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

The SVOM mission currently under development will carry various instruments, and in particular the coded-mask telescope ECLAIRs, with a large field of view of about 2 sr, operating in the 4-150 keV energy band. The main goal of ECLAIRs is to detect high-energy transients such as gamma-ray bursts. Its onboard trigger software will search for new hard X-ray sources appearing in the sky, as well as peculiar behaviour (e.g. strong outbursts) from known sources, in order to repoint the satellite to perform follow-up observations with its onboard narrow-field-of-view instruments. The presence of known X-ray sources must be disentangled from the appearance of new sources. This is done with the help of an onboard source catalogue, which we present in this paper. As an input we use catalogues of X-ray sources detected by Swift/BAT and MAXI/GSC and we study the influence of the sources on ECLAIRs' background level and on the quality of the sky-image reconstruction process. We show that the influence of the sources depends on the pointing direction on the sky, on the energy band, and on the exposure time. In the Galactic centre, the contribution from known sources largely dominates the cosmic X-ray background, which is, on the contrary, the main background in sky regions lacking strong sources. We also demonstrate the need to clean the contributions of these sources in order to maintain a low noise level in the sky images and to maintain a low threshold for the detection of new sources without introducing false triggers. We briefly describe one of our cleaning methods and its challenges. Finally, we present the overall structure of the onboard catalogue and the way it will be used to perform the source cleaning and disentangle detections of new sources from outbursts of known sources.