The environmental dependence of X-ray AGN activity at $z\sim0.4$
  We present an analysis of the X-ray Active Galactic Nucleus (AGN) population
in a sample of seven massive galaxy clusters in the redshift range
$0.35<z<0.45$. We utilize high-quality Chandra X-ray imaging to robustly
identify AGN and precisely determine cluster masses and centroids. Follow-up
VIMOS optical spectroscopy allows us to determine which AGN are cluster
members. Studying the subset of AGN with 0.5-8 keV luminosities
$>6.8\times10^{42}~\mathrm{erg~s^{-1}}$, within $r\leq2r_{500}$ (approximately
the virial radius), we find that the cluster AGN space density scales with
cluster mass as $\sim M^{-2.0^{+0.8}_{-0.9}}$. This result rules out zero mass
dependence of the cluster X-ray AGN space density at the 2.5$\sigma$ level. We
compare our cluster X-ray AGN sample to a control field with identical
selection and find that the cluster AGN fraction is significantly suppressed
relative to the field when considering the brightest galaxies with $V<21.5$.
For fainter galaxies, this difference is not present. Comparing the X-ray
hardness ratios of cluster member AGN to those in the control field, we find no
evidence for enhanced X-ray obscuration of cluster member AGN. Lastly, we see
tentative evidence that disturbed cluster environments may contribute to
enhanced AGN activity.
