Simulated AGN log N − log S relation. The black curve is the AGN model in the 0.5 − 2.0 keV energy band from Lehmer et al. (2012); the blue dotted curve represents one realization of an AGN population used for the simulation, while the orange dots are the detected AGNs. The zoom-in box shows the log N − log S relation at the faint end, which indicates ATHENA cannot detect any sources fainter than (7 × 10⁻¹⁷ ergs s⁻¹ cm⁻²) in an 80 ks exposure time observation.