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The term X-ray luminous galaxy cluster is generally used for a galaxy cluster discovered in an X-ray astronomical survey. An X-ray source can be identified as a likely galaxy cluster if it does not vary over observation time (much too rapid for such a large source), if it is an extended source, and if suitable counterparts in optical or infrared are found. The results will naturally be clusters that are bright in X-rays, and can include some at substantial redshift unlikely to be found otherwise. Identifying them in this manner allows the creation of data sets of distant galaxy clusters for study.