SIMBAD references

We directly compare X-ray and optical techniques of cluster detection by combining Sloan Digital Sky Survey photometric data with a wide-field (∼1.6 deg²) XMM-Newton survey near the North Galactic Pole region. The optical cluster detection procedure is based on merging two independent selection methods: a smoothing + percolation technique and a matched filter algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the Science Analysis System (sas) v.5.3 package. The final optical sample counts nine candidate clusters with estimated Automatic Plate Measuring like richness of more than 20 galaxies, while the X-ray based cluster candidates total four. Three out of these four X-ray cluster candidates are also optically detected. We argue that the cause is that the majority of the optically detected clusters are relatively poor X-ray emitters, with X-ray fluxes fainter than the flux limit (for extended sources) of our survey, f_x (0.3-2 keV) ≃ 2x10^–14 erg/cm2/s.