AGN Triggering in the Infall Regions of Distant X-Ray Luminous Galaxy Clusters at 0.9 < z less than or similar to 1.6

Observational constraints on the average radial distribution
profile of AGN in distant galaxy clusters can provide important
clues on the triggering mechanisms of AGN activity in dense
environments and are essential for a completeness evaluation of
cluster selection techniques in the X-ray and mm wavebands. The aim
of this work is a statistical study with XMM-Newton of the presence
and distribution of X-ray AGN in the large-scale structure
environments of 22 X-ray luminous galaxy clusters in the redshift
range 0.9 < z less than or similar to 1.6 compiled by the
XMM-Newton Distant Cluster Project (XDCP). To this end, the X-ray
point source lists from detections in the soft band (0.35-2.4 keV)
and full band (0.3-7.5 keV) were stacked in cluster-centric
coordinates and compared to average background number counts
extracted from three independent control fields in the same
observations. A significant full-band (soft-band) excess of similar
to 78 (67) X-ray point sources is found in the cluster fields
within an angular distance of 8' (4 Mpc) at a statistical
confidence level of 4.0 sigma (4.2 sigma), corresponding to an
average number of detected excess AGN per cluster environment of
3.5 +/- 0.9 (3.0 +/- 0.7). The data point towards a rising radial
profile in the cluster region (r < 1Mpc) of predominantly
low-luminosity AGN with an average detected excess of about one
point source per system, with a tentative preferred occurrence
along the main cluster elongation axis. A second statistically
significant overdensity of brighter soft-band-detected AGN is found
at cluster-centric distances of 4'-6' (2-3 Mpc), corresponding to
about three times the average cluster radius R-200 of the systems.
If confirmed, these results would support the idea of two different
physical triggering mechanisms of X-ray AGN activity in dependence
of the radially changing large-scale structure environment of the
distant clusters. For high-z cluster studies at lower spatial
resolution with the upcoming eROSITA all-sky X-ray survey, the
results suggest that cluster-associated X-ray AGN may impose a bias
in the spectral analysis of high-z systems, while their detection
and flux measurements in the soft band may not be significantly
affected.