Parallax Systematics and Photocenter Motions of Benchmark Eclipsing Binaries in Gaia EDR3

TL;DR

This study assesses Gaia EDR3 parallaxes using benchmark eclipsing binaries, finds improved accuracy with reduced systematics, and reveals a strong correlation between RUWE and photocenter motion, indicating RUWE's potential as an unresolved binary indicator.

Contribution

It provides the first detailed analysis of Gaia EDR3 parallaxes for benchmark eclipsing binaries and links RUWE to photocenter motions, enhancing understanding of Gaia's astrometric systematics.

Findings

Gaia EDR3 parallaxes show reduced systematics compared to DR2.

RUWE correlates strongly with photocenter motion, even within the 'good' range.

Unresolved binaries can be identified using RUWE values slightly above 1.0.

Abstract

Previous analyses of various standard candles observed by the Gaia satellite have reported statistically significant systematics in the parallaxes that have improved from $\sim$250 $\mu$as in the first data release (DR1) to 50--80 $\mu$as in the second data release (DR2). Here we examine the parallaxes newly reported in the Gaia early third data release (EDR3) using the same sample of benchmark eclipsing binaries (EBs) we used to assess the DR1 and DR2 parallaxes. We find a mean offset of $-37\pm20$ $\mu$as (Gaia$-$EB), which decreases to $-15\pm18$ $\mu$as after applying the corrections recommended by the Gaia Mission team; global systematics in the Gaia parallaxes have clearly improved and are no longer statistically significant for the EB sample, which spans $5\lesssim{G}\lesssim12$ in brightness and 0.03--3 kpc in distance. We also find that the RUWE goodness-of-fit statistic reported in Gaia EDR3 is highly sensitive to unresolved companions (tertiaries in the case of our EB sample) as well as to photocenter motion of the binaries themselves. RUWE is nearly perfectly correlated ($r^2=0.82$) with photocenter motions down to $\lesssim$0.1 mas, and surprisingly this correlation exists entirely within the nominal "good" RUWE range of 1.0--1.4. This suggests that RUWE values even slightly greater than 1.0 may signify unresolved binaries in Gaia, and that the RUWE value can serve as a quantitative predictor of the photocenter motion.