  We present here the results of a detailed study of the X-ray properties of
the cluster of galaxies Abell 3128 (z=0.06), based on the analysis of deep (100
ks) XMM-Newton data. The most obvious feature of the X-ray morphology of A3128
is the presence of two X-ray peaks separated by ~12'. By detecting the
redshifted Fe K line, we find that the Northeast (NE) X-ray peak observed
toward A3128 is a distant luminous cluster of galaxies at redshift z=0.44. Our
subsequent optical spectroscopic observation of a distant radio bright galaxy
in the centre of the NE X-ray peak with the Magellan telescope also revealed a
redshift of z=0.44, confirming the association of the galaxy with the cluster
seen in X-rays. We detect a gravitational arc around the galaxy. The properties
of this galaxy indicate that it is the cD galaxy of the cluster in the
background. The properties of the Southwest X-ray peak suggest that it is the
core of a group merging with A3128 along our line of sight. Based on 2D maps of
thermodynamic properties of the intra-cluster medium determined after
subtracting a model for the background cluster, we conclude that an enhanced
surface brightness region at a distance of ~2.8' from the centre of the galaxy
distribution is the centre of the gravitational potential of the cluster A3128.
The unrelaxed nature of A3128 can be attributed to its location in the high
density environment of the Horologium-Reticulum supercluster.
