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A long-standing problem of strong lensing by galaxy clusters regards theobserved high rate of giant gravitational arcs as compared to the predictionsin the framework of the "standard" cosmological model. Recently, few otherinconsistencies between theoretical expectations and observations have beenclaimed which regard the large size of the Einstein rings and the highconcentrations of few clusters with strong lensing features. All of theseproblems consistently indicate that observed galaxy clusters may begravitational lenses stronger than expected. We use clusters extracted from theMareNostrum Universe to build up mock catalogs of galaxy clusters selectedthrough their X-ray flux. We use these objects to estimate the probabilitydistributions of lensing cross sections, Einstein rings, and concentrations forthe sample of 12 MACS clusters at $z>0.5$ presented in Ebeling et al. (2007)and discussed in Zitrin et al. (2010). We find that simulated clusters produce$\sim 50%$ less arcs than observed clusters do. The medians of thedistributions of the Einstein ring sizes differ by $\sim 25%$ betweensimulations and observations. We estimate that, due to cluster triaxiality andorientation biases affecting the lenses with the largest cross sections, theconcentrations of the individual MACS clusters inferred from the lensinganalysis should be up to a factor of $\sim 2$ larger than expected from the$\Lambda$CDM model. The arc statistics, the Einstein ring, and theconcentration problems in strong lensing clusters are mitigated but not solvedon the basis of our analysis. Nevertheless, due to the lack of redshifts formost of the multiple image systems used for modeling the MACS clusters, theresults of this work will need to be verified with additional data. Theupcoming CLASH program will provide an ideal sample for extending ourcomparison (abridged).

Comparison of an X-ray selected sample of massive lensing clusters with the MareNostrum Universe LCDM simulation