Origin of the Gaia XP blue-wavelength systematic

Determine the origin of the known colour-dependent systematic bias in Gaia XP spectra at wavelengths below 400 nm.

Background

Gaia XP spectra exhibit a colour-dependent systematic effect at wavelengths shorter than 400 nm, a region that is particularly important for synthetic photometry in the SDSS and LSST u bands. The paper identifies the effect observationally but does not establish its physical or instrumental cause. Resolving its origin could improve the calibration of Gaia XP spectra and reduce reliance on empirical corrections or stellar-atmosphere-model extensions for blue photometric bands.

References

Specifically, there is a known colour-dependent systematic at $\lambda < 400$ nm, the exact origin of which remains uncertain; a magnitude-dependent systematic at wavelengths just before the region of overlap of the BP and RP spectra ($500 < \lambda < 600$ nm), likely caused by imperfect calibration in this transition region; and a magnitude-dependent systematic at $\lambda > 950$ nm.

— Vera C. Rubin LSST Synthetic Magnitudes derived from Gaia XP Spectra  (2608.17922 - Razim et al., 18 Aug 2026) in Section 2.1, “Gaia XP spectra”