  We present the Chandra analysis of the galaxy cluster A521 (z=0.247). The
high resolution of the Chandra observation has allowed us to refine the
original merging scenario proposed for A521, and to reveal new features in its
X-ray emission. A521 has strongly substructured ICM density and temperature
maps. Its X-ray diffuse emission is elongated along a NW/SE direction (SX2) and
shows two major components, a main cluster and a northern group of galaxies.
This latter is in turn substructured, showing a clump of cold and very dense
gas centred on the BCG, and a northern tail aligned in the SX2 direction. A
compression of the X-ray isophotes is also observed South of the BCG. We
conclude that the northern group is infalling onto the main cluster along the
NW/SE direction. This hypothesis is corroborated by the presence of a hot bar
in the ICM temperature map located between the southern and northern regions,
as the gas could be compressionally heated due to the subclusters' collision.
The hot region corresponds to the eastern part of an over-dense ridge of
galaxies, along which it was originally suggested that a merging of subclusters
has recently occurred along the line of sight. An alternative hypothesis about
the origin of the hot central bar is that we could observe in projection the
shock fronts due to this older cluster-cluster collision. Two other structures
possibly interacting with the main cluster are detected on the West and
North-East sides of the BCG. We also uncover the presence of two northern edges
in the ICM density. A521 is a spectacular example of a multiple merger cluster
made up by several substructures converging at different epochs towards the
centre of the system. The very pertubed dynamical state of this cluster is also
confirmed by our discovery of a radio relic in its South-East region.
