Cataclysmic variables from Gaia XP spectra: The Gaia emission-line magnitude (GEM) diagram

Gaia DR3 opens a new window for the census of Galactic stellar populations in all-sky surveys, specifically for identifying cataclysmic variables (CVs). While the majority of CVs are distributed below the main sequence on the Gaia HR diagram, identifying them among dominant background stellar populations remains a significant challenge. Here, we present the Gaia Emission-line Magnitude (GEM) diagram - a tool designed to isolate CV candidates from background stars located below the main sequence. Using Gaia photometric data and low-resolution BP/RP (XP) spectra, we construct the GEM diagram by plotting the extinction-corrected absolute magnitude (M G0) against the pseudo-equivalent width of the H alpha line (pgEW H alpha), which is computed via a Gaussian line-profile approximation. We find that known CVs and major stellar background populations, including white dwarfs, hot subdwarfs, and white dwarf-main sequence binaries, occupy distinct regions on the GEM diagram, reflecting both their physical properties and the low-resolution nature of the Gaia XP spectra. We define an empirical selection line on the GEM diagram to separate true H alpha-emitting CVs from background contaminants. As a proof of concept, we apply the GEM diagram to a Gaia volume-limited sample within 250 pc (G<17.5 mag) distributed below the main sequence, recovering known CVs. Additionally, we identify three new CV candidates whose accreting nature is supported by archival X-ray data and optical variability. Compared to traditional optical color-based selection techniques, the GEM diagram uses Gaia's low-resolution spectra for initial target identification in all-sky surveys. This provides a more reliable framework for the statistical selection of CV candidates prior to ground-based spectroscopic follow-up and can be scaled to future Gaia data releases to map Galactic emission-line star populations.