Seog-Tae Han

EARLY SCIENCE WITH THE KOREAN VLBI NETWORK: EVALUATION OF SYSTEM PERFORMANCE

We report the very long baseline interferometry (VLBI) observing performance of the Korean VLBI Network (KVN). The KVN is the first millimeter-dedicated VLBI network in East Asia. The KVN consists of three 21 m radio telescopes with baseline lengths in a range of 305-476 km. The quasi-optical system equipped on the antennas allows simultaneous observations at 22, 43, 86, and 129 GHz. The first fringes of the KVN were obtained at 22 GHz on 2010 June 8. Test observations at 22 and 43 GHz on 2010 September 30 and 2011 April 4 confirmed that the full cycle of VLBI observations works according to specification: scheduling, antenna control system, data recording, correlation, post-correlation data processing, astrometry, geodesy, and imaging analysis. We found that decorrelation due to instability in the hardware at times up to 600 s is negligible. The atmosphere fluctuations at KVN baseline are partly coherent, which allows us to extend integration time under good winter weather conditions up to 600 s without significant loss of coherence. The post-fit residuals at KVN baselines do not exhibit systematic patterns, and the weighted rms of the residuals is 14.8 ps. The KVN is ready to image compact radio sources both in snapshot and full-track modes with residual noise in calibrated phases of less than 2 deg at 22 and 43 GHz and with dynamic ranges of ~300 for snapshot mode and ~1000 for full-track mode. With simultaneous multi-frequency observations, the KVN can be used to make parsec-scale spectral index maps of compact radio sources.

Verification of the Astrometric Performance of the Korean VLBI Network, Using Comparative SFPR Studies with the VLBA at 14/7 mm

The Korean VLBI Network (KVN) is a new mm-VLBI dedicated array with capability for simultaneous observations at multiple frequencies, up to 129 GHz. The innovative multi-channel receivers present significant benefits for astrometric measurements in the frequency domain. The aim of this work is to verify the astrometric performance of the KVN using a comparative study with the VLBA, a well established instrument. For that purpose, we carried out nearly contemporaneous observations with the KVN and the VLBA, at 14/7 mm, in April 2013. The KVN observations consisted of simultaneous dual frequency observations, while the VLBA used fast frequency switching observations. We used the Source Frequency Phase Referencing technique for the observational and analysis strategy. We find that having simultaneous observations results in a superior performance for compensation of all atmospheric terms in the observables, in addition to offering other significant benefits for astrometric analysis. We have compared the KVN astrometry measurements to those from the VLBA. We find that the structure blending effects introduce dominant systematic astrometric shifts and these need to be taken into account. We have tested multiple analytical routes to characterize the impact of the low resolution effects for extended sources in the astrometric measurements. The results from the analysis of KVN and full VLBA datasets agree within 2-

100-GHZ BAND TEST OBSERVATIONS OF THE KVN 21-M RADIO TELESCOPES

\sigma

THE FIRST VERY LONG BASELINE INTERFEROMETRY IMAGE OF A 44 GHz METHANOL MASER WITH THE KVN AND VERA ARRAY (KaVA)

of the thermal error estimate. We interpret the discrepancy as arising from the different resolutions. We find that the KVN provides astrometric results with excellent agreement, within 1-

Observations of SiO (v = 0, 1, 2) J = 3-2 and J = 2-1 Emission in Late-Type Stars

\sigma

Construction of the Korean VLBI network (KVN)

, when compared to a VLBA configuration which has a similar resolution. Therefore this comparative study verifies the astrometric performance of KVN using SFPR at 14/7 mm, and validates the KVN as an astrometric instrument.

A 100-GHz-band heterodyne sis receiver for the trao telescope

We carry out 100-GHz band test observations with the newly-constructed KVN 21-m radio telescopes in order to evaluate their performance. The three telescopes have similar performance parameters. The pointing accuracies are about 4 00 rms for the entire sky. The main beam sizes are about 30 00 (FWHMs), which is nearly the difiraction limit of the telescopes at the observing frequency (97 GHz). The measured aperture and main-beam e‐ciencies are about 52% and 46%, respectively, for all three telescopes. The estimated moon e‐ciency is »84% for the KVN Tamna telescope. The flrst sidelobes appear 50 00 (»1.6£FWHM) from the main beam centers and the levels are on average i14 dB.

MEASURING THE CORE SHIFT EFFECT IN AGN JETS WITH THE EXTENDED KOREAN VLBI NETWORK

We have carried out the first very long baseline interferometry (VLBI) imaging of a 44 GHz classI methanol maser (70‐61A + ) associated with a millimeter core MM2 in a massive star-forming region IRAS 18151−1208 with KaVA (KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in imaging compact maser features with a synthesized beam sizeof2.7milliarcseconds ×1.5milliarcseconds(mas).Thesefeaturesaredetectedatalimitednumber ofbaselines within the length of shorter than ≈ 650 km corresponding to 100 Mλ in the uv-coverage. The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line. The minimum component size among the maser features is ∼5mas×2mas, which corresponds to the linear size of ∼15 AU × 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ∼3.5 × 10 8 to 1.0 × 10 10 K, which are higher than the estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ∼50 mas. The 44 GHz classI methanol maser in IRAS 18151−1208 is found to be associated with the MM2 core, which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz classII methanol maser.

Development of SRAO 3MM SIS Receiver

Observations of SiO v = 0, 1, 2, J = 3-2 transitions were carried out for 42 late-type stars containing SiO v = 1, J = 2-1 maser emission with the 14 m radio telescope at Taeduk Radio Astronomy Observatory (TRAO) during 1995 February and 1996 February. Observations of the v = 0, 1, 2, J = 2-1 transitions in the same objects were performed within 40 days of the former observations using the same telescope. For SiO v = 1, J = 3-2 masers, the line was detected in 23 stars giving a detection rate of 55%. Seventeen of them were new detections. For SiO v = 2, J = 3-2 masers, the line was detected in seven stars, four of them being new detections. The v = 2, J = 3-2 line intensities were found at the expected intensity, unlike the anomalously weak emission seen previously in the rare v = 2, J = 2-1 maser. We also report the intensity ratios observed within the vibrational and rotational states.

The Development of a Dual Channel SIS Receiver of Trao Telescope

Koreas new VLBI project to construct the Korean VLBI network (KVN) started in 2001, as a 7-year project that is fully funded by our government. We plan to build 3 new high-precision radio telescopes of 21-m diameter in 3 places in Korea which will be exclusively used for VLBI observations. We will install the 2/8, 22 and 43 GHz HEMT receivers within 2007 as a first target, and later we will expand the receiving frequency up to 86 and 129 GHz for astronomical, geodetic, and Earth science VLBI research. The millimeter-wave VLBI will be the ultimate goal of KVN. For the front-ends we are going to install a multi-channel receiver system that employs low-pass filters within a quasi-optical beam transportation system. This receiver system will give reliable phase calibrations for millimeter-wave VLBI as well as enable simultaneous multi-frequency band observations. The new hard-disk type mark 5 will be used as the main recorder of KVN. We have completed the design of the KVN DAS system of 2 Gsps sampling rate, which will use 4 data streams to meet the multi-channel requirement. A VERA type DAS modified for the mark 5 recorder is also under consideration. A new correlator project for KVN was recently approved from our government, and will start in the second half of this year.