  We present the results of a detailed analysis of the XMM-Newton observation
of the galaxy cluster Abell 3921. The X-ray morphology of the cluster is
elliptical, with the centroid offset from the brightest cluster galaxy by 17
arcsec, and with a pronounced extension toward the NW. Subtraction of a 2D beta
model fit to the main cluster emission reveals a large scale, irregular
residual structure in the direction of the extension, containing both diffuse
emission from the intra cluster medium, and extended emission from the second
and third-brightest cluster galaxies (BG2 and BG3). The greatest concentration
of galaxies in the subcluster lies at the extreme northern edge of the
residual. The cluster exhibits a remarkable temperature structure, in
particular a bar of significantly hotter gas, oriented SE-NW and stretching
from the centre of the cluster towards BG2 and BG3. Our detailed study of the
morphological and thermal structure points to an off-axis merger between a main
cluster and a less massive galaxy cluster infalling from the SE. From
comparison of the temperature map with numerical simulations, and with
independent calculations based on simple physical assumptions, we conclude that
the merging event is ~0.5 Gyr old. The cluster is thus perhaps the best X-ray
observed candidate so far of an intermediate mass ratio, moderate impact
parameter merger.
