Evidence for a systematic offset of $-$0.25 mas in the Gaia DR1 parallaxes

TL;DR

This study identifies a systematic offset of approximately -0.25 milliarcseconds in Gaia DR1 parallaxes, with the offset varying by ecliptic latitude and possibly by distance, affecting distance measurements.

Contribution

It provides the first empirical quantification of Gaia DR1 parallax offsets and reveals their dependence on ecliptic latitude and distance, improving calibration accuracy.

Findings

Gaia DR1 parallaxes are systematically underestimated by about 0.25 mas.

The offset varies with ecliptic latitude, being larger in the north.

Possible dependence of offset on distance, negligible below 1 mas.

Abstract

We test the parallaxes reported in the Gaia first data release using the sample of eclipsing binaries with accurate, empirical distances from Stassun \& Torres (2016). We find an average offset of $-$0.25$\pm$0.05 mas in the sense of the Gaia parallaxes being too small (i.e., the distances too long). The offset does not depend strongly on obvious parameters such as color or brightness. However, we find with high confidence that the offset may depend on ecliptic latitude: the mean offset is $-$0.38$\pm$0.06 mas in the ecliptic north and $-$0.05$\pm$0.09 mas in the ecliptic south. The ecliptic latitude dependence may also be represented by the linear relation, $\Delta\pi \approx -0.22(\pm0.05) -0.003(\pm0.001)\times\beta$ mas ($\beta$ in degrees). Finally, there is a possible dependence of the parallax offset on distance, with the offset becoming negligible for $\pi\lesssim 1$ mas; we discuss whether this could be caused by a systematic error in the eclipsing binary distance scale, and reject this interpretation as unlikely.