Mean standard deviation of S/N values in sky images from different regions in the sky: within the B1 law without cleaning (dark blue) and after cleaning (light blue, see Sect. 3.5); within the Galactic centre (box of ±15° in longitude and latitude) without cleaning (red), and after cleaning (orange, see Sect. 3.5). Solid lines show the simulation of the full catalogue, whereas the dotted line shows the simulation of the 53 bright sources only. The four plots correspond to energy bands foreseen for the onboard trigger algorithm.