Korean VLBI Network (KVN)

Recent Papers using KVN

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  • 2019-11
    Exploring the Morphology and Origins of the 4C 38.41 Jet
    J. C. Algaba1,2,3, B. Rani4,5,6, S. S. Lee3,7, M. Kino8,9, Jongho Park2,10, and Jae-Young Kim11
    Title Exploring the Morphology and Origins of the 4C 38.41 Jet
    Date 2019-11
    Publication The Astrophysical Journal, Volume 886, Number 2, 85
    Authors J. C. Algaba1,2,3, B. Rani4,5,6, S. S. Lee3,7, M. Kino8,9, Jongho Park2,10, and Jae-Young Kim11
    Affiliation 1Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia;algaba@um.edu.my2Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea3Korea Astronomy & Space Science Institute, 776, Daedeokdae-ro, Yuseong-gu, Daejeon, 305-348, Republic of Korea4Southeastern Universities Research Association, Washington, DC, USA5NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA6Center for Research and Exploration in Space Sciences and Technology, NASA/GSFC, Greenbelt, MD 20771, USA7Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea8Kogakuin University of Technology & Engineering, Academic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan9National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan10Academia Sinica, Institute of Astronomy and Astrophysics, P.O Box 23141, Taipei 10617, Taiwan11Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
    Abstract We study the properties of the innermost jet of the flat spectrum radio quasar 1633+382 (4C 38.41) based on very long baseline interferometry (VLBI) data from the radio monitoring observations of the Boston University VLBI program at 43 GHz. Analysis of the components suggests a semi-parabolic jet geometry with jet radius R following the relation R ∝ r 0.7 with distance r, with indications of a jet geometry break toward a conical geometry. Brightness temperature falls with distance following T B ∝ r −2.1. Combining this information, magnetic field and electron densities are found to fall along the jet as B ∝ r −1.5 and n ∝ r −1.1, respectively, suggesting that the magnetic configuration in the jet may be dominated by the poloidal component. Our analysis of the jet structure suggests that the innermost jet regions do not follow a ballistic trajectory and, instead, match a sinusoidal morphology, which could be due to jet precession from a helical pattern or Kelvin–Helmholtz instabilities.
    Link https://iopscience.iop.org/article/10.3847/1538-4357/ab4b45
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  • 2019-10
    Cloud G074.11+00.11: a stellar cluster in formation
    Mika Saajasto1, Jorma Harju1, Mika Juvela1, Liu Tie2,3, Qizhou Zhang4, Sheng-Yuan Liu5, Naomi Hirano5, Yuefang Wu6, Kee-Tae Kim2, Ken’ichi Tatematsu7, Ke Wang8 and Mark Thompson9
    Title Cloud G074.11+00.11: a stellar cluster in formation
    Date 2019-10
    Publication Astronomy & Astrophysics 630, A69
    Authors Mika Saajasto1, Jorma Harju1, Mika Juvela1, Liu Tie2,3, Qizhou Zhang4, Sheng-Yuan Liu5, Naomi Hirano5, Yuefang Wu6, Kee-Tae Kim2, Ken’ichi Tatematsu7, Ke Wang8 and Mark Thompson9
    Affiliation 1Department of Physics, PO Box 64, University of Helsinki, 00014 Helsinki, Finlande-mail:mika.saajasto@helsinki.fi2Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea3East Asian Observatory, 660 North A’ohoku Place, Hilo, HI 96720, USA4Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA5Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, Taipei 10617, Taiwan6Department of Astronomy, Peking University, Beijing 100871, PR China7National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan8Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, PR China9Centre for Astrophysics Research, School of Physics Astronomy&Mathematics, University of Hertfordshire, College Lane,Hatfield AL10 9AB, UK
    Abstract Context. We present molecular line and dust continuum observations of a Planck-detected cold cloud, G074.11+00.11. The cloud consists of a system of curved filaments and a central star-forming clump. The clump is associated with several infrared sources and H2O maser emission.

    Aims. We aim to determine the mass distribution and gas dynamics within the clump to investigate if the filamentary structure seen around the clump repeats itself on a smaller scale, and to estimate the fractions of mass contained in dense cores and filaments. The velocity distribution of pristine dense gas can be used to investigate the global dynamical state of the clump, the role of filamentary inflows, filament fragmentation, and core accretion.

    Methods. We used molecular line and continuum observations from single dish observatories and interferometric facilities to study the kinematics of the region.

    Results. The molecular line observations show that the central clump may have formed as a result of a large-scale filament collision. The central clump contains three compact cores. Assuming a distance of 2.3 kpc, based on Gaia observations and a three-dimensional extinction method of background stars, the mass of the central clump exceeds 700 M⊙, which is roughly ~25% of the total mass of the cloud. Our virial analysis suggests that the central clump and all identified substructures are collapsing. We find no evidence for small-scale filaments associated with the cores.
    Conclusions. Our observations indicate that the clump is fragmented into three cores with masses in the range [10, 50] M⊙ and that all three are collapsing. The presence of an H2O maser emission suggests active star formation. However, the CO lines show only weak signs of outflows. We suggest that the region is young and any processes leading to star formation have just recently begun.
    Link https://www.aanda.org/articles/aa/abs/2019/10/aa34991-18/aa34991-18.html
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  • 2019-09
    Simultaneous 22 GHz Water and 44 GHz Methanol Maser Survey of Ultracompact H ii Regions
    Won-Ju Kim1,2,3, Kee-Tae Kim1,4, and Kwang-Tae Kim2
    Title Simultaneous 22 GHz Water and 44 GHz Methanol Maser Survey of Ultracompact H ii Regions
    Date 2019-09
    Publication The Astrophysical Journal Supplement Series, Volume 244, Number 1, 2
    Authors Won-Ju Kim1,2,3, Kee-Tae Kim1,4, and Kwang-Tae Kim2
    Affiliation 1Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea2Chungnam National University, Yuseong-gu, Daejeon 34134, Republic of Korea3Instituto de Radioastronomía Milimétrica, Avenida Divina Pastora 7, E-18012 Granada, Spain4University of Science and Technology, Korea(UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
    Abstract We have carried out a multi-epoch simultaneous survey of 22 GHz H2O and 44 GHz Class I CH3OH masers toward 103 ultracompact H ii regions (UCH iis) between 2010 and 2017. We detected H2O and CH3OH maser emission in 74 (72%) and 55 (53%) UCH iis, respectively. Among them, three H2O and 27 CH3OH maser sources are new detections. These high detection rates suggest that the occurrence periods of both maser species are significantly overlapped with the UCH ii phase of massive star formation. The CH3OH maser lines always have small (<10 km s−1) relative velocities with respect to the natal molecular cores, while H2O maser lines often show larger relative velocities. Twenty four H2O maser-detected sources have maser lines at relative velocities >30 km s−1, and 13 of them show extremely high-velocity (>50 km s−1) components. The appearance and disappearance of H2O maser lines are quite frequent over six-month or one-year time intervals. In contrast, CH3OH maser lines usually do not exhibit significant variations in the line intensity, velocity, or shape for the same periods. The isotropic luminosities of both masers appear to correlate with the bolometric luminosities of the central stars. This correlation becomes much stronger in the case where data points in the low- and intermediate-mass regimes are added. The maser luminosities also tend to increase with the radio continuum luminosities of UCH iis and the submillimeter continuum luminosities of the associated dense cores.
    Link https://iopscience.iop.org/article/10.3847/1538-4365/ab2fc9
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  • 2019-09
    Time variations of H2O and SiO masers in the proto-Planetary Nebula OH231.8+4.2
    Jaeheon Kim,1‹S.-H. Cho,2,3V. Bujarrabal,4H. Imai,5,6R. Dodson,7D.-H. Yoon2and B. Zhang1
    Title Time variations of H2O and SiO masers in the proto-Planetary Nebula OH231.8+4.2
    Date 2019-09
    Publication Monthly Notices of the Royal Astronomical Society, Volume 488, Issue 1, 1427
    Authors Jaeheon Kim,1‹S.-H. Cho,2,3V. Bujarrabal,4H. Imai,5,6R. Dodson,7D.-H. Yoon2and B. Zhang1
    Affiliation 1Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China2Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Daejeon 34055, Republic of Korea3Department of Astronomy, Yonsei University, 50 Yonsei-ro Seodaemun-gu, Seoul 03722, Republic of Korea4Observatorio Astron ́omico Nacional (OAN-IGN), Ap. 112, E-28803 Alcal ́a de Henares, Spain5Center for General Education, Institute for Comprehensive Education, Kagoshima University, 1-21-30 Korimoto, Kagoshima 890-0065, Japan6Amanogawa Galaxy Astronomy Research Center, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima890-0065, Japan
    Abstract H2O (22 GHz) and SiO masers (43, 86, 129 GHz) in the bipolar proto-planetary nebulaOH 231.8+4.2 were simultaneously monitored using the 21-m antennas of the Korean VLBINetwork in 2009–2015. Both species exhibit periodic flux variations that correlate with thecentral star’s optical light curve, with a phase delay of up to 0.15 for the maser flux variationswith respect to the optical light curve. The flux densities of SiOv=2,J=1→0 and H2Omasers decrease with time, implying that they may disappear in 10–20 yr. However, thereseems to have been a transient episode of intense H2O maser emission around 2010. We alsofound a systematic behaviour in the velocity profiles of these masers. The velocities of theH2O maser components appear to be remarkably constant, suggesting ballistic motion forthe bipolar outflow in this nebula. On the other hand, those of the SiO maser clumps showa systematic radial acceleration of the individual clumps, converging to the outflow velocityof the H2O maser clumps. Measuring the full widths at zero power of the detected lines,we estimated the expansion velocities of the compact bipolar outflow traced by H2O maserand SiO thermal line, and discussed the possibility of the expanding SiO maser region in theequatorial direction. All of our analyses support that the central host star of OH231.8 is closeto the tip of the AGB phase and that the mass-loss rate recently started to decrease because ofincipient post-AGB evolution.
    Link https://watermark.silverchair.com/stz1830.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAAmUwggJhBgkqhkiG9w0BBwagggJSMIICTgIBADCCAkcGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMpI6406R3IvVFAjMZ
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  • 2019-08
    An International Survey of Front-end Receivers and Observing Performance of Telescopes for Radio Astronomy
    P. Bolli1, A. Orfei2, A. Zanichelli2, R. Prestage3, S. J. Tingay4, M. Beltrán1, M. Burgay5, C. Contavalle2, M. Honma6, A. Kraus7,M. Lindqvist8, J. Lopez Perez9, P. Marongiu5, T. Minamidani10, S. Navarro11, T. Pisanu5, Z.-Q. Shen12, B. W. Sohn13,C. Stanghellini2, T. Tzioumis14, and G. Zacchiroli2
    Title An International Survey of Front-end Receivers and Observing Performance of Telescopes for Radio Astronomy
    Date 2019-08
    Publication Publications of the Astronomical Society of the Pacific, Volume 131, Number 1002, 085002
    Authors P. Bolli1, A. Orfei2, A. Zanichelli2, R. Prestage3, S. J. Tingay4, M. Beltrán1, M. Burgay5, C. Contavalle2, M. Honma6, A. Kraus7,M. Lindqvist8, J. Lopez Perez9, P. Marongiu5, T. Minamidani10, S. Navarro11, T. Pisanu5, Z.-Q. Shen12, B. W. Sohn13,C. Stanghellini2, T. Tzioumis14, and G. Zacchiroli2
    Affiliation 1INAF-OAA, Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Arcetri, Firenze, Italy2INAF-IRA, Istituto Nazionale di Astrofisica, Istituto di Radioastronomia, Bologna, Italy3NRAO-GBT, National Radio Astronomy Observatory, Green Bank Telescope, Green Bank, USA4International Centre for Radio Astronomy Research, Curtin University, Bentley, Australia5INAF-OAC, Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Cagliari, Cagliari, Italy6NAOJ, National Astronomical Observatory of Japan, Japan7MPIfR, Max Planck Institut für Radioastronomie, Bonn, Germany8OSO, Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden9IGN-CAY, Instituto Geográfico Nacional, Centro Astronómico de Yebes, Guadalajara, Spain10NAOJ-NRO, National Astronomical Observatory of Japan, Nobeyama Radio Observatory, Japan11IRAM, Instituto de Radioastronomía Milimétrica, Granada, Spain12CAS-ShAO, Chinese Academy of Sciences, Shanghai Astronomical Observatory, Shanghai, People’s Republic of China13KASI, Korea Astronomy and Space Science Institute, Daejeon, Republic of Korea14CSIRO-ATNF, Commonwealth Scientific and Industrial Research Organization, Australia Telescope National Facility, Epping, Australia
    Abstract This paper presents a survey of microwave front-end receivers installed at radio telescopes throughout the world. This unprecedented analysis was conducted as part of a review of front-end developments for Italian radio telescopes, initiated by the Italian National Institute for Astrophysics in 2016. Fifteen international radio telescopes have been selected to be representative of the instrumentation used for radio astronomical observations in the frequency domain from 300 MHz to 116 GHz. A comprehensive description of the existing receivers is presented and their characteristics are compared and discussed. The observing performances of the complete receiving chains are also presented. An overview of ongoing developments illustrates and anticipates future trends in front-end projects to meet the most ambitious scientific research goals.
    Link https://iopscience.iop.org/article/10.1088/1538-3873/ab1f7e
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  • 2019-06
    Ejection of Double Knots from the Radio Core of PKS 1510–089 during the Strong Gamma-Ray Flares in 2015
    Jongho Park1, Sang-Sung Lee2,3, Jae-Young Kim4, Jeffrey A. Hodgson2, Sascha Trippe1, Dae-Won Kim1, Juan-Carlos Algaba1,2,5, Motoki Kino6,7, Guang-Yao Zhao2, Jee Won Lee2
    Title Ejection of Double Knots from the Radio Core of PKS 1510–089 during the Strong Gamma-Ray Flares in 2015
    Date 2019-06
    Publication The Astrophysical Journal, Volume 877, Number 2, 106
    Authors Jongho Park1, Sang-Sung Lee2,3, Jae-Young Kim4, Jeffrey A. Hodgson2, Sascha Trippe1, Dae-Won Kim1, Juan-Carlos Algaba1,2,5, Motoki Kino6,7, Guang-Yao Zhao2, Jee Won Lee2
    Affiliation 1Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Republic of Korea;jhpark@astro.snu.ac.kr2Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Republic of Korea3Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea4Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany5Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia6National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan7Kogakuin University of Technology & Engineering, Academic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan8Center for Astrophysics|Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
    Abstract PKS 1510–089 is a bright and active γ-ray source that showed strong and complex γ-ray flares in mid-2015 during which the Major Atmospheric Gamma Imaging Cerenkov telescopes detected variable very high energy (photon energies >100 GeV) emission. We present long-term multifrequency radio, optical, and γ-ray light curves of PKS 1510–089 from 2013 to 2018, and results of an analysis of the jet kinematics and linear polarization using 43 GHz Very Long Baseline Array data observed between late 2015 and mid-2017. We find that a strong radio flare trails the γ-ray flares in 2015, showing an optically thick spectrum at the beginning and becoming optically thin over time. Two laterally separated knots of emission are observed to emerge from the radio core nearly simultaneously during the γ-ray flares. We detect an edge-brightened linear polarization near the core in the active jet state in 2016, similar to the quiescent jet state in 2008–2013. These observations indicate that the γ-ray flares may originate from compression of the knots by a standing shock in the core and the jet might consist of multiple complex layers showing time-dependent behavior, rather than of a simple structure of a fast jet spine and a slow jet sheath.
    Link https://iopscience.iop.org/article/10.3847/1538-4357/ab1b27
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  • 2019-06
    Jet kinematics of the quasar 4C+21.35 from observations with the KaVA very long baseline interferometry array
    Taeseok Lee,1Sascha Trippe,1‹Motoki Kino,2,3Bong Won Sohn,4Jongho Park,1Junghwan Oh,4Kazuhiro Hada,5,6Kotaro Niinuma,7Hyunwook Ro,4,8Taehyun Jung,4Guang-Yao Zhao,4Sang-Sung Lee,4Juan-Carlos Algaba,1,9Kazunori Akiyama,10Kiyoaki Wajima,4Satoko Sawada-Satoh,11Fumie Tazaki,5Ilje Cho,4,12Jeffrey Hodgson,4Jeong Ae Lee,1Yoshiaki Hagiwara,13Mareki Honma,3,6Shoko Koyama,14Ta o A n,15,16Yuzhu Cui,3Hyemin Yoo,8Noriyuki Kawaguchi,15Duk-Gyoo Roh,4Se-Jin Oh,4Jae-Hwan Yeom,4Dong-Kyu Jung,4Chungsik Oh,4Hyo-Ryoung Kim,4Ju-Yeon Hwang,4Do-Young Byun,4Se-Hyung Cho,4Hyun-Goo Kim,4Hideyuki Kobayashi,5Katsunori M. Shibata,3,5Zhiqiang Shen,15,16Wu Jiang15,16and Jee Won Lee4
    Title Jet kinematics of the quasar 4C+21.35 from observations with the KaVA very long baseline interferometry array
    Date 2019-06
    Publication Monthly Notices of the Royal Astronomical Society, Volume 486, Issue 2, June 2019, Pages 2412
    Authors Taeseok Lee,1Sascha Trippe,1‹Motoki Kino,2,3Bong Won Sohn,4Jongho Park,1Junghwan Oh,4Kazuhiro Hada,5,6Kotaro Niinuma,7Hyunwook Ro,4,8Taehyun Jung,4Guang-Yao Zhao,4Sang-Sung Lee,4Juan-Carlos Algaba,1,9Kazunori Akiyama,10Kiyoaki Wajima,4Satoko Sawada-Satoh,11Fumie Tazaki,5Ilje Cho,4,12Jeffrey Hodgson,4Jeong Ae Lee,1Yoshiaki Hagiwara,13Mareki Honma,3,6Shoko Koyama,14Ta o A n,15,16Yuzhu Cui,3Hyemin Yoo,8Noriyuki Kawaguchi,15Duk-Gyoo Roh,4Se-Jin Oh,4Jae-Hwan Yeom,4Dong-Kyu Jung,4Chungsik Oh,4Hyo-Ryoung Kim,4Ju-Yeon Hwang,4Do-Young Byun,4Se-Hyung Cho,4Hyun-Goo Kim,4Hideyuki Kobayashi,5Katsunori M. Shibata,3,5Zhiqiang Shen,15,16Wu Jiang15,16and Jee Won Lee4
    Affiliation 1Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Korea2Kogakuin University of Technology & Engineering, Academic SupportCenter, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan3National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka,Tokyo 181-8588, Japan4Korea Astronomy and Space Science Institute, Yuseong-gu, Daejeon 34055,Korea5Mizusawa VLBI Observatory, National Astronomical Observatory ofJapan, Osawa, Mitaka, Tokyo 181-8588, Japan6Department of Astronomical Science, The Graduate University for Ad-vanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588,Japan7Graduate School of Sciences and Technology for Innovation, YamaguchiUniversity, Yoshida 1677-1, Yamaguchi, Yamaguchi 753-8512, Japan8Department of Astronomy, Yonsei University, 134 Shinchon-dong,Seodaemun-gu, Seoul 120-749, Republic of Korea9Department of Physics, Faculty of Science, University of Malaya, 50603Kuala Lumpur, Malaysia10Massachusetts Institute of Technology, Haystack Observatory, 99 Mill-stone Road, Westford, MA 01886, USA11Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima-shi, Kagoshima 890-0065, Japan12Department of Astronomy & Space Science, University of Science &Technology, 217 Gajeong-ro, Daejeon, Republic of Korea13Toyo University, 5-28-20 Hakusan, Bunkyo-ku, Tokyo 112-8606, Japan14Institute of Astronomy & Astrophysics, Academia Sinica, P.O. Box 23-141,Taipei 10617, Taiwan15Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80Nandan Road, Shanghai 200030, China16Key Laboratory of Radio Astronomy, Chinese Academy of Sciences,Nanjing 210008, China
    Abstract We present the jet kinematics of the flat spectrum radio quasar (FSRQ) 4C+21.35 using time-resolved KaVA very long baseline interferometry array radio maps obtained from 2014 September to 2016 July. During two out of three observing campaigns, observations were performed bi-weekly at 22 and 43 GHz quasi-simultaneously. At 22 GHz, we identified three jet components near the core with apparent speeds up to (14.4 ± 2.1)c. The timing of the ejection of a new component detected in 2016 is consistent with a γ-ray flare in 2014 November. At 43 GHz, we found four inner jet (<3 mas) components with speeds from (3.5 ± 1.4)c to (6.8 ± 1.5)c. Jet component speeds tend to be higher with increasing distances from the core. We compared our data with archival Very Long Baseline Array (VLBA) data from the Boston University (BU) 43 GHz and the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) 15.4 GHz monitoring programmes. Whereas MOJAVE data and our data are in good agreement, jet speeds obtained from the BU programme data in the same time period are about twice as high as the ones we obtain from the KaVA data. The discrepancy at 43 GHz indicates that radio arrays with different angular resolution identify and trace different jet features even when the data are obtained at the same frequency and at the same time. The flux densities of jet components decay exponentially, in agreement with a synchrotron cooling time-scale of ∼1 yr. Using known electron Lorentz factor values (∼9000), we estimate the magnetic field strength to be ∼1–3 μT. When adopting a jet viewing angle of 5°, the intrinsic jet speed is of order 0.99c.
    Link https://academic.oup.com/mnras/article/486/2/2412/5432371
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  • 2019-04
    CORE-JET BLENDING EFFECTS IN ACTIVE GALACTIC NUCLEI UNDER THE KOREAN VLBI NETWORK VIEW AT 43 GHZ
    Juan-Carlos Algaba1,2, Jeffrey Hodgson3, Sin-Cheol Kang3,4, Dae-Won Kim2, Jae-Young Kim5, Jee Won Lee3, Sang-Sung Lee3,4, and Sascha Trippe2
    Title CORE-JET BLENDING EFFECTS IN ACTIVE GALACTIC NUCLEI UNDER THE KOREAN VLBI NETWORK VIEW AT 43 GHZ
    Date 2019-04
    Publication Journal of The Korean Astronomical Society (천문학회지) Volume 52, Issue 2
    Authors Juan-Carlos Algaba1,2, Jeffrey Hodgson3, Sin-Cheol Kang3,4, Dae-Won Kim2, Jae-Young Kim5, Jee Won Lee3, Sang-Sung Lee3,4, and Sascha Trippe2
    Affiliation 1Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia; algaba@um.edu.my 2Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Korea
    3Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Korea 4Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea 5Max-Planck-Institut fu ̈r Radioastronomie, Auf dem Hu ̈gel 69, D-53121 Bonn, Germany
    Abstract 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 τ=1
    τ=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.
    Link http://koreascience.or.kr/article/JAKO201914439302918.page
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  • 2019-04
    First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
    The Event Horizon Telescope Collaboration
    Title First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
    Date 2019-04
    Publication The Astrophysical Journal Letters, 875: L1
    Authors The Event Horizon Telescope Collaboration
    Affiliation
    Abstract When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42±3 μas, which is circular and encompasses a central depression in brightness with a flux ratio 10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M=(6.5±0.7)×109 Me. Our radio- wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.
    Link https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7
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  • 2019-04
    First M87 Event Horizon Telescope Results. II. Array and Instrumentation
    The Event Horizon Telescope Collaboration
    Title First M87 Event Horizon Telescope Results. II. Array and Instrumentation
    Date 2019-04
    Publication The Astrophysical Journal Letters, 875:L1
    Authors The Event Horizon Telescope Collaboration
    Affiliation
    Abstract The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ∼1.3 mm, EHT angular resolution (λ/D) is ∼25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64gigabits−1, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.
    Link https://iopscience.iop.org/article/10.3847/2041-8213/ab0c96
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  • 2019-04
    First M87 Event Horizon Telescope Results. III. Data Processing and Calibration
    The Event Horizon Telescope Collaboration
    Title First M87 Event Horizon Telescope Results. III. Data Processing and Calibration
    Date 2019-04
    Publication The Astrophysical Journal Letters, 875:L2
    Authors The Event Horizon Telescope Collaboration
    Affiliation
    Abstract We present the calibration and reduction of Event Horizon Telescope (EHT) 1.3mm radio wavelength observations of the supermassive black hole candidate at the center of the radio galaxy M87 and the quasar 3C 279, taken during the 2017 April 5–11 observing campaign. These global very long baseline interferometric observations include for the first time the highly sensitive Atacama Large Millimeter/submillimeter Array (ALMA); reaching an angular resolution of 25 μas, with characteristic sensitivity limits of ∼1 mJy on baselines to ALMA and ∼10 mJy on other baselines. The observations present challenges for existing data processing tools, arising from the rapid atmospheric phase fluctuations, wide recording bandwidth, and highly heterogeneous array. In response, we developed three independent pipelines for phase calibration and fringe detection, each tailored to the specific needs of the EHT. The final data products include calibrated total intensity amplitude and phase information. They are validated through a series of quality assurance tests that show consistency across pipelines and set limits on baseline systematic errors of 2% in amplitude and 1° in phase. The M87 data reveal the presence of two nulls in correlated flux density at ∼3.4 and ∼8.3 Gλ and temporal evolution in closure quantities, indicating intrinsic variability of compact structure on atimescale of days, or several light-crossing times for afew billion solar-mass black hole. These measurements provide the first opportunity to image horizon-scale structure in M87.
    Link https://iopscience.iop.org/article/10.3847/2041-8213/ab0c57
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  • 2019-04
    First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
    The Event Horizon Telescope Collaboration
    Title First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
    Date 2019-04
    Publication The Astrophysical Journal Letters, 875:L4
    Authors The Event Horizon Telescope Collaboration
    Affiliation
    Abstract We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of ∼40 μas, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others’ work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions.
    Link https://iopscience.iop.org/article/10.3847/2041-8213/ab0e85
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  • 2019-04
    First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
    The Event Horizon Telescope Collaboration
    Title First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
    Date 2019-04
    Publication The Astrophysical Journal Letters, 875:L5
    Authors The Event Horizon Telescope Collaboration
    Affiliation
    Abstract The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87’s large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.
    Link https://iopscience.iop.org/article/10.3847/2041-8213/ab0f43
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  • 2019-04
    First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
    The Event Horizon Telescope Collaboration
    Title First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
    Date 2019-04
    Publication The Astrophysical Journal Letters, 875:L6
    Authors The Event Horizon Telescope Collaboration
    Affiliation
    Abstract We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data
    obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior
    brightness depressions) using two independent sampling algorithms that consider distinct representations of the
    visibility data. We show that the crescent family of models is statistically preferred over other comparably complex
    geometric models that we explore. We calibrate the geometric model parameters using general relativistic
    magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We
    further fit images generated from GRMHD models directly to the data. We compare the derived emission region and
    black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable
    consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from
    near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing
    direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of
    42±3μas and constrain its fractional width to be <0.5. Associating the crescent feature with the emission
    surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc2=3.8±0.4 μas. Folding in
    a distance measurement of 16.8+0.8 Mpc gives a black hole mass of M = 6.5  0.2∣  0.7∣ ́ 109 M . This -0.7 stat sys 
    measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.
    Link https://iopscience.iop.org/article/10.3847/2041-8213/ab1141
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  • 2019-02
    Source-Frequency Phase-Referencing Observation of AGNs with KaVA Using Simultaneous Dual-Frequency Receiving
    Guang-Yao Zhao1, Taehyun Jung1,2, Bong Won Sohn1,2,15, Motoki Kino3,4, Mareki Honma4,5, Richard Dodson6, Mar ́ıa Rioja6,7,8, Seog-Tae Han1, Katsunori Shibata4,5, Do-Young Byun1,2,15, Kazunori Akiyama9, Juan-Carlos Algaba10,17, Tao An11,12, Xiaopeng Cheng11,13, Ilje Cho1,2, Yuzhu Cui4, Kazuhiro Hada4,5, Jeffrey A. Hodgson1, Wu Jiang11,12, Jee Won Lee1, Jeong Ae Lee10, Kotaro Niinuma14, Jong-Ho Park10, Hyunwook Ro15, Satoko Sawada-Satoh16, Zhi-Qiang Shen11,12, Fumie Tazaki4, Sascha Trippe10, Kiyoaki Wajima1, and Yingkang Zhang11,13
    Title Source-Frequency Phase-Referencing Observation of AGNs with KaVA Using Simultaneous Dual-Frequency Receiving
    Date 2019-02
    Publication Journal of the Korean Astronomical Society(한국천문학회지) 52 1 23
    Authors Guang-Yao Zhao1, Taehyun Jung1,2, Bong Won Sohn1,2,15, Motoki Kino3,4, Mareki Honma4,5, Richard Dodson6, Mar ́ıa Rioja6,7,8, Seog-Tae Han1, Katsunori Shibata4,5, Do-Young Byun1,2,15, Kazunori Akiyama9, Juan-Carlos Algaba10,17, Tao An11,12, Xiaopeng Cheng11,13, Ilje Cho1,2, Yuzhu Cui4, Kazuhiro Hada4,5, Jeffrey A. Hodgson1, Wu Jiang11,12, Jee Won Lee1, Jeong Ae Lee10, Kotaro Niinuma14, Jong-Ho Park10, Hyunwook Ro15, Satoko Sawada-Satoh16, Zhi-Qiang Shen11,12, Fumie Tazaki4, Sascha Trippe10, Kiyoaki Wajima1, and Yingkang Zhang11,13
    Affiliation 1Korea Astronomy and Space Science Institute, Daejeon 34055 Korea; gyzhao@kasi.re.kr
    2University of Science and Technology, 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea
    3Kogakuin University, Academic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan 4National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 5Department of Astronomical Science, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan 6ICRAR, M468, University of Western Australia, 35 Stirling Hwy, Perth 6009, Australia
    7CSIRO Astronomy and Space Science, 26 Dick Perry Avenue, Kensington WA 6151, Australia
    8OAN (IGN), Alfonso XII, 3 y 5, 28014 Madrid, Spain
    9Massachusetts Institute of Technology, Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA 10Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Korea 11Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China 12Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, China 13University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, China
    14Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yoshida 1677-1, Yamaguchi, Yamaguchi 753-8512, Japan
    15Department of Astronomy, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 03722, Korea 16Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima-shi, Kagoshima 890-0065, Japan
    17Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
    Abstract The KVN(Korean VLBI Network)-style simultaneous multi-frequency receiving mode is demonstrated to be promising for mm-VLBI observations. Recently, other Very long baseline inter- ferometry (VLBI) facilities all over the globe start to implement compatible optics systems. Simultaneous dual/multi-frequency VLBI observations at mm wavelengths with international baselines are thus possi- ble. In this paper, we present the results from the first successful simultaneous 22/43 GHz dual-frequency observation with KaVA(KVN and VERA array), including images and astrometric results. Our analysis shows that the newly implemented simultaneous receiving system has brought a significant extension of the coherence time of the 43 GHz visibility phases along the international baselines. The astrometric results obtained with KaVA are consistent with those obtained with the independent analysis of the KVN data. Our results thus confirm the good performance of the simultaneous receiving systems for the non- KVN stations. Future simultaneous observations with more global stations bring even higher sensitivity and micro-arcsecond level astrometric measurements of the targets.
    Link http://koreascience.or.kr/article/JAKO201912262463288.page
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  • 2019-02
    A Broad HCO+ Absorption Line Associated with the Circumnuclear Torus of NGC 1052
    Satoko Sawada-Satoh1, Do-Young Byun2,3, Sang-Sung Lee2,3, Se-Jin Oh2, Duk-Gyoo Roh2, Seiji Kameno4,5, Jae-Hwan Yeom2, Dong-Kyu Jung2, Chungsik Oh2, Hyo-Ryoung Kim2, and Ju-Yeon Hwang2,6
    Title A Broad HCO+ Absorption Line Associated with the Circumnuclear Torus of NGC 1052
    Date 2019-02
    Publication The Astrophysical Journal Letters, Volume 872, Number 2, L21
    Authors Satoko Sawada-Satoh1, Do-Young Byun2,3, Sang-Sung Lee2,3, Se-Jin Oh2, Duk-Gyoo Roh2, Seiji Kameno4,5, Jae-Hwan Yeom2, Dong-Kyu Jung2, Chungsik Oh2, Hyo-Ryoung Kim2, and Ju-Yeon Hwang2,6
    Affiliation 1Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan;swdsth@gmail.com2Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong, Daejeon 34055, Republic of Korea3University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea4Joint ALMA Observatory, Alonso de Cordova 3107 Vitacura, Santiago 763 0355, Chile5National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan6SET system, 16-3 Gangnam-daero 8-gil, Seocho-gu, Seoul 06787, Republic of Korea
    Abstract We present the first subparsec-scale maps of HCO+ J = 1–0 absorption in the circumnuclear region of the nearby radio galaxy NGC 1052. Our λ3 mm very-long-baseline-interferometry (VLBI) observations with the Korean VLBI Network have spatially resolved the broad HCO+ absorption at a velocity range of 1350–1850 km s−1 against a double-sided nuclear jet, and have revealed that the HCO+ absorption is concentrated on the receding jet and the nuclear components. The distribution of the HCO+ absorbing gas strongly supports the circumnuclear torus surrounding the supermassive black hole. From the estimations of the column density and the volume density of molecular hydrogen, the size of the molecular gas region in the torus is at least 1 pc. The broad spectral profile of HCO+ is likely to be a blend of multiple gas clumps with various velocities. The HCO+ absorption of NGC 1052 could trace complex kinematics in the vicinity of the active galactic nucleus, such as inflow, outflow, turbulence, and so on.
    Link https://iopscience.iop.org/article/10.3847/2041-8213/ab0425
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  • 2019-02
    The magnetic field structure in CTA 102 from high-resolution mm-VLBI observations during the flaring state in 2016-2017
    Carolina Casadio1, Alan P. Marscher2, Svetlana G. Jorstad2,3, Dmitry A. Blinov4,3, Nicholas R. MacDonald1, Thomas P. Krichbaum1, Biagina Boccardi1,5, Efthalia Traianou1, José L. Gómez6, Iván Agudo6, Bong-Won Sohn7, Michael Bremer8, Jeffrey Hodgson7, Juha Kallunki9, Jae-Young Kim1, Karen E. Williamson2 and J. Anton Zensus1
    Title The magnetic field structure in CTA 102 from high-resolution mm-VLBI observations during the flaring state in 2016-2017
    Date 2019-02
    Publication Astronomy & Astrophysics
    Authors Carolina Casadio1, Alan P. Marscher2, Svetlana G. Jorstad2,3, Dmitry A. Blinov4,3, Nicholas R. MacDonald1, Thomas P. Krichbaum1, Biagina Boccardi1,5, Efthalia Traianou1, José L. Gómez6, Iván Agudo6, Bong-Won Sohn7, Michael Bremer8, Jeffrey Hodgson7, Juha Kallunki9, Jae-Young Kim1, Karen E. Williamson2 and J. Anton Zensus1
    Affiliation 1Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germanye-mail:casadio@mpifr-bonn.mpg.de2Institute for Astrophysical Research, Boston University, Boston, MA 02215, USA3Astronomical Institute, St. Petersburg State University, St. Petersburg 199034, Russia4University of Crete, Heraklion, Greece5INAF – OAS Bologna, Area della Ricerca CNR, Via Gobetti 101, 40129 Bologna, Italy6Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain7Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Korea8Institut de Radio Astronomie Millimétrique, 300 Rue de la Piscine, 38406 Saint-Martin-d’Hères, France9Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
    Abstract Context. Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to understanding the crucial physical processes giving rise to jet formation, as well as to their extraordinary radiation output up to γ-ray energies.

    Aims. We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm VLBI polarimetric observations, reaching an unprecedented resolution (∼50 μas). We also investigate the variability and physical processes occurring in the source during the observing period, which coincides with a very active state of the source over the entire electromagnetic spectrum.

    Methods. We perform the Faraday rotation analysis using 3 and 7 mm data and we compare the obtained rotation measure (RM) map with the polarization evolution in 7 mm VLBA images. We study the kinematics and variability at 7 mm and infer the physical parameters associated with variability. From the analysis of γ-ray and X-ray data, we compute a minimum Doppler factor value required to explain the observed high-energy emission.

    Results. Faraday rotation analysis shows a gradient in RM with a maximum value of ∼6 × 104 rad m−2 and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7 mm EVPA orientation is different when the component is exiting the core or crossing a stationary feature at ∼0.1 mas. The interaction between the superluminal component and a recollimation shock at ∼0.1 mas could have triggered the multi-wavelength flares. The variability Doppler factor associated with such an interaction is large enough to explain the high-energy emission and the remarkable optical flare occurred very close in time.
    Link https://www.aanda.org/articles/aa/abs/2019/02/aa34519-18/aa34519-18.html
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Korean VLBI Network