Onboard catalog of known X-ray sources for EP-WXT

Abstract The Einstein Probe (EP) is dedicated to explore the dynamic X-ray universe and capture transient events in real time with its Wide-field X-ray Telescope (WXT). However, WXT’s unprecedentedly large instantaneous field of view—exceeding 3,600 square de- grees—simultaneously encompasses numerous known X-ray emitters. Distinguishing genuine novel transients from these persistent sources is a critical observational chal- lenge. To resolve this, EP-WXT incorporates a dedicated reference catalog of known X- ray sources directly into its onboard data processing and triggering system. In this paper, we detail the compilation of this onboard catalog. By merging data from the ROSAT All- Sky Survey, the MAXI source list, and a curated stellar flare candidate catalog, we con- structed a robust baseline database of 9,000 sources. This catalog provides coordinates, baseline count rates, and spatial veto radii for exceptionally bright emitters. Real-time cross-matching against this database effectively decouples known background sources from the transient alert stream. Recent in-orbit operations validate the high stability and efficiency of this catalog-driven trigger system, proving its essential role in ensuring EP’s rapid and accurate response to genuine astrophysical discoveries.