Squared angular distance (θ²) to the NGC 4151 position distribution showing the number of gamma-ray candidate events as a function of the squared angular separation between the reconstructed gamma-ray direction and the NGC 4151 position. The distribution of θ², defined relative to the position of NGC 4151, is indicated with the black points, while the gray histogram corresponds to the θ² distribution defined relative to one of the background estimation points (Fomin et al. 1994). The vertical dashed line defines the signal region within which the detection significance is computed.