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The SVOM mission under development will carry various instruments, and inparticular the coded-mask telescope ECLAIRs, with a large field of view ofabout 2 sr, operating in the 4--150 keV energy band, whose goal is to detecthigh energy transients such as gamma-ray bursts. The trigger software onboardECLAIRs will search for new hard X-ray sources appearing in the sky, as well aspeculiar behaviour (e.g. strong outbursts) from known sources, in order torepoint the satellite to perform follow-up observations with its onboard narrowfield of view instruments. The presence of known X-ray sources must bedisentangled from the appearance of new sources. This is done with the help ofan onboard source catalogue, which we present in this paper. As an input we usecatalogues of X-ray sources detected by Swift/BAT and MAXI/GSC and we study theinfluence of the sources on ECLAIRs' background level and on the quality of thesky image reconstruction process. We show that the influence of the sourcesdepends on the pointing direction on the sky, on the energy band and on theexposure time. In the Galactic centre, the known sources contribution largelydominates the cosmic X-ray background, which is, on the contrary, the mainbackground in sky regions empty of strong sources. We also demonstrate the needto clean the sources contributions in order to maintain a low noise level inthe sky images and to keep the threshold applied for the detection of newsources as low as possible, without introducing false triggers. We brieflydescribe one of our cleaning methods and its challenges. Finally, we presentthe overall structure of the onboard catalogue and the way it will be used toperform the source cleaning and disentangle the detections of new sources fromoutbursts of known sources.

Onboard catalogue of known X-ray sources for SVOM/ECLAIRs