Best-fit functions obtained from the distance distributions between X-ray sources and Moraux et al. catalog counterparts. We separately computed the distributions corresponding to X-ray sources located at different off-axis angle θ in the Chandra FoV. The two components of each best-fit function, the Gaussian and the linear function, are indicated with dotted lines in the inset plot. The areas of the solid triangles indicate the expected number of spurious identification, assuming a matching radius of 3σ for each off-axis interval.