2XMM J083026+524133: the most X-ray luminous cluster at redshift 1

Aims. 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 greater than or similar to 1. Methods. 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. Results. We determined the redshift 0.99 +/- 0.03 for 2XMM J083026.2+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8 x 10(45) erg s(-1) this is the most X-ray luminous cluster at redshifts z >= 1. We measured a gas temperature of 8.2 +/- 0.9 keV and estimate a cluster mass M-500 = 5.6 x 10(14) M-circle dot. The optical imaging revealed a rich cluster of galaxies.