Towards the ICRF3: astrometric comparison of the USNO 2016A VLBI solution with ICRF2 and Gaia DR1

TL;DR

This paper evaluates the astrometric accuracy of the USNO 2016A VLBI solution, comparing it with ICRF2 and Gaia DR1, highlighting improvements and identifying significant radio-optical offsets for some sources.

Contribution

It provides the first detailed comparison of the USNO 2016A VLBI solution with ICRF2 and Gaia DR1, demonstrating improvements and identifying new sources with significant offsets.

Findings

U16A shows 0.1 mas offsets from ICRF2.

U16A reduces VLBI-Gaia offsets by 21%.

20 new sources have significant radio-optical offsets.

Abstract

The VLBI USNO 2016A (U16A) solution is part of a work-in-progress effort by USNO towards the preparation of the ICRF3. Most of the astrometric improvement with respect to the ICRF2 is due to the re-observation of the VCS sources. Our objective in this paper is to assess U16A's astrometry. A comparison with ICRF2 shows statistically significant offsets of size 0.1 mas between the two solutions. While Gaia DR1 positions are not precise enough to resolve these offsets, they are found to be significantly closer to U16A than ICRF2. In particular, the trend for typically larger errors for Southern sources in VLBI solutions are decreased in U16A. Overall, the VLBI-Gaia offsets are reduced by 21%. The U16A list includes 718 sources not previously included in ICRF2. Twenty of those new sources have statistically significant radio-optical offsets. In two-thirds of the cases, these offsets can be explained from PanSTARRS images.