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.
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
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.
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
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.
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
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.
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
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).Publication: Interferometric Monitoring of Gamma-Ray Bright AGNs: S5 0716+714, 2017, ApJ 841, 119
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.Publication: Interferometric Monitoring of Gamma-Ray Bright AGNs. I. The Results of Single-epoch Multifrequency Observations, 2016, ApJS 227, 8
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.Publication: Spatially Resolved HCN Absorption Features in the Circumnuclear Region of NGC 1052, 2016, ApJL 830, 3