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We have analyzed the differences in positions of 9081 matched sources betweenthe Gaia DR2 and VLBI catalogues. The median position uncertainty of matchedsources in the VLBI catalogue is a factor of two larger than the medianposition uncertainty in the Gaia DR2. There are 9% matched sources withstatistically significant offsets between both catalogues. We found thatreported positional errors should be re-scaled by a factor of 1.3 for VLBI and1.06 for Gaia, and in addition, Gaia errors should be multiplied by the squareroot of chi square per degree of freedom in order to best fit the normalizedposition differences to the Rayleigh distribution. We have established that themajor contributor to statistically significant position offsets is the presenceof optical jets. Among the sources for which the jet direction was determined,the position offsets are parallel to the jet directions for 62% of theoutliers. Among the matched sources with significant proper motion, thefraction of objects with proper motion directions parallel to jets is a factorof 3 greater than on average. Such sources have systematically higher chisquare per degree of freedom. We explain these proper motions as amanifestation of the source position jitter caused by flares that we havepredicted earlier. Therefore, the assumption that quasars are fixed points andtherefore, differential proper motions determined with respect to quasarphotocenters can be regarded as absolute proper motions, should be treated witha great caution.

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