GaiaEarly Data Release 3

Abstract

Context. Gaia Early Data Release 3 (Gaia EDR3) gives trigonometric parallaxes for nearly 1.5 billion sources. Inspection of the EDR3 data for sources identified as quasars reveals that their parallaxes are biased, that is, they are systematically offset from the expected distribution around zero, by a few tens of microarcseconds. Aims. We attempt to map the main dependences of the parallax bias in EDR3. In principle, this could provide a recipe for correcting the EDR3 parallaxes. Methods. Quasars provide the most direct way for estimating the parallax bias for faint sources. In order to extend this to brighter sources and a broader range of colours, we used differential methods based on physical pairs (binaries) and sources in the Large Magellanic Cloud. The functional forms of the dependences were explored by mapping the systematic differences between EDR3 and DR2 parallaxes. Results. The parallax bias is found to depend in a non-trivial way on (at least) the magnitude, colour, and ecliptic latitude of the source. Different dependences apply to the five- and six-parameter solutions in EDR3. While it is not possible to derive a definitive recipe for the parallax correction, we give tentative expressions to be used at the researcher’s discretion and point out some possible paths towards future improvements.

Gaia Early Data Release 3

“…A more comprehensive analysis and cleaning of the observation-to-source matching results led to less sources with highly significant negative parallaxes or too large parallaxes (see Appendix C in Lindegren et al 2018), which also removes spuriously high proper motions. The treatment of close source pairs was improved to deal with the pairs with separations below 400 mas which were erroneously considered duplicate sources in Gaia DR2.…”

“…The astrometric processing for Gaia EDR3 is described in Lindegren et al (2021a), who note the following major improvements with respect to the processing for Gaia DR2. The basic inputs for the Astrometric Global Iterative Solution (AGIS; Lindegren et al 2012) are the source image locations in the Gaia CCD pixel stream, translated to observation times and across-scan locations.…”

“…The details are described in Rowell et al (2021). It is shown in Lindegren et al (2021a) that for sources fainter than G = 13 the chromatic effects on image locations are almost completely removed during the image parameter determination stage, thus mostly eliminating an important source of systematic errors. Eventually for future Gaia data releases this should remove the need for colour terms in the astrometric calibrations.…”

“…In addition, the image fluxes estimated as part of the IPD are improved, which in turn improves the G-band photometric processing. A diagram illustrating this data processing flow can be found in Lindegren et al (2021a).…”

“…The VBAC model introduces additive corrections to the basic angle variation corrections calculated on the basis of the basic angle monitor measurements (cf. Lindegren et al 2018). Lindegren et al (2021a) describe the successful attempt to fit the coefficient of the cos Ω term, which leads to a reduction in the size of the overall parallax zero point, which for Gaia EDR3 is −17 µas (compared to −29 µas for Gaia DR2), as estimated from quasar parallaxes.…”