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The Survey Science Centre of the XMM-Newton satellite released the firstincremental version of the 2XMM catalogue in August 2008 . With more than220,000 X-ray sources, the 2XMMi was at that time the largest catalogue ofX-ray sources ever published and thus constitutes an unprecedented resource forstudying the high-energy properties of various classes of X-ray emitters suchas AGN and stars. The advent of the 7th release of the Sloan Digital Sky Surveyoffers the opportunity to cross-match two major surveys and extend the spectralenergy distribution of many 2XMMi sources towards the optical bands. We herepresent a cross-matching algorithm based on the classical likelihood ratioestimator. The method developed has the advantage of providing trueprobabilities of identifications without resorting to Monte-Carlo simulations.Over 30,000 2XMMi sources have SDSS counterparts with individual probabilitiesof identification higher than 90%. Using spectroscopic identifications from theSDSS DR7 catalogue supplemented by extraction from other catalogues, we buildan identified sample from which the way the various classes of X-ray emittersgather in the multi dimensional parameter space can be analysed. We investigatetwo scientific use cases. In the first example we show how thesemulti-wavelength data can be used to search for new QSO2s. Although no specificrange of observed properties allows us to identify Compton Thick QSO2s, we showthat the prospects are much better for Compton Thin AGN2 and discuss severalpossible multi-parameter selection strategies. In a second example, we confirmthe hardening of the mean X-ray spectrum with increasing X-ray luminosity on asample of over 500 X-ray active stars and reveal that on average X-ray active Mstars display bluer $g-r$ colour indexes than less active ones (abridged).

Cross-correlation of the 2XMMi catalogue with Data Release 7 of the Sloan Digital Sky Survey