Korean VLBI Network (KVN)

KVN Science Highlights

Capabilities and prospects of the East Asia Very Long Baseline Interferometry Network

The very long baseline interferometry (VLBI) technique offers angular resolutions superior to any other instruments at other wavelengths, enabling unique science applications of high-resolution imaging of radio sources and high-precision astrometry. The East Asia VLBI Network (EAVN) is a collaborative effort in the East Asian region. The EAVN currently consists of 21 tele- scopes with diverse equipment configurations and frequency setups, allowing flexible subarrays for specific science projects. The EAVN provides the highest resolution of 0.5 mas at 22 GHz, allowing the fine imaging of jets in active galactic nuclei, high-accuracy astrometry of masers and pulsars, and precise spacecraft positioning. The soon-to-be-operational Five-hundred- meter Aperture Spherical radio Telescope (FAST) will open a new era for the EAVN. This state-of-the-art VLBI array also pro- vides easy access to and crucial training for the burgeoning Asian astronomical community. This Perspective summarizes the status, capabilities and prospects of the EAVN.

Geographic distribution of the EAVN telescopes and correlators.
Geographic distribution of the EAVN telescopes and correlators. The EAVN can be flexibly operated using subarrays to support diverse science projects. The coloured circles show telescopes affiliated with each subarray. The small coloured dots above each telescope name indicate their available frequency bands. Note that only four common frequency bands (6.7, 8.4, 22 and 43 GHz) of the EAVN are marked here, but some individual telescopes have broader frequency coverage (see details in Table 2). Credit: Tamna 21 m and Yonsei 21 m, KASI; Sejong 22 m, NGII; KASI/KJCC and Ulsan 21 m, KASI; Mizusawa 20 m, VERA; Takahagi 32 m and Hitachi 32 m, Ibaraki University; Kashima 34 m, NICT; Ogasawara 20 m, VERA; Nobeyama 45 m, Nobeyama Radio Observatory; Usuda 64 m, JAXA; Gifu 11 m, Gifu University; Ishigakijima 20 m and Iriki 20 m, VERA; Yamaguchi 32 m, Yamaguchi University; FAST 500 m, NAOC; Kunming 40 m, SHAO/DIFX, Sheshan 25 m, Tianma 65 m, Miyun 50 m and Nanshan 26 m, CVN; background map, NASA.
Comparison of u–v coverages on a 12-hour track of the EAVN with the EVN and the VLBA at 22 GHz frequency band.
Comparison of u–v coverages on a 12-hour track of the EAVN with the EVN and the VLBA at 22 GHz frequency band. The target is M87, a giant radio galaxy lying in the heart of the Virgo cluster. Each dot represents a u–v sampling on a certain baseline in an integration time of five minutes. Telescopes used in creating the u–v coverages: a,d, EVN: Effelsberg (Germany), Onsala (Sweden), Torun (Poland), Jodrell Bank, Cambridge (UK), Sardinia, Medicina and Noto (Italy), Metsähovi (Finland), Sheshan (China), Urumqi (China), Yebes (Spain), Svetloe, Zelenchukskaya, Badary (Russia); b,e, EAVN: Gifu, Hitachi, Iriki, Ishigakijima, Kashima, Mizusawa, Nobeyama, Ogasawara, Sejong, Sheshan, Tahahagi, Tamna, Tianma, Ulsan, Urumqi, Yonsei; c,f, VLBA : Brewster, Fort Davis, Hancock, Kitt Peak, Los Alamos, Mauna Kea, North Liberty, Owens Valley, Pie Town, St. Croix. Panels d–f highlight the inner 2,000 km baselines. It is prominent that the EAVN shows a denser sampling in short and intermediate u–v spacing. With the availability of a large number of stations, good u–v coverage of the EAVN obtained on long and short baselines enables imaging of both compact and extended celestial structures.

Publication: Capabilities and prospects of the East Asia Very Long Baseline Interferometry Network, 2018, Nature Astronomy 2, 118
Authors: T. An, B. W. Sohn, H. Imai

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Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii

The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers. However, previous VLBI observations have been performed separately, which makes it difficult to spatially trace the outward transfer of the material consecutively. Here we present the astrometrically registered, simultaneous maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward VX Sagittarii. The H2O masers detected above the dust-forming layers have an asymmetric distribution. The multi-transition SiO masers are nearly circular ring, suggesting spherically symmetric wind within a few stellar radii. These results provide the clear evidence that the asymmetry in the outflow is enhanced after the smaller molecular gas clump transform into the inhomogeneous dust layers. The 129.3 GHz maser arises from the outermost region compared to that of 43.1/42.8/86.2 GHz SiO masers. The ring size of the 129.3 GHz maser is maximized around the optical maximum, suggesting that radiative pumping is dominant.

Astrometrically registered velocity-integrated intensity maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers obtained from the SFPR technique toward VX Sgr.
Astrometrically registered velocity-integrated intensity maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers obtained from the SFPR technique toward VX Sgr. The rms noise levels on the maps are 115.4, 18.01, 15.45, 33.69, and 11.59 Jy beam−1 m s−1, respectively, in order of the maser transition. The peak flux values are 573.62, 55.65, 38.08, 61.70, and 1.85 Jy beam−1 km s–1. The contour levels are adopted at 10 12 15 20 25 30 50... for multiples of each rms value

Publication: Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii, 2018, Nature Communications 9, 2534
Authors: Dong-Hwan Yoon, Se-Hyung Cho, Youngjoo Yun, Yoon Kyung Choi, Richard Dodson, María Rioja, Jaeheon Kim, Hiroshi Imai, Dongjin Kim, Haneul Yang, Do-Young Byun

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Simultaneous Survey of Water and Class I Methanol Masers toward Red MSX Sources

Simultaneous KVN single-dish surveys are presented for 22 GHz H2O and 44 and 95 GHz class I CH3OH masers toward 299 Red Midcourse Space Experiment Sources in the protostellar stage. The detection rates are 45% at 22 GHz, 28% at 44 GHz, and 23% at 95 GHz. There are 15, 53, and 51 new discoveries at 22, 44, and 95 GHz, respectively. We detect high-velocity (>30 km s−1) features in 27 H2O maser sources. The 95 GHz maser emission is detected only in 44 GHz maser sources. The two transitions show strong correlations in the peak velocity, peak flux density, and isotropic maser luminosity, indicating that they are likely generated in the same sites by the same mechanisms. The 44 GHz masers have much narrower distributions than 22 GHz masers in the relative peak velocity and velocity range, while 6.7GHz class II CH3OH masers have distributions intermediate between the two. The maser luminosity significantly correlates with the parental clump mass, while it correlates well with the bolometric luminosity of the central protostar only when data of the low-mass regime from the literature are added. Comparison with the results of previous maser surveys toward massive star-forming regions suggests that the detection rates of 22 and 44 GHz masers tend to increase as the central objects evolve. This is contrary to the trends found in low- and intermediate-mass star-forming regions. Thus, the occurrence of both masers might depend on the surrounding environments as well as on the evolution of the central object.

22 GHz H2O (Upper panel), 44 GHz CH3OH (middle panel), and 95 GHz CH3OH (lower panel) maser spectra of the sources detected in all three transitions.
22 GHz H2O (Upper panel), 44 GHz CH3OH (middle panel), and 95 GHz CH3OH (lower panel) maser spectra of the sources detected in all three transitions. Red and black represent the first and second epochs, respectively. The source name is given in the top left corner of the upper panel, and “M” is attached for the source associated with 6.7 GHz CH3OH maser emission. The observing dates are shown in the top right corner. The vertical dotted line indicates the systemic velocity, which was mostly determined from the NH3 line observations.

Publication: Simultaneous Survey of Water and Class I Methanol Masers toward Red MSX Sources,2018 ApJS 236 31
Authors: Chang-Hee Kim, Kee-Tae Kim , and Yong-Sun Park

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Revealing the Nature of Blazar Radio Cores through Multifrequency Polarization Observations with the Korean VLBI Network

The linear polarization of the radio cores of eight blazars are observed simultaneously at 22, 43, and 86 GHz with the Korean VLBI Network (KVN) in three epochs between late 2016 and early 2017 in the frame of the Plasma-physics of Active Galactic Nuclei project. We investigate the Faraday rotation measure (RM) of the cores; the RM is expected to increase with observing frequency if core positions depend on frequency owing to synchrotron self-absorption. We find a systematic increase of RMs at higher observing frequencies in our targets. The RM–ν relations follow power laws with indices distributed around 2, indicating conically expanding outflows serving as Faraday rotating media. Comparing our KVN data with contemporaneous optical polarization data from the Steward Observatory for a few sources, we find indications that the increase of RM with frequency saturates at frequencies of a few hundred gigahertz. This suggests that blazar cores are physical structures rather than simple τ = 1 surfaces. A single region, e.g., a recollimation shock, might dominate the jet emission downstream of the jet-launching region. We detect a sign change in the observed RMs of CTA 102 on a timescale of ≈1 month, which might be related to new superluminal components emerging from its core undergoing acceleration/deceleration and/or bending. We see indications for quasars having higher core RMs than BL Lac objects, which could be due to denser inflows/outflows in quasars.

CLEAN maps and Rotation Measure of blazars.
CLEAN maps and Rotation Measure of blazars. Left: contours show naturally weighted CLEAN maps. The lowest contour level starts at 3–5 times the rms noise in the maps. Colors show the degree of linear polarization in units of %. Red ticks show distribution of EVPAs with their lengths proportional to polarized intensity (not degree of polarization). The maps at different observing frequencies are shifted by 20 mas along the x-axis. The gray shaded ellipses show the beam size of the KVN at 22 GHz. All maps from different frequencies are convolved with this beam. Observation dates are noted in the upper right corner of each panel. Middle: EVPA (black diamonds, values on the left axis) and spectral index (blue asterisks, values on the right axis) at the core as a function of λ2 (geometrical mean λ2 for spectral index). Right: RM as a function of geometric mean observing frequency. Each data point is obtained from two adjacent data points for EVPAs in the corresponding middle panel. The dashed lines are power-law functions fitted to the data points; the best-fit power-law indices α are noted. All RM values are rest-frame values.

Publication: Revealing the Nature of Blazar Radio Cores through Multifrequency Polarization Observations with the Korean VLBI Network, 2018, ApJ 860, 112
Authors: Jongho Park, Minchul Kam, Sascha Trippe , Sincheol Kang, Do-Young Byun , Dae-Won Kim, Juan-Carlos Algaba , Sang-Sung Lee , Guang-Yao Zhao, Motoki Kino, Naeun Shin, Kazuhiro Hada, Taeseok Lee, Junghwan Oh, Jeffrey A. Hodgson, Bong Won Sohn

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B-field environments of a blazar S5 0716+714

A KVN Key Science Program, the iMOGABA, has monitored a very compact, variable blazar S5 0716+714 simultaneously at four cm-mm bands (13mm-2.5mm) on sptial resolutions of milli-arcseconds. 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. The turnover frequency was found to vary between 21 and 69 GHz during the observations. By assuming a synchrotron self-absorption model for the relativistic jet emission in S5 0716+714, the authors 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).

Variation in peak flux densitie s (1), turnover frequencies (2), magnetic fields (3), KVN flux densities (4), and deconvolved core sizes (5) in the time domain.
Variation in peak flux densities (1), turnover frequencies (2), magnetic fields (3), KVN flux densities (4), and deconvolved core sizes (5) in the time domain.
Publication: Interferometric Monitoring of Gamma-Ray Bright AGNs: S5 0716+714, 2017, ApJ 841, 119
Authors: Jee Won Lee, Sang-Sung Lee, Jeffrey A. Hodgson, Dae-Won Kim, Juan-Carlos Algaba,Sincheol Kang, Jiman Kang, Sungsoo S. Kim Go to the top

Single-epoch results of a KVN KSP: iMOGABA

The iMOGABA program continues to aim at revealing the origins of the gamma-ray flares that are often detected in AGNs. One of the observing sessions in the program was conducted in 2013 on November 19 and 20, yielding 32 sources that were detected and imaged at all frequency bands with or without the use of the FPT technique. The first 129 GHz VLBI images were made from most of the sources, establishing a 129 GHz VLBI image database with a size of 32 sources. Thanks to simultaneous multifrequency VLBI observations, the authors are able to investigate the spectral properties, free from any source variability. We find that the sources become fainter at higher frequencies, yielding optically thin spectra at mm wavelengths.

 Spectra of the CLEAN flux density (black dot) and core flux density (red circle) for sources imaged at 22 129 GHz. The black solid lines are the best fitting power law.
Spectra of the CLEAN flux density (black dot) and core flux density (red circle) for sources imaged at 22 129 GHz. The black solid lines are the best fitting power law.
Publication: Interferometric Monitoring of Gamma-Ray Bright AGNs. I. The Results of Single-epoch Multifrequency Observations, 2016, ApJS 227, 8
Authors: Sang-Sung Lee, Kiyoaki Wajima, Juan-Carlos Algaba, Guang-Yao Zhao, Jeffrey A. Hodgson, Dae-Won Kim, Jongho Park, Jae-Young Kim, Atsushi Miyazaki, Do-Young Byun, Sincheol Kang, Jeong-Sook Kim, Soon-Wook Kim, Motoki Kino, Sascha Trippe Go to the top

Spatially Resolved HCN Absorption Features in NGC 1052

The first VLBI detection of HCN molecular absorption in the nearby active galactic nucleus NGC 1052 was achieved by the KVN, spatially resolving the HCN absorption against a double-sided nuclear jet structure. Two velocity features of HCN absorption are detected significantly at the radial velocity of 1656 and 1719 km/s, redshifted by 149 and 212 km/s with respect to the systemic velocity of the galaxy. The absorption features show high optical depth localized on the receding jet side, where the free– free absorption occurred due to the circumnuclear torus. The redshifted velocities of the HCN absorption features imply that HCN absorbing gas traces ongoing infall motion inside the circumnuclear torus onto the central engine.

Possible model of the oriented double-sided jet and the circumnuclear torus. (A) A circumnuclear torus. (B) Eastern approaching jet. (C) HCN clouds. (D) Warm molecular region. (E) Plasma region. (F) Western receding jet.
Possible model of the oriented double-sided jet and the circumnuclear torus. (A) A circumnuclear torus. (B) Eastern approaching jet. (C) HCN clouds. (D) Warm molecular region. (E) Plasma region. (F) Western receding jet.
Publication: Spatially Resolved HCN Absorption Features in the Circumnuclear Region of NGC 1052, 2016, ApJL 830, 3
Authors: Satoko Sawada-Satoh, Duk-Gyoo Roh, Se-Jin Oh, Sang-Sung Lee, Do-Young Byun, Seiji Kameno, Jae-Hwan Yeom, Dong-Kyu Jung, Hyo-Ryoung Kim, and Ju-Yeon Hwang

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Korean VLBI Network (KVN)