Distant clusters of galaxies in the 2XMM/SDSS footprint: follow-up
  observations with the LBT
  Context: Galaxy clusters at high redshift are important to test cosmological
models and models for the growth of structure. They are difficult to find in
wide-angle optical surveys, however, leaving dedicated follow-up of X-ray
selected candidates as one promising identification route. Aims: We aim to
increase the number of galaxy clusters beyond the SDSS-limit, z ~ 0.75.
Methods: We compiled a list of extended X-ray sources from the 2XMMp catalogue
within the footprint of the Sloan Digital Sky Survey. Fields without optical
counterpart were selected for further investigation. Deep optical imaging and
follow-up spectroscopy were obtained with the Large Binocular Telescope,
Arizona (LBT), of those candidates not known to the literature. Results: From
initially 19 candidates, selected by visually screening X-ray images of 478
XMM-Newton observations and the corresponding SDSS images, 6 clusters were
found in the literature. Imaging data through r,z filters were obtained for the
remaining candidates, and 7 were chosen for multi-object (MOS) spectroscopy.
Spectroscopic redshifts, optical magnitudes, and X-ray parameters (flux,
temperature, and luminosity) are presented for the clusters with spectroscopic
redshifts. The distant clusters studied here constitute one additional redshift
bin for studies of the L-T relation, which does not seem to evolve from high to
low redshifts. ...
