Effective survey volume as a function of X-ray luminosity. The survey volume was calculated for three different cut-off redshifts, z = 0.8, z = 0.3, z = 0.22 for our reference cosmology (h = 0.7, Ω_m = 0.3 and Ω_Λ = 0.7). We also determined the same set of curves for a cosmology with the parameters h = 0.7, Ω_m = 0.26 and Ω_Λ = 0.74 shown as dashed curves. The difference between the set of curves has been amplified by a factor of 10, to make the offset more visible. This affects the dashed curves that are shown slightly offset from the original position.