Best-fit parameters obtained by fitting simultaneously all the event grades for a flux level of 200 mCrab using the nominal PSF (left) and the modified PSF (right, as explained in Sect. 4.2), for t_exp = 2 ks and 10 ks (blue circles and red diamonds, respectively). The error bars correspond to the 90% confidence level. Bottom panel: deviation of the best-fit C-stat from the expected value estimated by following the analytic prescription presented by Kaastra (2017).