###### Abstract

The East Asian VLBI Network (EAVN) is an international VLBI facility in East
Asia and is operated under mutual collaboration between East Asian countries,
as well as part of Southeast Asian and European countries. EAVN currently
consists of 16 radio telescopes and three correlators located in China, Japan,
and Korea, and is operated mainly at three frequency bands, 6.7, 22, and 43
GHz with the longest baseline length of 5078 km, resulting in the highest
angular resolution of 0.28 milliarcseconds at 43 GHz. One of distinct
capabilities of EAVN is multi-frequency simultaneous data reception at nine
telescopes, which enable us to employ the frequency phase transfer technique
to obtain better sensitivity at higher observing frequencies. EAVN started its
open-use program in the second half of 2018, providing a total observing time
of more than 1100 hours in a year. EAVN fills geographical gap in global VLBI
array, resulting in enabling us to conduct contiguous high-resolution VLBI
observations. EAVN has produced various scientific accomplishments especially
in observations toward active galactic nuclei, evolved stars, and star-forming
regions. These activities motivate us to initiate launch of the ’Global VLBI
Alliance’ to provide an opportunity of VLBI observation with the longest
baselines on the earth.

###### keywords:

active galactic nucleus; astrometry; evolved star; star-forming region; very
long baseline interferometry

1 1 0 Academic Editor: https://doi.org/ Overview of the Observing System and
Initial Scientific Accomplishments of the East Asian VLBI Network (EAVN)
Overview of the Observing System and Initial Scientific Accomplishments of the
East Asian VLBI Network (EAVN) Kazunori Akiyama 1,2,3, Juan-Carlos Algaba 4,
Tao An 5,6, Keiichi Asada 7, Kitiyanee Asanok 8, Do-Young Byun 9,10, Thanapol
Chanapote 8, Wen Chen 11, Zhong Chen 5, Xiaopeng Cheng 9, James O. Chibueze
12,13, Ilje Cho 14, Se-Hyung Cho 9,15, Hyun-Soo Chung 9, Lang Cui 16,6, Yuzhu
Cui 17, Akihiro Doi 18,19, Jian Dong 5, Kenta Fujisawa 20,21, Wei Gou 5, Wen
Guo 5, Kazuhiro Hada 2,22, Yoshiaki Hagiwara 23,24,25, Tomoya Hirota 2,22,
Jeffrey A. Hodgson 26, Mareki Honma 2,22,27, Hiroshi Imai 28,29, Phrudth
Jaroenjittichai 8, Wu Jiang 5,6, Yongbin Jiang 5, Yongchen Jiang 5, Takaaki
Jike 2,22, Dong-Kyu Jung 9, Taehyun Jung 9, Noriyuki Kawaguchi 2, Dong-Jin Kim
1,30, Hyo-Ryoung Kim 9, Jaeheon Kim 9, Jeong-Sook Kim 31,32, Kee-Tae Kim 9,10,
Soon-Wook Kim 9, Motoki Kino 2,33, Hideyuki Kobayashi 2, Shoko Koyama 34,7,
Busaba H. Kramer 8,30, Jee-Won Lee 9, Jeong Ae Lee 9, Sang-Sung Lee 9,10, Sang
Won Lee 35, Bin Li 5, Guanghui Li 16, Xiaofei Li 16, Zhixuan Li 11, Qinghui
Liu 5, Xiang Liu 16,6, Ru-Sen Lu 5,6,30, Kazuhito Motogi 21,20, Masanori
Nakamura 36,7, Kotaro Niinuma 21,20, Chungsik Oh 9, Hongjong Oh 35, Junghwan
Oh 25,26, Se-Jin Oh 9, Tomoaki Oyama 2, Jongho Park 9,7, Saran Poshyachinda 8,
Hyunwook Ro 37,9, Duk-Gyoo Roh 9, Wiphu Rujopakarn 8, Nobuyuki Sakai 8, Satoko
Sawada-Satoh 38, Zhi-Qiang Shen 5,6, Katsunori M. Shibata 2, Bong Won Sohn
9,10,37, Boonrucksar Soonthornthum 8, Koichiro Sugiyama 8, Yunxia Sun 5, Mieko
Takamura 2,27, Yoshihiro Tanabe 39, Fumie Tazaki 40, Sascha Trippe 15, Kiyoaki
Wajima 9,10,*, Jinqing Wang 5, Na Wang 16,6, Shiqiang Wang 16, Xuezheng Wang
5, Bo Xia 5, Shuangjing Xu 9,5, Hao Yan 16, Wenjun Yang 16, Jae-Hwan Yeom 9,
Kunwoo Yi 15, Sang-Oh Yi 35, Yoshinori Yonekura 39, Hasu Yoon 35, Linfeng Yu
5, Jianping Yuan 16, Youngjoo Yun 9, Bo Zhang 5, Hua Zhang 16, Yingkang Zhang
5, Guang-Yao Zhao 14, Rongbing Zhao 5, Weiye Zhong 5, on behalf of the East
Asian VLBI Network Collaboration East Asian VLBI Network Collaboration,
Kiyoaki Wajima Akiyama, K.; Algaba, J.-C.; An, T.; Asada, K.; Asanok, K.;
Byun, D.-Y.; Chanapote, T.; Chen, W.; Chen, Z.; Cheng, X.; et al.
Correspondence<EMAIL_ADDRESS>
##  1 Introduction

The very long baseline interferometry (VLBI) technique enables us to obtain
extremely high angular resolution of milliarcsecond (mas) to microarcsecond
($\mu$as) scales Thompson (2017). In VLBI, higher angular resolution can be
achieved by employing longer baseline length between each telescope or by
conducting an observation at higher observing frequency. Longer baseline
lengths can be obtained with global-scale VLBI array such as, for example, the
European VLBI Network (EVN) Zensus (2015), the Long Baseline Array (LBA) in
Australia Edwards (2015), the Very Long Baseline Array (VLBA) Napier (1994),
and the Global Millimeter-VLBI Array (GMVA) Hodgson (2012), or can also be
achieved with so-called ‘space VLBI’ technique by employing one (or more)
satellite(s) and conducting a VLBI observation using a satellite and ground-
based radio telescopes. Space VLBI observations were realized by the VLBI
experiment with the Tracking and Data Relay Satellite System (TDRSS) Levy
(1986); Linfield (1989, 1990), the VLBI Space Observatory Programme (VSOP)
with the HALCA spacecraft Hirabayashi (1998, 2000), and the RadioAstron
project Kardashev (2013). On the other hand, one of the ultimate goals in the
angular resolution with higher observing frequency was accomplished by a
series of VLBI observations of a black hole shadow in our Galactic center Sgr
A∗ and a nearby active galaxy M87 at 230 GHz led by the Event Horizon
Telescope (EHT) project with its angular resolution of about 25 $\mu$as EHT
Collaboration (2019a, 2022a).

VLBI is an important tool for studying Galactic and extragalactic
astrophysical phenomena mainly on the basis of mas- to $\mu$as-scale images
Verschuur (1988), while it also has various applications in geophysics, such
as, for example, measurement of the Earth orientation parameter (EOP; Schuh
(2012); Nothnagel (2017) and references therein), determination of the
International Celestial Reference Frame (ICRF; Ma (1998); Fey (2015); Charlot
(2020); de Witt (2022)) and the International Terrestrial Reference Frame
(ITRF; Altamimi (2016) and references therein), and study on the Earth’s
tectonic plate (Sovers (1998) and references therein). Capabilities of these
applications can be developed in terms of an accuracy of measurement by
conducting high-cadence monitoring with high angular resolution by employing
intercontinental VLBI observations (e.g., Ghaderpour (2021)). These examples
thus indicate the importance of close international collaboration in VLBI.

Since the first success of a VLBI experiment in 1967 Broten (1967); Kellermann
(2001), a lot of domestic and international VLBI networks have been developed
and it is also the case in East Asian countries. In Japan, pioneering domestic
VLBI experiments were carried out using Kashima 34 m and Nobeyama 45 m
telescopes called as ‘the Kashima–Nobeyama Interferometer’ (KNIFE) at 22 and
43 GHz early in 1990s Miyoshi (1993). Nobeyama also joined the
intercontinental millimeter-wavelength VLBI experiment at 100 GHz Bååth
(1992). Series of VLBI experiments with KNIFE led launch of the first
Japanese-domestic VLBI array entitled ‘the Japanese VLBI Network’ (J-Net) in
1994 consisting of four telescopes (Kashima 34 m, Nobeyama 45 m, Mizusawa 10
m, and Kagoshima 6 m). J-Net made couple of new findings in its kinematics
with observations of water-vapor masers in and around star-forming regions and
protostars (e.g., Omodaka (1999); Imai (1999)) and evolved stars (e.g., Imai
(1997); Ishitsuka (2001)). J-Net also took an important role in a Galactic-
plane survey of extragalactic radio sources for a positional reference of
Galactic maser sources Honma (2000a) as a pathfinder of the VLBI Exploration
of Radio Astrometry (VERA) Honma (2000b); Kobayashi (2003). VERA is also a key
partner of ‘the (renewed) Japanese VLBI Network’ (JVN) Doi (2006) consisting
of several telescopes operated by universities and institutes in which each
telescope is located (Tomakomai 11 m operated by Hokkaido University Sorai
(2008); Hitachi 32 m and Takahagi 32 m operated by Ibaraki University Yonekura
(2016); Tsukuba 32 m operated by Geospatial Information Authority of Japan
(GSI) Kokado (2008); Kashima 34 m operated by the National Institute of
Information and Communications Technology (NICT), Usuda 64 m operated by Japan
Aerospace Exploration Agency (JAXA), Gifu 11 m operated by Gifu University,
and Yamaguchi 32 m operated by Yamaguchi University Fujisawa (2002))Tomakomai
11 m, Tsukuba 32 m, and Kashima 34 m telescopes have already been
deconstructed..

China participated in international VLBI experiments in 1980s. The first radio
telescope in China was constructed at Sheshan, a suburban area of Shanghai,
with its diameter of 25 m in 1987 Liang (1989). Sheshan is a telescope which
participated in EVN as a full member at an early stage of EVN in 1990. After
that many telescopes in China started participation in EVN, Nanshan 25 m in
1994 (main reflector of the telescope has been refurbished with its diameter
of 26 m in 2015) Zhang (2019), Kunming 40 m in 2009 Hao (2010), and Tianma 65
m in 2015 Bolli (2019). These telescopes constitute ‘the Chinese VLBI Network’
(CVN) Li (2008) with the longest baseline length of 3300 km between Sheshan
and Nanshan telescopes. In addition to astronomical VLBI observations, CVN is
utilized for determining an orbit of a series of Chang’E lunar explorers (Liu
(2021) and references therein) and the Tianwen-1 Mars explorer Yang (2022),
and for making a precise position determination of landers for these
spacecrafts Klopotek (2019).

The first radio telescope in Korea, Taeduk Radio Astronomy Observatory (TRAO)
14-m telescope, was constructed in 1986 with its major observing frequencies
of around and greater than 100 GHz Jeong (2019). TRAO 14 m conducted the first
astronomical VLBI observation with Nobeyama 45 m at 86 GHz in 2001 and
successful fringe detection of SiO $v$ = 1, $J$ = 2–1 maser emission was made
toward an evolved star VY Canis Majoris Shibata (2004). This experiment was an
important demonstration for the construction of a domestic VLBI array ‘the
Korean VLBI Network’ (KVN) from 2001 Wajima (2005); Lee (2011, 2014). One of
the most unique capabilities of KVN is simultaneous multifrequency data
receiving system for four observing frequencies (22, 43, 86, and 129 GHz),
which is the world’s first system equipped in a radio telescope Han (2013).
Antenna’s basic performance of KVN telescopes, including the pointing accuracy
and beam pattern measurement, was confirmed using a temporary receiver at 100
GHz before starting the open-use program of KVN in 2013 Kim (2011). KVN
started participation in EVN as an associate member in 2014.

As mentioned above, domestic VLBI arrays in each country in East Asia have
their own geographical uniqueness. Japan (VERA and JVN) shows a wide
distribution of a lot of radio telescopes in the country with the maximum
baseline length of 2300 km, whereas KVN is a compact array with the range of
its baseline length of 305–480 km, resulting in making very high-sensitive
array with shorter baseline lengths. Some of CVN telescopes locate at the
near-Easternmost and near-Westernmost parts of its very big country. The
features found in each VLBI array are complementary in terms of an angular
resolution and an imaging quality. Those situations motivate us to launch a
new international VLBI array in East Asia called ‘the East Asian VLBI Array’
(EAVN) Wajima (2016); An (2018). The very early concept of EAVN, however, was
discussed in the mm-VLBI workshop held in 2003 in China and the committee of
the East Asian VLBI Consortium was established at the 6th East Asian Meeting
on Astronomy (EAMA) in 2004 Inoue (2005). This consortium was transferred to a
task force for EAVN as one of working groups under the East Asian Core
Observatories Association (EACOA)https://www.eacoa.net/, accessed on 1
December 2022 in 2005 and started its practical activity from 2013 including
creation of a governing body of EAVN and evaluation of EAVN’s array
capability. Prior to launch of EAVN, bilateral VLBI array between Korea and
Japan, the KVN and VERA Array (KaVA; e.g., Matsumoto (2014); Niinuma (2014)),
has been started its operation in 2013 and its open-use program in 2014. KaVA
has conducted more than 650 observing sessions with the total observing time
of about 4700 hours by the middle of 2018. After completing a series of fringe
test observations Wajima (2016), evaluation of data calibration capability Cho
(2017), and pioneering scientific experiments with EAVN (e.g., Fujisawa
(2014); Sugiyama (2016)), open-use program of EAVN has been launched in the
second half of 2018.

This paper provides basic information on the overall observing system of EAVN
and reviews major scientific results obtained by EAVN to date. EAVN is a
unique VLBI array not only for filling a geographical gap between existing
VLBI arrays on the Earth but also for providing an opportunity to conduct an
observation using the multi-frequency simultaneous data reception system with
global-scale VLBI for the first time, both of which are described in Section
2. Astronomers all over the world can utilize these capabilities through the
EAVN open-use program, which is described in Section 3. Thanks to the unique
capabilities mentioned above, EAVN has made scientific findings toward active
galactic nuclei (AGNs), evolved stars, and star-forming regions, as well as
various Galactic and extragalactic radio sources, on the basis of high-cadence
VLBI monitoring and simultaneous data reception of two or more radio signals
at cm- to mm-wavelength regime, which are shown in Section 4. EAVN is also
grown as one of the key VLBI arrays in terms of future international
collaboration and those activities are summarized in Section 5. Note that part
of scientific results shown in this paper are derived with not only EAVN but
also bilateral VLBI collaboration between East Asian countries, such as KaVA.

##  2 Overview of the Array

This section describes an overview of the observation system of EAVN. The
system is always updated and its detailed specifications can be found in ‘the
EAVN Status Report’ on the EAVN websitehttps://eavn.kasi.re.kr/, accessed on 1
December 2022.

###  2.1 Telescope

Figure 1 shows the location map for each participating telescope in EAVN which
consists of 16 radio telescopes and three correlator sites located in each
East Asian country (Daejeon in Korea, Shanghai in China, and Mizusawa in
Japan). EAVN composes of several independent VLBI arrays and radio telescopes.
VERA consists of four telescopes (Iriki, Ishigakijima, Mizusawa, and
Ogasawara) and is a core VLBI array in Japan. Other Japanese telescopes except
Nobeyama are joint partners for JVN. Four Chinese telescopes (Kunming,
Nanshan, Sheshan, and Tianma) constitute CVN. KVN consists of three telescopes
(Seoul, Ulsan, and Jeju). Sejong 22-m telescope has been constructed in 2013
with space geodetic survey being as a main task, while the telescope joined
the open-use program for both KVN and EAVN in 2022.

Figure 1: Concept map of EAVN including 16 active telescopes listed in Table
1, three potential telescopes (Miyun 50 m in China, Gifu 11 m, and Usuda 64 m
in Japan), and three correlator sites in China, Japan, and Korea. Photographs
of each radio telescope and correlator site are overlaid on ‘the Blue Marble’
image (credit of the ground image: NASA Earth Observatory
(https://earthobservatory.nasa.gov/, accessed on 1 December 2022).

Figure 2 shows examples of the spatial frequency coverage (($u$, $v$)
coverage) of EAVN at each observing frequency. The maximum baseline length of
EAVN is composed of Nanshan and VERA-Ogasawara stations with its baseline
length of 5078 km, corresponding to an angular resolution of 1.82, 0.55, and
0.28 mas at 6.7, 22, and 43 GHz, respectively.

Figure 2: Examples of the ($u$, $v$) coverage for an EAVN observation with
full array configuration at 6.7 (upper panels), 22 (middle panels), and 43 GHz
(lower panels) with the source’s declination of $+60^{\circ}$ (left panels),
$+20^{\circ}$ (center panels), and $-20^{\circ}$ (right panels). $\lambda$
represents the observing wavelength. Total observing duration of 10 hours and
the antenna’s lower elevation limit of $15^{\circ}$ are assumed for all cases.

Table 1 indicates basic observing parameters for participating telescopes of
EAVN. EAVN is operated using 11, 12, and 11 telescopes at three frequency
bands, 6.7, 22, and 43 GHz, respectively. The number of participating
telescopes shown above includes Nanshan at 6.7 and 43 GHz in which performance
of the cooled receiver at these frequencies is under evaluation. The system
equivalent flux density (SEFD) shown in Table 1 is defined as follows;

$SEFD=\frac{2k_{\rm B}T_{{\rm sys},\nu}}{A_{{\rm e},\nu}},$ (1)

where $k_{\rm B}$ is Boltzmann’s constant, $T_{{\rm sys},\nu}$ and $A_{{\rm
e},\nu}$ are the system noise temperature and the effective aperture area at
an observing frequency $\nu$, respectively.

Table 1: Participating arrays and telescopes in EAVN.

Country | Array/Telescope |  Enrollment a | Diameter [m] | SEFD b [Jy]
---|---|---|---|---
|  |  |  | 6.7 GHz | 22 GHz | 43 GHz
China | Kunming | 2021 |  40 |  307 | – | –
| Nanshan | 2018 |  26 | *** d |  364 | *** d
| Sheshan | 2021 |  25 | 1740 | – | –
| Tianma | 2018 |  65 |  55 |  100 |  122
Japan | Hitachi | 2021 |  32 |  158 | – | –
| Nobeyama | 2018 |  45 | – |  285 |  655
| Takahagi | 2020 |  32 | – |  343 | –
| VERA | 2018 | $4\times 20$ | 2155 | 2110 | 4393
| Yamaguchi | 2021 |  32 |  286 | – | –
Korea | KVN | 2018 | $3\times 21$ | 4241 c | 1328 | 1992
| Sejong | 2022 |  22 | – | 1231 | 2055

Note. a Year of enrolment in EAVN. b The system equivalent flux density (SEFD)
for VERA and KVN is a value for a single antenna. c Only Ulsan station can
conduct an observation at 6.7 GHz. d The system performance is under
evaluation.

###  2.2 Overview of Data Acquisition Flow

Figure 3 shows a schematic diagram of data flow of an EAVN observation. Radio
signal received is digitized and formatted using digital signal processing
(DSP) system, and recorded with a disk storage at each telescope. Recorded
data are transferred to the Korea–Japan Correlation Center (KJCC), in which
the main correlator for EAVN is equipped, via FTP. Data at Nobeyama and three
JVN stations (Hitachi, Takahagi, and Yamaguchi) are recorded by the OCTADISK
recorder Oyama (2012) and those data are once transferred to the digital
baseband converter (DBBC) at Mizusawa VLBI Observatory to perform digital
filtering before sending them to KJCC. Details of this process are mentioned
in Section 2.4, while note that the process becomes unnecessary for future
EAVN observations with the data recording rate of 2 Gbps or higher.

Figure 3: Schematic diagram of data flow of current EAVN observations. ‘DAS’
represents data acquisition system.

###  2.3 Frontend

Receiver noise temperature of EAVN telescopes for each observing frequency is
shown in Table 2. Cooled HEMT receiver for all observing frequencies of EAVN
is equipped at all EAVN telescopes, except a 6.7 GHz receiver at all VERA
stations, KVN-Ulsan, and Sheshan being operated under a room temperature. Data
reception with dual polarization is available for all receivers except a 6.7
GHz receiver at VERA and KVN-Ulsan and a 43 GHz receiver at Nobeyama being
available for only left-hand circular polarization.

One of the most unique capabilities of EAVN is a multi-frequency data
receiving system in which an observation data can be obtained at two or more
frequencies simultaneously. Currently such system is installed at nine
stations, KVN (22, 43, 86, and 129 GHz), VERA (22 and 43 GHz), Sejong (22 and
43 GHz), and Nobeyama (22, 43, and 86 GHz). This unique capability enables us
to obtain better baseline sensitivity by employing the ‘frequency phase
transfer’ (FPT) technique Algaba (2015); Zhao (2018), and to determine a
relative position between target and reference source by employing the
‘source-frequency phase-referencing’ (SFPR) technique Dodson (2014); Zhao
(2019). Details of the system installed at Nobeyama, the HINOTORI (Hybrid
Installation Project in Nobeyama, Triple-band Oriented) system, is provided by
another paper Okada (2020).

It is also important not only to equip a high-performance receiver but to
provide a source catalog optimized for an observation at 22 GHz or higher,
which are main observing frequency with EAVN, in order to raise the
detectability of target sources and to provide an appropriate environment for
calibration of the data obtained by EAVN since VLBI calibrators at higher
frequency of greater than 20 GHz are less known compared to those at lower
frequency. A series of VLBI calibrator survey containing more than 1500
extragalactic radio sources with its declination of higher than
$-32.5^{\circ}$ are carried out using KVN at 22 and 43 GHz, resulting in
providing an important information for calibration of the flux density with
KaVA and EAVN Lee (2017).

Table 2: Performance of the receiver and the data acquisition system at each
EAVN station.

Array/Telescope |  $T_{\rm RX}$ | DSP | Recorder
---|---|---|---
| 6.7 GHz | 22 GHz | 43 GHz |  |
Kunming |  20 | — | — | DBBC, CDAS | Mark 5B, Mark 6
Nanshan | *** a | 15 | *** a | DBBC, CDAS | Mark 5B, Mark 6
Sheshan | 100 | — | — | DBBC, CDAS | Mark 5B, Mark 6
Tianma |  20 | 35 |  50 | DBBC, CDAS | Mark 5B, Mark 6
Hitachi |  20 | — | — | ADS3000+ | OCTADISK
Nobeyama | — | 85 | 110 | ADS3000+ | OCTADISK
Takahagi | — | 30 | — | ADS3000+ | OCTADISK
VERA | 100 | 50 |  90 | VERA, OCTAD | OCTADISK
Yamaguchi |  20 | — | — | ADS3000+ | OCTADISK
KVN | 300 | 40 |  50 | VERA, OCTAD | Mark 5B, Mark 6, OCTADISK
Sejong | — | 30 |  90 | DBBC | Mark 6

Note. a The system performance is under evaluation.

###  2.4 Digital Signal Processing and Recorder

Table 2 shows an equipment for DSP and a recorder used at each telescope. EAVN
is currently operated with the data recording rate of 1024 Mbps, corresponding
to the total bandwidth of 256 MHz for 2-bit sampling. Due to a difference in
the historical background of each VLBI array constituting EAVN, DSP and
recorder installed are somewhat different.

To keep compatibility of digital data sequence between KVN and VERA for
conducting joint observations with KaVA, similar type of digital data
acquisition system Iguchi (2005) is introduced at KVN stations Oh (2011). CVN
telescopes, on the other hand, introduce the DBBC system partly because they
have started participation in EVN prior to EAVN. Another type of DSP system,
the Chinese VLBI Data Acquisition System (CDAS), is developed uniquely in
China and installed at all CVN telescopes Zhang (2010).

Observation data at four Japanese telescopes (Hitachi, Nobeyama, Takahagi, and
Yamaguchi) are digitized by the ADS3000+ sampler Takefuji (2010) and recorded
using the OCTADISK system Oyama (2016), both of which were developed uniquely
in Japan, while ADS3000+ does not have a same digital filter mode to that of
VERA DAS and DBBC. Data obtained at these telescopes are thus recorded by
OCTAD with the data output rate of 2 Gbps and transferred to the Mizusawa VLBI
Observatory to convert the data output rate (1 Gbps) and the channel
assignment of the data suitable for correlation with other telescopes’ data.
Note that EAVN is planning to move to regular operation with the data
recording rate of 2 Gbps or higher in the near future, data processing at
Mizusawa, shown in Figure 3, will thus become unnecessary.

###  2.5 Correlator

EAVN is operated using correlators located in China, Japan, and Korea, as
shown in Table 3. All correlators are utilized for examining data quality and
various observing modes of EAVN, whereas all observation data on the open-use
program are correlated using the Daejeon Correlator, which is hardware-based,
at the headquarters of Korea Astronomy and Space Science Institute (KASI).
DiFX correlator at Daejeon and Shanghai is mainly used for correlation of data
obtained by KVN and CVN, respectively. The ’SoftCos’ correlator was assembled
by National Astronomical Observatory of Japan (NAOJ) Oyama (2016) on the basis
of the GICO3 software correlator as a main engine which was developed by NICT
Kimura (2002).

Table 3: Correlators used for EAVN observations.  Site |  Correlator |  Reference
---|---|---
Daejeon (KASI) |  Daejeon Correlator, DiFX |  Lee (2015); Deller (2007)
Shanghai (SHAO) |  DiFX |  Deller (2007); Jiang (2018)
Mizusawa (NAOJ) |  SoftCos |  Kimura (2002); Oyama (2016)

Table 4 represents basic specifications of the Daejeon Correlator. Results of
performance evaluation of the Daejeon Correlator can be found in the
literature Lee (2015). The Daejeon Correlator has 16 data input ports, named
as the antenna unit, corresponding to a single polarization (right-hand
circular polarization (RHCP) or left-hand circular polarization (LHCP)) at a
single frequency for one telescope. This results in the maximum number of
telescopes for data correlation at a single path being 16 for an observation
with a single polarization at a single frequency, or 8 for an observation with
dual polarization at a single frequency.

Table 4: Basic specifications of the Daejeon Correlator.  Maximum number of stations |  16
---|---
Maximum delay compensation |  $\pm 36000$ km
Maximum fringe tracking |  1.075 kHz
Maximum data input rate |  2048 Mbps
Maximum number of spectral points per IF channel |  8192
Integration time |  25.6 ms–10.24 s

##  3 Summary of the EAVN Open-Use Program

As mentioned in Sections 1 and 2, a domestic VLBI array is operated in each
East Asian country, JVN and VERA in Japan, CVN in China, and KVN in Korea, and
the open-use program is also carried out by each VLBI array. Although each
VLBI array has its own advantage in both scientific and technical point of
view, it is a natural expansion to constitute a larger VLBI network by
combining adjacent domestic VLBI arrays. On the basis of scientific and
technical accomplishments with KaVA and initial performance evaluation with
EAVN shown in Section 1, the EAVN open-use program has been started from the
second half of 2018.

EAVN consists of 16 telescopes in East Asian countries, while the observation
time for EAVN to be provided by each telescope time is different between each
other, as shown in Table 5. Proposers can choose any combination of an array
configuration so that the configuration becomes a suitable one for their
scientific purposes, although KaVA shall participate in all EAVN observations
in principle.

Table 5: Total observation time of EAVN provided by each telescope for the open-use program in the 2022B semester.  Country |  Array/Telescope |  Observing Time for EAVN [h]
---|---|---
China |  Kunming |  50
|  Nanshan |  150
|  Tianma/Sheshan |  150
Japan |  VERA |  500
|  Hitachi |  50
|  Nobeyama |  48
|  Takahagi |  50
|  Yamaguchi |  50
Korea |  KVN |  500
|  Sejong |  100

Table 6 represents the total observing time of EAVN for each semester. EAVN
offers opportunities of a VLBI observation to astronomers all over the world
from September 1 to June 15 with 2.5 months of the system maintenance, while
the term of observation shown above is divided into two semesters from
September 1 to January 15 next year (the ‘B’ semester) and from January 16 to
June 15 (the ‘A’ semester). As shown in Table 6, EAVN is providing the
observing time of about 1100 hours during two adjacent semesters. The
observing session of EAVN is basically allocated every two weeks with the
total observing term of one week each. EAVN thus provides opportunities of
VLBI observations with the total observing duration of two weeks per month
from September to June, while note that EAVN observations are not always
allocated to whole observing term tightly since the total observing time
provided by each telescope is limited, as shown in Table 5, and target sources
tend to concentrate on a particular right ascension range. These result in the
total observing time for a year ($\sim$ 1100 h) shown in Table 6 being much
smaller than the total potential observing time ($\sim$ 3000 h) of EAVN.

Table 6: Total observing time of EAVN for each semester.

Semester | GOT a [h] | Session | ToO b [h] | Session | PE c [h] | Session | Total Time [h] | Total Session
---|---|---|---|---|---|---|---|---
2018B |  487.5 |  78 | — | — |  32.0 |  2 |  519.5 |  80
2019A |  439.5 |  66 |  116.0 | 16 |  75.0 |  7 |  630.5 |  89
2019B |  472.0 |  70 |  0.0 |  0 |  55.0 |  6 |  527.0 |  76
2020A |  539.5 |  79 |  24.0 |  3 |  63.0 |  6 |  626.5 |  88
2020B |  327.5 |  58 |  44.0 | 10 |  71.5 | 10 |  443.0 |  78
2021A |  581.0 |  94 |  16.0 |  2 |  26.5 |  5 |  623.5 | 101
2021B |  483.5 |  72 |  15.5 | 10 |  49.0 |  7 |  548.0 |  89
2022A |  522.0 |  84 |  13.0 | 11 |  34.0 |  2 |  569.0 |  97
Total | 3852.5 | 601 | 228.5 | 52 | 406.0 | 45 | 4481.0 | 698

Note. a General observing time. b Target of opportunity. c Performance
evaluation.

As of the 2022B semester, EAVN open-use program provides opportunities of
observations at 6.7, 22, and 43 GHz with single polarization and with the data
recording rate of 1 Gbps. Dual-polarization capability is opened for the open-
use program from the 2023A semester. EAVN also provides opportunities of the
dual-frequency simultaneous data recording mode at 22 and 43 GHz for 9
telescopes (KaVA, Nobeyama, and Sejong), as mentioned in Section 2.3. The
‘multi-frequency data recording mode’, on the other hand, is also offered as
one of the most distinct modes of EAVN in which KVN records the data of not
only 22 and 43 GHz but also 86 and 129 GHz simultaneously. This mode thus
enables us to compare the data at different frequencies directly.

From the 2019A semester, EAVN offers a director’s discretionary time for
target of opportunity (ToO) observations to track transient or unexpected
phenomena swiftly. As of the 2022A semester, we have conducted 52 ToO sessions
with the total observing time of 228.5 hours with maser bursts associated with
star-forming regions, Galactic black hole binary, and gamma-ray-emitted
extragalactic objects being major targets.

Part of EAVN observations were conducted with non-EAVN telescopes in a
framework of EAVN open-use program. As of the 2022A semester, following non-
EAVN telescopes have participated in EAVN observation one or more sessions;
Hobart 26 m, Tidbinbilla 70 m (Australia), Effelsberg 100 m (Germany), Noto 32
m, Medicina 32 m, Sardinia 64 m (Italy), Badary 32 m, Svetloe 32 m,
Zelenchukskaya 32 m (Russia), and Yebes 40 m (Spain). These are also a good
demonstration for promoting global VLBI activities, such as the EATING VLBI or
the Global VLBI Alliance, both of which are explained in Section 5.

##  4 Scientific Accomplishments by EAVN

Thanks to its high angular resolution at multiple frequencies and plenty of
observing time of more than 1100 hours in a year, EAVN is a suitable array to
conduct high-cadence and long-term VLBI monitoring toward particular targets.
To make full use of the capabilities of EAVN for deriving initial scientific
results and for considering future potential science cases with EAVN, three
science working groups (SWGs), AGNs, evolved stars, and star-forming regions,
were organized in 2011 before launching KaVA’s operation and the large
observing programs led by these SWGs have been conducted using KaVA and EAVN
from 2015 to 2019 by providing about 50% of whole observing time of KaVA/EAVN.
Galactic Astrometry SWG was also organized in 2017 for promoting astrometric
VLBI observations to investigate dynamics of our galaxy. In this section, let
us introduce overview of activities and scientific achievements to date led by
these SWGs.

###  4.1 Active Galactic Nuclei

Key issues in AGN science are to investigate the accretion process of the
matter onto supermassive black holes (SMBHs), and physical processes on the
mechanism of jet formation and its collimation and acceleration in the
vicinity of SMBHs. To approach these important subjects, EAVN AGN SWG is
focusing on two targets, our Galactic center Sagittarius A∗ which contains a
SMBH with its BH mass of $\approx 4.0\times 10^{6}M_{\odot}$ (e.g., Do (2019);
EHT Collaboration (2022a)), and a nearby radio galaxy M87 with its BH mass of
$\approx 6.5\times 10^{9}M_{\odot}$ (e.g., Gebhardt (2011); EHT Collaboration
(2019b)), both of which are main targets of the EHT project.

The first-epoch observation of M87 was conducted in 2013 using KaVA as a pilot
experiment of the large program of KaVA and the monitoring program was
continued with the observing interval of two or three weeks. The first
10-epoch observations in 2013 and 2014 revealed superluminal motions and a
trend of gradual acceleration of multiple jet components with its physical
scale of 0.1–2 pc, corresponding to 140–2800 Schwarzschild radii Hada (2017).
M87 has been continuously monitored through a large program of KaVA and EAVN
since then and those observations revealed the kinematics of the M87 jet on
the basis of a detailed velocity profile of the apparent jet speed ($v_{\rm
app}$) from the vicinity of the jet base ($\approx 0.5$ mas) with $v_{\rm
app}\approx 0.3c$ to $v_{\rm app}\approx 2.7c$ at $\approx 20$ mas from the
jet base (Park (2019), see also Figure 4). EAVN observations also
reconstructed the fine structure of both approaching and receding jets of M87
with the image dynamic range of around 4000 at both 22 and 43 GHz Cui (2021).
EAVN observing campaign of M87 in 2017 spring was carried out as part of
quasi-simultaneous multi-wavelength campaign from radio to TeV $\gamma$-ray,
which was conducted concurrently with the EHT campaign. EAVN provided clear
images showing knotty jet structure EHT Collaboration (2021), as well as the
flux information at cm-wavelengths EHT Collaboration (2019b). Dense monitoring
of M87 with five consecutive month in 2016 by KaVA at 22 and 43 GHz quasi-
simultaneously could also estimate the magnetic field strength along the jet
precisely at an angular scale of 2–10 mas (or 900–4500 Schwarzschild radii for
M87) from the core on the basis of detailed determination of the spectral
index between two observing frequencies Ro (2022).

Figure 4: (Left) CLEAN images of M87 obtained with KaVA at 22 GHz. The images
are obtained at eight epochs from 2016 February 26 to June 15. Circular
Gaussian components obtained by the modelfit analysis are overlaid on the
images. (Right) Apparent jet speed as a function of projected distance from
the core of M87. Symbols shown with magenta and blue colors represent the data
obtained at 22 and 43 GHz, respectively. Adapted from the work in Park (2019)
with permission from the American Astronomical Society (AAS).

Another major target of the EHT project, Sgr A∗, is also one of the best
laboratories for investigating mass accretion processes and time variability
of physical properties in the vicinity of SMBH thanks to its proximity. EAVN
AGN SWG is conducting a long-term monitoring of Sgr A∗, including an intensive
observation program during the EHT campaign every year as another target of
the large program of KaVA and EAVN. Radio emission from Sgr A∗ is affected by
interstellar scattering effect, resulting in scatter-broadening of its size as
the square of the observing wavelength. This becomes a disadvantage of an
observation at a frequency of, e.g., 22 GHz or lower. Detection of Sgr A∗, on
the other hand, is also difficult at 43 GHz or higher due to its lower flux
density, although Sgr A∗ still has a rising spectrum at 43 GHz. In this point,
KaVA is a suitable array for observing Sgr A∗ since KaVA can provide a denser
($u$, $v$) coverage corresponding to the significant radio structure thanks to
its compact array configuration with the baseline length of 300–2300 km.
Moreover, KaVA has a better angular resolution in the north–south direction at
43 GHz than that of VLBA, thus KaVA can estimate the minor axis size of Sgr A∗
more precisely. On the basis of these advantages, results of precise size
measurements of Sgr A∗ with its major-axis angular size of $704\pm 102$
$\upmu$as and $300\pm 25$ $\upmu$as at 22 and 43 GHz, respectively, can be
obtained by KaVA and EAVN Johnson (2018); Cho (2022). Those results enable us
to compare with the accretion flow dominated model in terms of the
distribution of thermal/non-thermal electron and the viewing angle of Sgr A∗,
both of which can be determined by a theoretical prediction. EAVN also
provided results of the flux density measurement of Sgr A∗ at 22 and 43 GHz
through the multi-wavelength observation campaign from radio to X-ray, which
was conducted concurrently with the EHT campaign in 2017 EHT Collaboration
(2022b).

KaVA/EAVN is participating in an international multi-frequency observing
campaign of AGNs besides the EHT campaign mentioned above. Multi-frequency
monitoring of a gravitationally lensed blazar B0218+357, for example, was
performed during 2016 and 2020 from radio to GeV $\gamma$-ray and KaVA
provided clear images and flux density information at 22 and 43 GHz for the
monitoring, in which two lensed images show clear core-jet structure although
a distinct jet motion cannot be seen Hada (2020); Acciari (2022).

KaVA/EAVN is carrying out individual proposal-based observations toward
various AGNs as well. A series of KaVA observations of a nearby bright radio
galaxy 3C 84 have revealed the existence and its structure of a circumnuclear
plasma disk Wajima (2020) and strong jet–cloud interaction on pc-scales Kino
(2018, 2021). Short- and long-term monitoring observations with KaVA/EAVN show
new findings in physical properties of the jet structure and its motion toward
several AGNs, e.g., confirming a clear connection between $\gamma$-ray flare
and the ejection of a new jet component in the flat spectrum radio quasar
4C+21.35 at 22 and 43 GHz Lee (2019), determining the rebrightened regions in
the jet of a nearby radio galaxy NGC 315 at 22 GHz, which is similar to M87
Park (2021), detecting a misaligned jet component in a transient GeV
$\gamma$-ray blazar J1544$-$0649 with its maximum viewing angle of 7.4∘ at 6.7
GHz suggesting the misaligned blazar scenario for this source Shao (2022), and
finding a tight linear relation between accretion luminosity and jet
luminosity in a sample of the Swift BAT (Burst Alert Telescope)-detected local
AGNs with their redshift of smaller than 0.05 Baek (2019). It should also be
notable that EAVN is conducting observations toward extragalactic radio
sources which are non-traditional VLBI targets, such as a cluster galaxy Baek
(2022) and a gamma-ray burst object An (2020), thanks to an excellent imaging
sensitivity of EAVN although the gamma-ray burst object 190114C could not be
detected with EAVN, maybe due to rapid decay of the radio flux.

###  4.2 Evolved Stars

Evolved stars tend to show strong maser emission of the hydroxyl radical (OH),
water vapor (H2O), and silicon monoxide (SiO) at the circumstellar envelope in
the asymptotic giant branch (AGB) and post-AGB phases. Those masers are
associated with gas and dust clumps around an evolved star and, thus, a useful
tool to reveal the mass loss processes of evolved stars, although its motion
is very complex. To conduct high-cadence VLBI monitoring of a sample of
evolved stars for approaching those issues, EAVN Evolved Stars SWG has
launched the ‘ESTEMA’ (Expanded Study on Stellar Masers) project using KaVA
Oyadomari (2016) and the ‘(renewed) ESTEMA’ (EAVN Synthesis of Stellar Maser
Animations) project using EAVN. The most distinguishing strategy of ESTEMA for
approaching various scientific objects is to conduct (quasi-)simultaneous
observation at 22, 43, 86, and 129 GHz, corresponding to maser emission H2O
maser at 22 GHz, and SiO $J$ = 1–0, 2–1, and 3–2 maser at 43, 86, and 129 GHz,
respectively, thanks to a unique capability of multi-frequency simultaneous
data reception at KaVA, Sejong, and Nobeyama.

ESTEMA aims to produce animations of circumstellar H2O and SiO masers toward
10 variable stars by dense monitoring during its pulsation period ($P$) of 1–3
years to investigate three-dimensional kinematics of maser gas clumps, while
they are focusing on two objects, a Mira variable BX Cam ($P=486$ days) and a
red supergiant NML Cyg ($P\sim 1000$ days), as a first step. Prior to launch
of ESTEMA, pilot observations of several evolved stars showing distinct maser
features at different maser species were conducted using KaVA. For one of
those samples, an OH/IR star WX Psc, single-epoch KaVA observation could
reconstruct superposed images of the $v=1$ and 2, $J$ = 1–0 SiO masers with
keeping its relative position by employing astrometric analysis. Those images
clearly show a typical ring-like structure in both SiO masers and the
difference in their distribution with $v=2$ masers being located in an inner
region of about 0.5 mas compared to $v=1$ masers (Yun (2016), see also Figure
5). As for one of the major targets of ESTEMA, BX Cam, long-term monitoring
with EAVN from 2018 May to 2021 June, including 37 epochs clearly described
three-dimensional kinematics of H2O masers associated with the circumstellar
envelope with providing the animation of H2O masers. These results help us to
understand the mechanism of the propagation of shock waves around evolved
stars Xu (2022).

Long-term monitoring of those masers will be a good tool for revealing the
physical environments of the circumstellar environment and the pumping
mechanism of SiO maser. Simultaneous observations of H2O and SiO masers at
four frequencies (22, 43, 86, and 129 GHz) toward several evolved stars, which
are not included in the target of ESTEMA, have also conducted using KVN, with
EAVN Evolved Stars SWG being collaborated (e.g., Yoon (2018); Kim (2019); Yang
(2020)).

Figure 5: (Left) Spatial distribution of the SiO masers around WX Psc on 2012
April 1. Blue and red colors represent the $v=1$ and 2, $J$ = 1–0 SiO maser.
(Right) Superposed map of SiO masers for $v=1$ and 2, $J$ = 1–0 in blue and
red colors, respectively. Dashed circles show the average size of the ring-
like structure for each SiO maser spot. Adapted from the work in Yun (2016)
with permission from AAS.

Some of evolved stars in AGB or post-AGB phases are identified as showing fast
bipolar outflows traced by H2O masers, called water fountain sources (e.g.,
Imai (2002)). EAVN Evolved Stars SWG is carrying out follow-up KaVA
observations of a sample of ESTEMA targets in which high-velocity components
of H2O masers are detected by Nobeyama 45 m telescope in order to confirm the
spatial distribution of H2O masers. They found that H2O masers in IRAS
18286$-$0959, one of the ESTEMA targets, show a symmetric spatial distribution
with extremely high expansion velocities of greater than 200 km s-1,
suggesting that H2O masers are associated with the bipolar outflow Imai
(2020).

###  4.3 Star-Forming Regions

To reveal the process of massive star formation with its mass of greater than
8$M_{\odot}$ is an important issue for understanding basic astrophysics, such
as formation of heavy elements and galaxy formation, while it is still poorly
understood mainly because of large distance from us and relatively short
evolutionary timescale. Large number of high-mass young stellar objects
(HMYSOs) are known to associate strong emission of class I and II methanol
(CH3OH) masers at 44 and 6.7 GHz, respectively, and water vapor (H2O) maser at
22 GHz. Those masers are associated with various scales of structures in
HMYSOs, such as outflows, disks, and H II regions, VLBI is thus one of the
best tools for investigating the process related to high-mass star formation
since VLBI can directly obtain images of HMYSOs and trace three-dimensional
motions of masers.

EAVN Star-Forming Regions SWG has started large program for long-term
monitoring of H2O masers at 22 GHz and class I CH3OH masers at 44 GHz for a
sample of HMYSOs containing 25 primary candidates Kim (2018). The project was
conducted mainly using KaVA since target masers tend to resolve with longer
baseline lengths by EAVN than that by KaVA, instead the project contains
follow-up observations with JVN, VERA, JVLA (Karl G. Jansky Very Large Array),
and ALMA (Atacama Large Millimeter/submillimeter Array). Prior to launch of
EAVN, imaging test observations of EAVN (VERA + JVN + Sheshan) at 6.7 GHz had
been conducted toward 36 HMYSOs which emit class II CH3OH masers in 2010 and
2011. These observations provide us a basic information on maser distribution
and its morphology (elliptical, arched, linear, paired, and complex) for a
sample of HMYSOs Fujisawa (2014); Sugiyama (2016). It is also notable that a
KaVA observation of a high-mass star-forming region IRAS 18151$-$1208 detected
class I CH3OH methanol masers with the baseline lengths of shorter than 650
km, suggesting that the maser component has an extended structure with its
angular size of larger than 4 mas or $\sim$12 AU in the linear size of the
source Matsumoto (2014). This is the first detection of class I CH3OH maser at
44 GHz with VLBI, indicating that KaVA is a powerful instrument to study
physical properties of HMYSOs.

Another good example for studying the process of high-mass star formation with
KaVA and ALMA was demonstrated toward a high-mass cluster-forming region
G25.82$-$0.17, detecting continuum emission showing a velocity gradient
indicating a rotating disk by ALMA at 230 GHz and strong H2O maser sources
associated with the root of bipolar outflows in the continuum source by KaVA
at 22 GHz. These results suggest a similar star-formation process between low-
mass and high-mass stars (Kim (2020), see also Figure 6). KaVA also provides
precise measurements of proper motion of H2O masers associated with bipolar
outflows toward several HMYSOs and high-mass star-forming regions (e.g.,
Chibueze (2021); Trinidad (2021)) thanks to a capability of dense monitoring
by KaVA. These results are of help to us for understanding basic physical
processes of HMYSOs.

Figure 6: Spatial distribution of H2O masers obtained with KaVA, which are
shown in colored points. Blue and red contours indicate the integrated
intensity map of the SiO 5–4 line and and gray scale represents the continuum
emission at 230 GHz, both of which were obtained with ALMA Band 6. Adapted
from the work in Kim (2020) with permission from AAS.

###  4.4 Astrometry

Precise astrometric measurement for detecting parallax is one of the most
distinct features in VLBI thanks to its high angular resolution Reid (2014).
In East Asia, various studies on measurement of distance to Galactic H2O and
SiO maser sources, and the three-dimensional structure and dynamics of the
Milky Way have been accomplished (VERA (2020) and references therein) based on
astrometric measurements with VERA, which is a dedicated VLBI array for
Galactic astrometry with the unique capability of the dual-beam system Honma
(2003). On the basis of accomplishments made mainly by VERA, EAVN Galactic
Astrometry SWG examined the capability of astrometric measurements and EAVN
opened the astrometry mode for measuring parallax using KaVA 7 telescopes for
the open-use program in the 2019B semester. Station coordinates of KaVA have
been monitored by 22-GHz geodetic VLBI observations since 2014 and individual
baseline lengths are determined with an accuracy of $\sim$3 mm Jike (2018); Xu
(2021). For atmospheric calibration of KaVA astrometry mode (i.e., estimation
of zenith wet excess path delays), the data provided by GPS and JMA (Japan
Meteorological Agency meso-scale analysis data for numerical weather
predictionhttps://www.jma.go.jp/jma/jma-eng/jma-
center/nwp/outline2019-nwp/index.htm, accessed on 1 December 2022) are
applied. Those examinations result in suppressing the tropospheric zenith
delay residual ($c\Delta\tau_{\rm{trop}}$) of less than $\sim$2 cm Nagayama
(2015).

To demonstrate the capability of the astrometry mode, Galactic trigonometric
parallax measurement of 22 GHz H2O maser sources associated with a famous
star-forming region W3(OH) was conducted with KaVA. The resultant parallax is
measured to be $0.460\pm 0.035$ mas, which is consistent with a result of
$0.489\pm 0.017$ mas measured by VLBA Hachisuka (2006) within the margin of
error (see Figure 7). More than 10 proposals with the total observing time of
greater than 300 hours have already been accepted for astrometric study with
KaVA, which covers a wide range of scientific topics including studies of the
structure of extreme outer Galaxy Sakai (2022), microquasars, jet swinging in
AGN, astrometry of radio stars, and geodetic VLBI observations with
simultaneous data reception at 22 and 43 GHz.

Figure 7: Results of parallax measurement and proper-motion fitting for a
star-forming region W3(OH). The position offsets of W3(OH) are plotted with
respect to the background quasar J0244+6228 in the east ($\alpha$cos$\delta$)
and north ($\delta$) directions as a function of time. For clarity, the north
direction data is plotted offset from the east direction data. (Left) The
best-fit models in the east and north directions are shown as continuous and
dashed curves, respectively. (Right) Same as the left panel, but the component
of proper motion is removed.

##  5 Future Growth into the Global VLBI

As mentioned in Section 1, higher angular resolution can be obtained by
employing longer baseline length between each telescope. It is also important
to expand the array capability of EAVN by, for example, inviting telescopes
and existing VLBI arrays located near and far from EAVN. International
collaboration in VLBI is thus a natural extension to obtain better array
capability and we enhance international collaboration in VLBI research with
non-EAVN facilities as shown below as examples.

###  5.1 Collaboration with Southeast Asian Countries

Radio telescope facilities and the radio astronomy community are spreading not
only in East Asia but also in Southeast Asia. In Thailand, construction of 40
m radio telescope (Thai National Radio Telescope; TNRT) has been completed in
Chiang Mai led by National Astronomical Research Institute of Thailand (NARIT)
and will be operational at 1.0–1.8 and 18.0–26.5 GHz in 2022, while an
observable frequency range will be expanded to be 300 MHz–115 GHz in the
future Jaroenjittichai (2018). Key science topics to be achieved with TNRT are
summarized in the white paper Jaroenjittichai (2022). Two new radio telescopes
with their diameter of 13 m are also under construction in Chiang Mai and
Songkhla, both of which conform to the specifications of the VLBI Global
Observing System (VGOS). For another 32 m telecommunication antenna at the
campus of CAT Telecom Public Company Limited in Chonburi, the conversion into
a radio telescope is also being considered. These telescopes will constitute
the Thai VLBI Network (TVN) with the longest baseline length of 1330 km.
Indonesia plans to convert a 32 m telecommunication antenna into radio
telescope. A new 7 m radio telescope is operated by the Universiti Pendidikan
Sultan Idris (UPSI)–Universiti Malaya (UM) Radio Astronomical Observatory in
Malaysia. These are potential telescopes of EAVN for developing array
capabilities thanks to its unique location.

For future development in radio astronomy and VLBI in Southeast Asian
countries and for close collaboration in these fields between EAVN and those
countries, a new memorandum of understanding (MoU) for EAVN has been concluded
on 2021 October 1 by inviting NARIT, as well as Yunnan Observatories (YNAO) in
China, and National Geographic Information Institute (NGII) in Korea, as a new
member institute of EAVN. This MoU is a renewal of existing one which had been
concluded on 2018 September 6 between KASI, NAOJ, Shanghai Astronomical
Observatory (SHAO), and Xinjiang Astronomical Observatory (XAO), and will
accelerate further practical collaboration in wide fields of VLBI sciences in
Asian region.

###  5.2 EATING VLBI

To further connect EAVN to global baselines, let us introduce an example of a
practical collaborative VLBI project entitled ‘East Asia To Italy: Nearly
Global VLBI’ (EATING VLBI). Project overview and initial scientific results of
EATING VLBI is provided in another review paper in this Special Issue.

EATING VLBI was launched as a trilateral collaborative project between Italy,
Japan, and Korea in 2012 and became a regular session of KVN open-use program
in 2020. Current EATING VLBI consists of three Italian telescopes, Medicina 32
m, Noto 32 m, and Sardinia 64 m, and EAVN telescopes shown in Table 1, while
it operates with the observing frequency of 22 GHz which is a common one for
all Italian telescopes and many EAVN telescopes. The maximum baseline length
of EATING VLBI is composed of Sardinia and VERA-Ogasawara stations with its
baseline length of 11164 km, corresponding to an angular resolution of 0.25
mas at 22 GHz. Although a big geographical gap in a ($u$, $v$) plane appears
for the original trilateral EATING VLBI, better image sensitivity and ($u$,
$v$) coverage can be obtained for renewed EATING VLBI thanks to a big
telescope and a unique location in China, such as Tianma 65 m and Nanshan 26
m, respectively.

As of the first half of 2022, EAVN has conducted EATING VLBI observations with
the number of sessions of 37 and with the total observing time of 275 hours
since the launch of EAVN open-use program in 2018. About 49% (135 hours) of
those observations involved any of Italian telescopes so that they can secure
the common sky coverage with EAVN. It should also be noted that part of the
EATING VLBI sessions were conducted in conjunction with the Quasar network
(Badary 32 m, Svetloe 32 m, and Zelenchukskaya 32 m) in Russia Shuygina
(2019). Major targets of EATING VLBI conducted to date are M87 and the nearest
gamma-ray emitting narrow-line Seyfert 1 (NLS1) galaxy 1H 0323+342. It is
notable that a series of EATING VLBI observations could provide images with
the highest angular resolution to date toward 1H 0323+342 compared to previous
studies with VLBI (e.g., Wajima (2014); Fuhrmann (2016); Doi (2018); Hada
(2018)), which enable us to investigate a gamma-ray emission mechanism of
NLS1. Imaging capability and initial scientific results of these targets are
shown in another review paper of this Special Issue.

Three Italian telescopes are planning installation of a new compact triple-
band receiver (CTR) Han (2017), in which simultaneous reception of radio
signal at 22, 43, and 86 GHz is available. Design of the quasi-optics and
assembly of CTR is carried out at KASI and installation to each telescope will
be conducted in 2023.

###  5.3 Global VLBI Alliance

The open-use program is operated by a lot of domestic and international VLBI
arrays besides EAVN, such as EVN, LBA, and VLBA, as mentioned in Section 1. It
is thus a natural extension of VLBI array capability by integrating these
arrays into a single VLBI array to obtain 100 $\mu$as-scale angular resolution
at cm- to mm-wavelengths. This motivates us to set up an idea of the Global
VLBI Alliance (GVA)See also the following site for more details of GVA:
http://gvlbi.evlbi.org/, accessed on 1 December 2022 Colomer (2021). GVA aims
not only providing an opportunity of obtaining access to the global VLBI array
but also sharing various knowledge related to VLBI, such as scientific
strategies, technical developments between each VLBI array, and discussion on
observational campaigns using GVA. The official working group for GVA has
already been established under the commission B4 (radio astronomy) of the
International Astronomical Union in
2020https://www.iau.org/science/scientific_bodies/working_groups/324/,
accessed on 1 December 2022 and started practical discussion with four VLBI
arrays shown above (EAVN, EVN, LBA, and VLBA) being currently a regular member
of GVA.

##  6 Conclusions

The paper describes overview of the observing system of the East Asian VLBI
Network (EAVN) and brief summary of scientific accomplishments to date with
EAVN. EAVN is currently operated using 16 radio telescopes and three
correlators located in China, Japan, and Korea, with the longest baseline
length of 5078 km, corresponding to the highest angular resolution of 1.82,
0.55, and 0.28 mas at 6.7, 22, and 43 GHz, respectively. EAVN is offering a
total observing time of about 1100 hours in a year for the open-use program
and occasional observations using the director’s discretionary time to
astronomers all over the world. Although EAVN is operated with limited number
of observing frequencies, EAVN provides opportunities of VLBI observations
with dual-frequency data receiving system at 22 and 43 GHz, which is one of
the most unique capabilities of EAVN, enabling us to obtain better baseline
sensitivity and to determine a relative source position by employing the
frequency phase transfer technique and the source-frequency phase-referencing
technique, respectively. EAVN conducts VLBI observations toward various
Galactic and extragalactic radio sources, while EAVN provides observing
opportunities to three major scientific objectives, active galactic nucleus,
evolved star, and star-forming region, through the large program of KaVA/EAVN.
EAVN will become one of the most important VLBI facilities for future global
VLBI thanks to its unique location and unique capability of multi-frequency
simultaneous data receiving system.

The corresponding author (K.W.) wrote all the manuscript. Each author
contributes to EAVN operation and scientific activities as shown below: EAVN
director (M.H., K.-T.K., Z.-Q.S., N.W.), EAVN AGN Science Working Group (K.A.
(Kazunori Akiyama), J.-C.A., T.A., K.A. (Keiichi Asada), Z.C., X.C., I.C.,
Y.C., A.D., J.D., W.G. (Wei Gou), W.G. (Wen Guo), K.H., Y.H., J.A.H., Y.J.
(Yongbin Jiang), Y.J. (Yongchen Jiang), N.K., M.K., S.K., J.-W.L., J.A.L.,
S.-S.L., B.L., X.L. (Xiaofei Li), R.-S.L., M.N., K.N., J.O., T.O., J.P., H.R.,
S.S.-S., B.W.S., Y.S., M.T., F.T., S.T., J.W., X.W., W.Y., K.Y., L.Y., Y.Z.,
G.-Y.Z., R.Z., W.Z.), EAVN Evolved Stars Science Working Group (J.O.C.,
S.-H.C., H.I., D.-J.K., J.K., Y.Y. (Youngjoo Yun)), EAVN Star-Forming Regions
Science Working Group (K.A. (Kitiyanee Asanok), D.-Y.B., T.H., K.S.), EAVN
Galactic Astrometry Science Working Group (T.J. (Taehyun Jung), S.-W.K., C.O.,
N.S., S.X., B.Z.), management and operation of the Korea-Japan Correlation
Center (D.-K.J., H.-R.K., S.-J.O., D.-G.R., J.-H.Y.), EAVN site operation and
management (W.C., H.-S.C., L.C., K.F., W.J., T.J. (Takaaki Jike), J.-S.K.,
H.K., S.W.L., G.L., Z.L., Q.L., X.L. (Xiang Liu), K.M., H.O., K.M.S., Y.T.,
S.W., B.X., H.Y. (Hao Yan), S.-O.Y., Y.Y. (Yoshinori Yonekura), H.Y. (Hasu
Yoon), J.Y., H.Z.), collaboration between EAVN and Southeast Asia (T.C., P.J.,
B.H.K., S.P., W.R., B.S.), overall coordination of EAVN (K.W.). All authors
have read and agreed to the published version of the manuscript.

This work is funded by following: JSPS (Japan Society for the Promotion of
Science) Grant-in Aid for Scientific Research (KAKENHI) (S) 18H05222 (T.H.),
(A) 16H02167 (H.I.), 18H03721 (K.N.), 21H04524 (H.I.), 22H00157 (K.H.), (B)
18KK0090 (K.H.), 21H01120 (Y.Y. (Yoshinori Yonekura)), (C) 17K05398 (T.H.),
19K03921 (K.S.), 21K03628 (S.S-S.), JSPS Grant-in-Aid for Scientific Research
on Innovative Areas (Research in a proposed research area) 21H00032 (Y.Y.
(Yoshinori Yonekura)), 21H00047 (H.I.), JSPS Grant-in-Aid for Transformative
Research Areas (A) 20H05845 (T.H.), the Grant of PIIF Heiwa Nakajima
Foundation in 2019 (K.S.), and the Mitsubishi Foundation 201911019 (K.H.).
K.A. (Kazunori Akiyama) is financially supported by grants from the National
Science Foundation (AST-1935980, AST-2034306, AST-2107681, AST-2132700,
OMA-2029670). L.C. is supported by the Chinese Academy of Sciences (CAS)
’Light of West China’ Program (No. 2021-XBQNXZ-005) and the National Science
Foundation of China (NSFC; No. U2031212 and 61931002). W.C. is supported by
NSFC (No. 11903079). Z.C. is supported by NSFC (No. U1931135). W.J. is
supported by NSFC (No. 12173074). R.-S.L. is supported by the Max Planck
Partner Group of the MPG and the CAS, the Key Program of the NSFC (No.
11933007), the Key Research Program of Frontier Sciences, CAS (No. ZDBS-LY-
SLH011), and the Shanghai Pilot Program for Basic Research - CAS, Shanghai
Branch (No. JCYJ-SHFY-2022-013). S.T. acknowledges financial support from the
National Research Foundation of Korea (NRF) through grant no.
2022R1F1A1075115. B.Z. is supported by the NSFC (No. U1831136 and U2031212)
and Shanghai Astronomical Observatory (N-2020-06-09-005).

All observing data obtained by EAVN (including KaVA), except the data in the
term of right of occupation by a principal investigator, are archived via the
following website.
https://radio.kasi.re.kr/kvn_arch/kvn_search.php

###### Acknowledgements.

This work is made use of the East Asian VLBI Network (EAVN), which is operated
under cooperative agreement by National Astronomical Observatory of Japan
(NAOJ), Korea Astronomy and Space Science Institute (KASI), Shanghai
Astronomical Observatory (SHAO), Xinjiang Astronomical Observatory (XAO),
Yunnan Observatories (YNAO), National Geographic Information Institute (NGII),
and National Astronomical Research Institute of Thailand (Public Organization)
(NARIT), with the operational support by Ibaraki University, Yamaguchi
University, and Kagoshima University. Operation of Hitachi 32 m, Takahagi 32
m, and Yamaguchi 32 m telescopes is partially supported by the inter-
university collaborative project ‘Japanese VLBI Network (JVN)’ of NAOJ. The
authors declare no conflicts of interest. Abbreviations The following
abbreviations are used in this manuscript:
AGB | Asymptotic Giant Branch
---|---
AGN | Active Galactic Nucleus
CTR | Compact Triple-Band Receiver
CVN | Chinese VLBI Network
DAS | Data Acquisition System
DBBC | Digital Base Band Converter
DSP | Digital Signal Processing
EACOA | East Asian Core Observatories Association
EAMA | East Asian Meeting on Astronomy
EATING VLBI | East Asia To Italy: Nearly Global VLBI
EAVN | East Asian VLBI Network
EHT | Event Horizon Telescope
EOP | Earth Orientation Paraeter
ESTEMA | Expanded Study on Stellar Masers / EAVN Synthesis of Stellar Maser Animations
EVN | European VLBI Network
FPT | Frequency Phase Transfer
GMVA | Global Millimeter-VLBI Array
GVA |  Global VLBI Alliance
---|---
HINOTORI |  Hybrid Installation Project in Nobeyama, Triple-Band Oriented
HMYSO |  High-Mass Young Stellar Object
ICRF |  International Celestial Reference Frame
ITRF |  International Terrestrial Reference Frame
J-Net |  Japanese VLBI Network
JVN |  Japanese VLBI Network
KaVA |  KVN and VERA Array
KJCC |  Korea-Japan Correlation Center
KNIFE |  Kashima–Nobeyama Interferometer
KVN |  Korean VLBI Network
LBA |  Long Baseline Array
LHCP |  Left-Hand Circular Polarization
RHCP |  Right-Hand Circular Polarization
SEFD |  System Equivalent Flux Density
SFPR |  Source-Frequency Phase-Referencing
SMBH |  Supermassive Black Hole
TDRSS |  Tracking and Data Relay Satellite System
TNRT |  Thai National Radio Telescope
ToO |  Target of Opportunity
TRAO |  Taeduk Radio Astronomy Observatory
TVN |  Thai VLBI Network
VERA |  VLBI Exploration of Radio Astrometry
VGOS |  VLBI Global Observing System
VLBA |  Very Long Baseline Array
VLBI |  Very Long Baseline Interferometry
VSOP |  VLBI Space Observatory Programme
[custom] References

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