The X-CLASS - redMaPPer galaxy cluster comparison: I. Identification
  procedures
  We performed a detailed and, for a large part interactive, analysis of the
matching output between the X-CLASS and redMaPPer cluster catalogues. The
overlap between the two catalogues has been accurately determined and possible
cluster positional errors were manually recovered. The final samples comprise
270 and 355 redMaPPer and X-CLASS clusters respectively. X-ray cluster matching
rates were analysed as a function of optical richness. In a second step, the
redMaPPer clusters were correlated with the entire X-ray catalogue, containing
point and uncharacterised sources (down to a few 10^{-15} erg s^{-1} cm^{-2} in
the [0.5-2] keV band). A stacking analysis was performed for the remaining
undetected optical clusters. Main results show that neither of the wavebands
misses any massive cluster (as coded by X-ray luminosity or optical richness).
After correcting for obvious pipeline short-comings (about 10% of the cases
both in optical and X-ray), ~50% of the redMaPPer (down to a richness of 20)
are found to coincide with an X-CLASS cluster; when considering X-ray sources
of any type, this fraction increases to ~ 80%; for the remaining objects, the
stacking analysis finds a weak signal within 0.5 Mpc around the cluster optical
centers. The fraction of clusters totally dominated by AGN-type emission
appears to be of the order of a few percent. Conversely ~ 40% of the X-CLASS
clusters are identified with a redMaPPer (down to a richness of 20) - part of
the non-matches being due to the fact that the X-CLASS sample extends further
out than redMaPPer (z<1 vs z<0.6); extending the correlation down to a richness
of 5, raises the matching rate to ~ 65%.
