Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position

TL;DR

Gaia EDR3 parallaxes exhibit systematic biases influenced by magnitude, colour, and position, and this study maps these dependencies to aid in bias correction, especially for faint sources like quasars.

Contribution

This work characterizes the complex dependencies of Gaia EDR3 parallax bias on multiple source parameters, providing tentative correction formulas for improved astrometric accuracy.

Findings

Parallax bias depends on magnitude, colour, and ecliptic latitude.

Bias varies between five- and six-parameter solutions.

Tentative correction expressions are proposed for research use.

Abstract

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 systematically offset from the expected distribution around zero, by a few tens of microarcsec. We attempt to map the main dependencies of the parallax bias in EDR3. In principle this could provide a recipe for correcting the EDR3 parallaxes. For faint sources the quasars provide the most direct way to estimate parallax bias. In order to extend this to brighter sources and a broader range of colours, we use differential methods based on physical pairs (binaries) and sources in the Large Magellanic Cloud. The functional forms of the dependencies are explored by mapping the systematic differences between EDR3 and DR2 parallaxes. 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 dependencies 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.