A quantitative analysis of systematic differences in positions and proper motions of Gaia DR2 with respect to VLBI

TL;DR

This study compares Gaia DR2 and VLBI source positions, revealing systematic differences, the influence of optical jets on offsets, and cautioning against assuming quasars as fixed points for proper motion measurements.

Contribution

It provides a detailed analysis of positional discrepancies between Gaia DR2 and VLBI, highlighting the impact of optical jets and proposing error rescaling methods.

Findings

9% of sources have significant positional offsets.

Optical jets contribute to 62% of offset directions.

Proper motions are often affected by source position jitter.

Abstract

We have analyzed the differences in positions of 9081 matched sources between the Gaia DR2 and VLBI catalogues. The median position uncertainty of matched sources in the VLBI catalogue is a factor of two larger than the median position uncertainty in the Gaia DR2. There are 9% matched sources with statistically significant offsets between both catalogues. We found that reported positional errors should be re-scaled by a factor of 1.3 for VLBI and 1.06 for Gaia, and in addition, Gaia errors should be multiplied by the square root of chi square per degree of freedom in order to best fit the normalized position differences to the Rayleigh distribution. We have established that the major contributor to statistically significant position offsets is the presence of optical jets. Among the sources for which the jet direction was determined, the position offsets are parallel to the jet directions for 62% of the outliers. Among the matched sources with significant proper motion, the fraction of objects with proper motion directions parallel to jets is a factor of 3 greater than on average. Such sources have systematically higher chi square per degree of freedom. We explain these proper motions as a manifestation of the source position jitter caused by flares that we have predicted earlier. Therefore, the assumption that quasars are fixed points and therefore, differential proper motions determined with respect to quasar photocenters can be regarded as absolute proper motions, should be treated with a great caution.