer (FPT) technique. The coherence time can thus be significantly extended. Ionospheric effects, which can still be significant, remain however uncalibrated after FPT as well as the instrumental effects. In this work, we implement a further phase transfer between two FPT residuals (i.e., so-called FPT-square) to calibrate the ionospheric effects based on their frequency dependence. We show that after FPT-square, the coherence time at 3 mm can be further extended beyond 8 hr and the residual phase errors can be sufficiently canceled by applying the calibration of another source, which can have a large angular separation from the target (> 20 degrees) and significant temporal gaps. Calibrations for all-sky distributed sources with a few calibrators are also possible after FPT-square. One of the strengths and uniqueness of this calibration strategy is the suitability for high-frequency all-sky survey observations including very weak sources. We discuss the introduction of a pulse calibration system in the future to calibrate the remaining instrumental effects, allowing the possibility of imaging the source structure at high frequencies with FPT-square, where all phases are fully calibrated without involving any additional sources.</P>
The Power of Simultaneous Multi-frequency Observations for mm-VLBI: Beyond Frequency Phase Transfer
Guang-Yao Zhao,Juan Carlos Algaba,Sang Sung Lee,Taehyun Jung,Richard Dodson,Maria Rioja,Do-Young Byun,Jeffrey Hodgson,Sincheol Kang,Dae-Won Kim,Jae-Young Kim,Jeong-Sook Kim,Soon-Wook Kim,Motoki Kino,A 한국천문학회 2017 天文學會報 Vol.42 No.1
Lee, Sang-Sung,Wajima, Kiyoaki,Algaba, Juan-Carlos,Zhao, Guang-Yao,Hodgson, Jeffrey A.,Kim, Dae-Won,Park, Jongho,Kim, Jae-Young,Miyazaki, Atsushi,Byun, Do-Young,Kang, Sincheol,Kim, Jeong-Sook,Kim, Soo American Astronomical Society 2016 The Astrophysical journal Supplement series Vol.227 No.1
<P>We present results of single-epoch very long baseline interferometry (VLBI) observations of gamma-ray bright active galactic nuclei (AGNs) using the Korean VLBI Network (KVN) at the 22, 43, 86, and 129 GHz bands, which are part of a KVN key science program, Interferometric Monitoring of Gamma-Ray Bright AGNs. We selected a total of 34 radio-loud AGNs of which 30 sources are gamma-ray bright AGNs with flux densities of >6 x 10(-10) ph cm(-2) s(-1). Single-epoch multifrequency VLBI observations of the target sources were 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 the imaging of 12 faint sources at 129 GHz, except for one source. Many of the target sources are resolved on milliarcsecond scales, yielding a core-jet structure, with the VLBI core dominating the synchrotron emission on a milliarcsecond scale. CLEAN flux densities of the target sources are 0.43-28 Jy, 0.32-21 Jy, 0.18-11 Jy, and 0.35-8.0 Jy in the 22, 43, 86, and 129 GHz bands, respectively. Spectra of the target sources become steeper at higher frequency, with spectral index means of -0.40, -0.62, and -1.00 in the 22-43 GHz, 43-86 GHz and 86-129 GHz bands, respectively, implying that the target sources become optically thin at higher frequencies (e.g., 86-129 GHz).</P>
THE MILLIMETER-RADIO EMISSION OF BL LACERTAE DURING TWO γ-RAY OUTBURSTS
김대원,Sascha Trippe,이상성,박종호,김재영,Juan Carlos Algaba Marcos,Jeffrey A. Hodgson,KINOMOTOKI,Guang-Yao Zhao,Kiyoaki Wajima,강신철,오정환,이태석,변도영,김순욱,김정숙 한국천문학회 2017 Journal of The Korean Astronomical Society Vol.50 No.6
We present a study of the inexplicit connection between radio jet activity and $\gamma$-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two $\gamma$-ray outbursts (in November 2013 and March 2015) can be seen in $\gamma$-ray light curves obtained from \emph{Fermi} observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of 411$\pm$85 days, 352$\pm$79 days, 310$\pm$57 days, and 283$\pm$55 days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths $B\sim2\,\mu$T and electron Lorentz factors $\gamma\sim10\,000$. Taking into account that our formal measurement errors include intrinsic variability and thus over-estimate the statistical uncertainties, we find that the decay timescale $\tau$ scales with frequency $\nu$ like $\tau\propto\nu^{-0.2}$. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the $\gamma$-ray outbursts. The spectral evolution is consistent with the `generalized shock model' of \citet{valtaoja1992}. A temporary increase in the core opacity and the emergence of a knot around the time of the second $\gamma$-ray event indicate that this $\gamma$-ray outburst might be an `orphan' flare powered by the `ring of fire' mechanism.
THE AUTOMATIC CALIBRATION OF KOREAN VLBI NETWORK DATA
Jeffrey A. Hodgson,이상성,Guang-Yao Zhao,Juan-Carlos Algaba,윤영주,정태현,변도영 한국천문학회 2016 Journal of The Korean Astronomical Society Vol.49 No.4
The calibration of Very Long Baseline Interferometry (VLBI) data has long been a time consuming process. The Korean VLBI Network (KVN) is a simple array consisting of three identical antennas. Because four frequencies are observed simultaneously, phase solutions can be transferred from lower frequencies to higher frequencies in order to improve phase coherence and hence sensitivity at higher frequencies. Due to the homogeneous nature of the array, the KVN is also well suited for automatic calibration. In this paper we describe the automatic calibration of single-polarisation KVN data using the KVN Pipeline and comparing the results against VLBI data that has been manually reduced. We find that the pipelined data using phase transfer produces better results than a manually reduced dataset not using the phase transfer. Additionally we compared the pipeline results with a manually reduced phase-transferred dataset and found the results to be identical.