Duk-Gyoo Roh

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-

Phase compensation experiments with the paired antennas method: 2. Millimeter‐wave fringe correction using centimeter‐wave reference

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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-

Time Monitoring Observations of SiO J = 2-1 and J = 3-2 Maser Emission toward Late-Type Stars

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A NEW HARDWARE CORRELATOR IN KOREA: PERFORMANCE EVALUATION USING KVN OBSERVATIONS

, 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.

AMPLITUDE CORRECTION FACTORS OF KOREAN VLBI NETWORK OBSERVATIONS

An experiment for compensating interferometer phase fluctuations due to the turbulent troposphere was conducted with the paired antennas method (PAM) using different sky frequencies for a target and reference source. A celestial source 3C279 was observed at 146.81 GHz with the Nobeyama millimeter array (NMA), while a geostationary satellite was observed as a reference source at 19.45 GHz with commercially available antennas. Each of the antennas was installed near one of the NMA antennas, thus giving us 10 pairs of nearly parallel baselines up to 316 m. Large fluctuations in the interferometer phase of 3C279 were mostly eliminated on almost all baselines by subtracting the reference phase multiplied by a ratio of the observing frequencies. The standard deviations of the compensated interferometer phase were under the level of 20° where the angular separation between the two sources was within 2°, while those of the original phase were typically at the level of 40°–50°. The phase compensation was much improved to the level of the differential excess path length of 0.06-mm rms (11° at 146.81GHz) in inserting a time lag proportional to the separation angle to the reference phase time series. These results have practical significance for the millimeter or submillimeter wave interferometry because there is rarely a suitable reference source in the vicinity of scientifically interesting sources at such high frequencies. The present experiment shows that it is quite effective in the PAM to use the reference phase at centimeter wave for compensating the millimeter-wave phase for future large millimeter-wave arrays.

Pilot KaVA monitoring on the M 87 jet: Confirming the inner jet structure and superluminal motions at sub-pc scales

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 Software Correlator for KJJVC

We present the results of simultaneous time monitoring observations of SiO J = 2-1 and J = 3-2 maser emission for 10 late-type stars (8 Mira variables, 1 OH/IR star, and 1 supergiant) with the 14 m radio telescope at Taeduk Radio Astronomy Observatory from 1999 January to 2001 February. The SiO v = 1, J = 2-1 and J = 3-2 maser emission was detected at almost all observational epochs. The SiO v = 2, J = 2-1 maser was detected from 4 late-type stars (VY CMa, R Cas, χ Cyg, R Leo) and the v = 2, J = 3-2 maser was detected from 7 stars (R Aqr, TX Cam, R Cas, χ Cyg, W Hya, R Leo, IK Tau). The v = 3, J = 2-1 and J = 3-2 masers were also detected from χ Cyg and TX Cam, respectively. Based on these observational data, line profile and peak velocity variations with respect to stellar velocity, antenna temperatures, and their ratio variations as a function of optical phase of central star were investigated. As main results, the line profile and the peak velocity variation of the v = 1, J = 3-2 maser with pulsation phase was found to differ from the v = 1, J = 2-1 transition. Similarly, the J = 2-1 and J = 3-2 transitions also differ between rovibrational transitions at a given pulsation phase. However, it is difficult to find significant correlations between the peak velocity variation relative to the stellar velocity of either the J = 3-2 or J = 2-1 transitions over pulsation phase, due to limited time sampling in these data. The peak and integrated antenna temperature (PT and IT) ratios among rotational ladders and vibrational states are investigated. These ratios between rotational ladders of the v = 1, J = 2-1, and J = 3-2 masers are averaged to be the peak antenna temperature ratio, PT(v = 1, J = 3-2)/PT(v = 1, J = 2-1) ≈ 0.29, and the integrated antenna temperature ratio, IT(v = 1, J = 3-2)/IT(v = 1, J = 2-1) ≈ 0.21, respectively. In the v = 2 state, these ratios are PT(v = 2, J = 3-2)/PT(v = 2, J = 2-1) ≈ 7.94 and IT(v = 2, J = 3-2)/IT(v = 2, J = 2-1) ≈ 8.50, respectively. The peak and integrated antenna temperature ratios between vibrational states are also averaged to be PT(v = 2, J = 3-2)/PT(v = 1, J = 3-2) ≈ 1.29, IT(v = 2, J = 3-2)/IT(v = 1, J = 3-2) ≈ 1.02, PT(v = 2, J = 2-1)/PT(v = 1, J = 2-1) ≈ 0.06, and IT(v = 2, J = 2-1)/IT(v = 1, J = 2-1) ≈ 0.05, respectively. These intensity ratios for the v = 2, J = 2-1 and v = 2, J = 3-2 masers suggest that line overlaps operating in the v = 2, J = 2-1 transition do not similarly affect the v = 2, J = 3-2 transition.

Time Monitoring Observations of SiO J=2-1 and J=3-2 Lines toward Orion KL

We report results of the performance evaluation of a new hardware correlator in Korea, the Daejeon correlator, developed by the Korea Astronomy and Space Science Institute (KASI) and the National Astronomical Observatory of Japan (NAOJ). We conduct Very Long Baseline Interferometry (VLBI) observations at 22 GHz with the Korean VLBI Network (KVN) in Korea and the VLBI Exploration of Radio Astrometry (VERA) in Japan, and correlated the aquired data with the Daejeon correlator. For evaluating the performance of the new hardware correlator, we compare the correlation outputs from the Daejeon correlator for KVN observations with those from a software correlator, the Distributed FX (DiFX). We investigate the correlated flux densities and brightness distributions of extragalactic compact radio sources. The comparison of the two correlator outputs shows that they are consistent with each other within < 8%, which is comparable with the amplitude calibration uncertainties of KVN observations at 22 GHz. We also find that the 8% difference in flux density is caused mainly by (a) the difference in the way of fringe phase tracking between the DiFX software correlator and the Daejeon hardware correlator, and (b) an unusual pattern (a double-layer pattern) of the amplitude correlation output from the Daejeon correlator. The visibility amplitude loss by the double-layer pattern is as small as 3%. We conclude that the new hardware correlator produces reasonable correlation outputs for continuum observations, which are consistent with the outputs from the DiFX software correlator.