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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 is a galaxy cluster candidate if it does not vary over observation time (which would be too rapid for such a large source) and is an extended source and naturally, a suitable counterpart in optical or infrared can confirm the candidate. The discoveries will naturally be clusters that are bright in X-rays, and can be at substantial redshift and unlikely to be found otherwise. Identifying galaxy clusters in this manner allows the creation of data sets of distant examples for study.