Vera C. Rubin LSST synthetic magnitudes derived from Gaia XP spectra

In the context of Milky Way studies, the Observatory’s Legacy Survey of Space and Time is often described as a deep extension of the survey. In the future, joint analyses of the and LSST data promise to bring new insights into the Galaxy’s structure and formation history. However, the relatively small overlap in the magnitude ranges of the two surveys raises the question of how to perform a joint calibration of these datasets. Vera C. Rubin Gaia Gaia We produce high-quality synthetic LSST photometry from XP low-resolution spectra and use SDSS and DES observed photometry to calibrate the spectra. Gaia We developed a method for empirically correcting XP spectra using SDSS Stripe 82 photometry. We projected sources for which XP data are available onto a magnitude--magnitude grid and calculated the residuals between uncorrected synthetic and observed SDSS magnitudes, which we then used to derive correction coefficients across spectral regions corresponding to each photometric band. Gaia Gaia The correction significantly reduces systematic trends and scatter in the synthetic magnitudes: the median residuals decrease by an order of magnitude (e.g. for the u band the improvement is from 0.038 to 0.002 mag), and the standard deviation of residuals typically reduces by factor of 2 (e.g. for the u band from 0.2 to 0.07 mag). Our correction approach improves the reliability of synthetic photometry derived from XP spectra and enables a joint analysis of and LSST data for studies of Galactic structure and stellar populations. Gaia Gaia