2008A&A...487L..33L

In the distant universe X-ray luminous clusters of galaxies are rare objects. Large area surveys are therefore needed to probe the high luminosity end of the cluster population at redshifts z>1. We correlated extended X-ray sources from the second XMM-Newton source catalogue (2XMM) with the SDSS in order to identify new clusters of galaxies. Distant cluster candidates in empty SDSS fields were imaged in the r- and z-bands with the Large Binocular Telescope. We extracted the X-ray spectra of the cluster candidates and fitted thermal plasma models to the data. We determined the redshift 0.99±0.03 for 2XMM J083026.2+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8x10⁴⁵erg/s this is the most X-ray luminous cluster at redshifts z ≥1. We measured a gas temperature of 8.2±0.9keV and estimate a cluster mass M₅₀₀=5.6x10¹⁴M_☉. The optical imaging revealed a rich cluster of galaxies.