  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+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8 x
10^45 erg/sec this is the most X-ray luminous cluster at redshifts z >= 1. We
measured a gas temperature of 8.2 +- 0.9 keV and and estimate a cluster mass
M(500) = 5.6 x 10^14 M(solar). The optical imaging revealed a rich cluster of
galaxies.
