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The SVOM mission under development will carry four instruments, and inparticular the coded-mask telescope named ECLAIRs, with a large field of viewof about 2 sr, operating in the 4-150 keV energy band. The trigger software onboard ECLAIRs will search for high-energy transients such as gamma-ray burstsand peculiar behaviour (e.g. strong outbursts) from known X-ray sources, inorder to repoint the satellite to perform follow-up observations with theonboard narrow field of view instruments. The image trigger, one of the twoalgorithms implemented in the software on board ECLAIRs, produces images overperiods of exposure ranging from 20 seconds to 20 minutes during which theEarth can cross the field of view. The CXB and contributions from known X-raysources are expected to dominate the ECLAIRs astrophysical and instrumentalbackground and must be taken into account and corrected prior to coded-maskimage deconvolution in order to optimise the sensitivity to faint transients.To correct these background components, we implemented and studied atraditional fitting method and a new method based on wavelet decomposition ofthe detector image. In order to study and to assess the performance of thesemethods, we performed a one-year simulation of the image trigger on boardECLAIRs. From the images produced during this realistic observation scenario ofthe SVOM mission, we also defined a way to analyse the sky images to search fornew sources. We present the algorithms behind the image trigger on boardSVOM/ECLAIRs. We show that the wavelet method we implemented provides similarresults in terms of cleaning performance compared to the traditional fittingmethod, and has the benefit of not requiring any assumption on the shape of thebackground on the detector. We also calibrate the detection threshold to beadaptive and based on the quality of the reconstructed sky image.

The SVOM/ECLAIRs image trigger with wavelet-based background correction optimised with a one-year simulation of observations