A Calibration Audit of a Gaia XP White-Dwarf Main-Sequence Binary Catalog: How Much the BP-Band Residual Contaminates at Its Bound and Above

Abstract

A Gaussian-process classifier on Gaia DR3 XP spectra produced $\sim$30,000 white-dwarf main-sequence binary candidates (Li et al. 2025), with probabilities but no likelihood. The corrected spectra agree with external libraries to better than 2% below 400 nm and 1% redward (Huang et al. 2024), and that residual sits where a hot white dwarf also adds flux. We asked whether it contaminates the selection at its bound. It does, weakly. The spurious rate runs about 1.7 times the baseline there. Put into the blue band, 2% of total flux is a median 55% local excess across our MS templates, 27 times that number read locally. We inject a blue taper clipped to that bound, under the templates' published per-pixel errors, with a $\Delta {\chi }^2$ threshold standing in for the classifier. The spurious rate is $0.09\pm 0.02$ across 20 seeds on a 0.053 baseline, 19 of 20 above it. The unclipped 2% taper, above the redward bound, gives $0.11\pm 0.03$. At that taper the amortized posterior's 90% coverage falls to 0.85 from a clean 0.88 on the flat-model leg, with PIT uniformity rejected at $p<10^{-5}$ in all four seeds. The selection fails in bulk only above a 10-15% local excess, and the rate reaches 0.96 near 50%. A model-comparison gate certifies binarity only above a white-dwarf flux share near 0.05. At share 0.01, near the catalog's median of 0.0096, the $\Delta {\chi }^2$ preference inverts at a 20% local excess. Outside Huang's color-magnitude box, 40% of the catalog, the post-correction residual is not measured.

18 pages, 3 figures, 1 table. v2: headline re-anchored on the Huang-consistent injection, title amended; see Data and code. Code, configurations, and the committed records behind every quoted number are at https://github.com/WizardEternal/wdms-xp-calibration