Distribution of the squared angular distance of γ-ray candidate events around the position of PKS 1510−089. The angular distribution (in terms of θ², the square of the angular distance between the source position and the reconstructed arrival direction) of events around the position of PKS 1510−089 is shown. On-source (signal+background) events are shown as hatched histogram, whereas the off-source (background) events are shown as points with error bars. The on-source region is defined by a  deg², shown by the vertical dotted line. The dashed curve is the PSF modeled as a Gaussian, and a constant scaled to the average background level.