The XXL Survey: XXXIII. Chandra Constraints on the AGN Contamination of
  z > 1 XXL Galaxy Clusters
  Context. The XMM-XXL survey uses observations from XMM-Newton to detect
clusters of galaxies over a wide range in mass and redshift. The moderate PSF
of XMM-Newton means that point sources within or projected onto a cluster may
not be separated from the cluster emission, leading to enhanced luminosities
and affecting the selection function of the cluster survey. Aims. We present
the results of short Chandra observations of 21 galaxy clusters and cluster
candidates at redshifts z $>$ 1 detected in the XMM-XXL survey in X-rays or
selected in the optical and infra-red. Methods. With the superior angular
resolution of Chandra we investigate whether there are any point sources within
the cluster region that were not detected by the XMM-XXL analysis pipeline, and
whether any point sources were misclassified as distant clusters. Results. Of
the 14 X-ray selected clusters, nine are free from significant point source
contamination, either having no previously unresolved sources detected by
Chandra, or with less than about 10% of the reported XXL cluster flux being
resolved into point sources. Of the other five sources, one is significantly
contaminated by previously unresolved AGN, and four appear to be AGN
misclassified as clusters. All but one of these cases are in the subset of less
secure X-ray selected cluster detections and the false positive rate is
consistent with that expected from the XXL selection function modelling. We
also considered a further seven optically-selected cluster candidates
associated with faint XXL sources that were not classed as clusters. By
decontaminating and vetting these distant clusters, we provide a pure sample of
clusters at redshift z $>$ 1 for deeper follow-up observations, and demonstrate
the utility of using Chandra snapshots to test for AGN in surveys with high
sensitivity but poor angular resolution.
