Determine the origin of the Gaia CRF parallax bias

Determine whether the predominantly negative parallax bias affecting distant Gaia Celestial Reference Frame objects originates from deterministic instrumental effects or has a purely stochastic measurement-error origin, in order to assess the validity of parallax-conditioned proper-motion corrections and their impact on low-degree vector spherical harmonic estimates.

Background

The paper examines how correlations between Gaia parallax and proper-motion measurements can be used to condition proper-motion estimates under a zero-parallax prior for distant quasars. The resulting corrections influence large-scale vector spherical harmonic terms, including the secular-aberration dipole used to estimate the acceleration of the solar-system barycenter.

The reliability of these corrections depends on whether the sky-correlated parallax offsets in Gaia CRF data are deterministic instrumental biases or include a substantial stochastic component. A deterministic bias propagated through the conditional-correction formula could produce an overcorrection, whereas a stochastic component would represent measurement-error realizations that can appropriately be removed using the observed parallax. The paper reports that this distinction remains unresolved and directly affects the interpretation of corrected low-degree vector spherical harmonic coefficients.

References

It is yet to be understood if this mostly negative bias (also called zero-point in the literature) originates from deterministic instrumental effects or has purely stochastic nature.

— A posteriori correction of Gaia CRF proper motions using conditional probability and zero-parallax prior  (2608.16767 - Makarov, 17 Aug 2026) in Section 3, “Determination of local acceleration of free fall with corrected CRF proper motions”