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KVN observations reveal multiple γ-ray emission regions in 3C 84?
Hodgson, Jeffrey A,Rani, Bindu,Lee, Sang-Sung,Algaba, Juan Carlos,Kino, Motoki,Trippe, Sascha,Park, Jong-Ho,Zhao, Guang-Yao,Byun, Do-Young,Kang, Sincheol,Kim, Jae-Young,Kim, Jeong-Sook,Kim, Soon-Wook Blackwell Publishing Ltd 2018 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.475 No.1
<P>3C 84 (NGC 1275) is a well-studied misaligned sctive galactic nucleus (AGN), which has been active in gamma-rays since at least 2008. We have monitored the source at four wavelengths (14, 7, 3, and 2mm) using the Korean VLBI network (KVN) since 2013 as part of the interferometric monitoring of gamma-ray bright AGN (iMOGABA) programme, and it exhibits bright radio emission both near the central SMBH and in a slowly moving feature located to the south known as C3. Other facilities have also detected these short-term variations above a slowly rising trend at shorter wavelengths, such as in gamma-ray and 1-mm total intensity light curves. We find that the variations in the gamma-rays and 1-mm total intensity light curves are correlated, with the gamma-ray leading and lagging the radio emission. Analysis of the 2-mm KVN data shows that both the gamma-rays and 1-mm total intensity short-term variations are better correlated with the SMBH region than C3, likely placing the short-term variations in Cl. We interpret the emission as being due to the random alignment of spatially separated emission regions. We place the slowly rising trend in C3, consistent with previous results. Spectral analysis of the gamma-ray data shows that the gamma-ray flaring is inconsistent with blazar-like gamma-ray emission. Additionally, we report that since mid-2015, a large mm-wave radio flare has been occurring in C3, with a large gamma-ray flare coincident with the onset of this flare at all radio wavelengths.</P>
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.
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.
Long-term millimeter VLBI monitoring of M 87 with KVN at milliarcsecond resolution: nuclear spectrum
Kim, Jae-Young,Lee, Sang-Sung,Hodgson, Jeffrey A.,Algaba, Juan-Carlos,Zhao, Guang-Yao,Kino, Motoki,Byun, Do-Young,Kang, Sincheol Springer-Verlag 2018 Astronomy and astrophysics Vol.610 No.-
<P>We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M 87 at ≲0.8 mas ~ 110<I>R</I>s spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index <I>α</I> of ≳−0.37 (<I>S</I> ∝ <I>ν</I><SUP>+<I>α</I></SUP>) between 22 and 129 GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15 GHz and taking the Event Horizon Telescope (EHT) 230 GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15 and 129 GHz, which could naturally connect the 230 GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency ≳ 100 GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at >100 GHz) and the relatively weakly magnetized (more extended, optically thin at <100 GHz) plasma components are colocated in the footprint of the relativistic jet.</P>
Interferometric Monitoring of Gamma-Ray Bright AGNs: S5 0716+714
Lee, Jee Won,Lee, Sang-Sung,Hodgson, Jeffrey A.,Kim, Dae-Won,Algaba, Juan-Carlos,Kang, Sincheol,Kang, Jiman,Kim, Sungsoo S. American Astronomical Society 2017 The Astrophysical Journal Vol.841 No.2
<P>We present the results of very long baseline interferometry (VLBI) observations of gamma-ray bright blazar S5 0716+714 using the Korean VLBI Network (KVN) at the 22, 43, 86, and 129 GHz bands, as part of the Interferometric Monitoring of Gamma-ray Bright active galactic nuclei (iMOGABA) KVN key science program. Observations were conducted in 29 sessions from 2013 January 16 to 2016 March 1, with the source being detected and imaged at all available frequencies. In all epochs, the source was compact on the milliarcsecond scale, yielding a compact VLBI core dominating the synchrotron emission on these scales. Based on the multiwavelength data between 15 GHz (Owens Valley Radio Observatory) and 230 GHz (Submillimeter Array), we found that the source shows multiple prominent enhancements of the flux density at the centimeter (cm) and millimeter (mm) wavelengths, with mm enhancements leading cm enhancements by -16 +/- 8 days. The turnover frequency was found to vary between 21 and 69 GHz during our observations. By assuming a synchrotron self-absorption model for the relativistic jet emission in S5 0716+714, we found the magnetic field strength in the mas emission region to be <= 5 mG during the observing period, yielding a weighted mean of 1.0 +/- 0.6 mG for higher turnover frequencies (e.g., > 45 GHz).</P>
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 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>