Quantify parallax bias in preliminary Gaia NSS astrometry

Determine the magnitude and characteristics of the parallax bias affecting the preliminary Gaia DR4 Non-Single Star (NSS 4.1) astrometric solutions and quantify its impact on derived astrometric mass functions, so that uncertainties for companion masses are correctly estimated.

Background

The paper cautions that Gaia parallaxes, as in previous releases, are affected by a small bias. For the preliminary NSS 4.1 astrometric solutions used in this work, the authors explicitly state they do not have enough information to quantify this bias at this stage.

Because the parallax enters directly into the astrometric mass function, an unquantified parallax bias leads to underestimated uncertainties on the mass function and therefore on the inferred mass of the dark companion. The authors partially mitigate this in a combined astrometric–spectroscopic solution by using the spectroscopic semi-major axis, but the underlying parallax bias in the preliminary astrometry remains to be characterized.