Integrated photon flux with respect to point-like emission at the center of the ROIs versus the number of co-added ROIs. The stacking is performed for a simulated source sample with mixed spectral shapes using analysis test sources with photon indices −2.0, −2.4 and −2.8. The grey and blue shaded areas correspond to the statistical uncertainties on the integrated flux values. All central sources are simulated with an integrated flux of 7.5 × 10^-10 ph/(cm² s), indicated by the dashed line.