The XMM Cluster Archive Super Survey ( X-CLASS ) is a serendipitously-detected X-ray-selected sample of 845 galaxy clusters based on 2774 XMM archival observations and covering approximately 90 deg ^ { 2 } spread across the high-Galactic latitude ( |b| > 20 \degr ) sky .
The primary goal of this survey is to produce a well-selected sample of galaxy clusters on which cosmological analyses can be performed .
This article presents the photometric redshift followup of a high signal-to-noise subset of 266 of these clusters with declination \delta < +20 \degr with GROND , a seven channel ( grizJHK ) simultaneous imager on the MPG 2.2m telescope at the ESO La Silla Observatory .
We use a newly developed technique based on the red sequence colour-redshift relation , enhanced with information coming from the X-ray detection to provide photometric redshifts for this sample .
We determine photometric redshifts for 236 clusters , finding a median redshift of z = 0.39 with an accuracy of \Delta z = 0.02 ( 1 + z ) when compared to a sample of 76 spectroscopically confirmed clusters .
We also compute X-ray luminosities for the entire sample and find a median bolometric luminosity of 7.2 \times 10 ^ { 43 } \mathrm { erg s ^ { -1 } } and a median temperature 2.9 keV .
We compare our results to the XMM-XCS and XMM-XXL surveys , finding good agreement in both samples .
The X-CLASS catalogue is available online at http : //xmm-lss.in2p3.fr:8080/l4sdb/ .