Second Epoch VLBA Calibrator Survey Observations - VCS-II

Area of Science:

• Astronomy and Astrophysics
• Geodesy
• Radio Astronomy

Background:

• The VLBA Calibrator Survey (VCS) has conducted six campaigns since 1994 to identify compact radio sources for VLBI phase reference calibration.
• Accurate source positions are crucial for precise astrometry and the construction of celestial reference frames.

Purpose of the Study:

• To report on the second epoch VLBA Calibrator Survey (VCS-II) observations.
• To improve the International Celestial Reference Frame (ICRF3) through re-observation of VCS sources.
• To enhance the utility of these sources as VLBI phase reference calibrators.

Main Methods:

• Re-observation of 2400 previously cataloged VCS sources using the Very Long Baseline Array (VLBA) at X and S bands.
• Detection of previously uncataloged sources.
• Analysis of positional uncertainties and flux densities.

Main Results:

• Revised positions for 2062 previously detected and 324 newly detected sources.
• Reduction in average position uncertainties by a factor of approximately 5 (from ~1.14/1.98 mas to ~0.24/0.41 mas in RA/Dec).
• Detection of sources with minimum flux values of ~15 mJy (X-band) and ~28 mJy (S-band), with median total fluxes of ~230/280 mJy.

Conclusions:

• VCS-II significantly improved the positional accuracy of radio sources essential for ICRF3 and VLBI.
• The survey identified a substantial number of new sources and refined positions for existing ones.
• The results confirm the suitability of these flat-spectrum sources for high-precision astrometric applications.