2006A&A...446..417F

We present the Chandra analysis of the rich galaxy cluster Abell 521 (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 Brightest Cluster Galaxy (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 for the origin of the hot central bar is that we observe in projection the shock fronts due to this older cluster-cluster collision. However, the two hypotheses are not mutually exclusive. Two other structures possibly interacting with the main cluster are detected on the West and North-East sides of the BCG. We also reveal the presence of two northern edges in the ICM density, which could be due to the ongoing merging events observed in the central field of the cluster, or even in its outer regions. A521 is a spectacular example of a multiple merger cluster made up of several substructures converging at different epochs towards the centre of the system. The very perturbed dynamical state of this cluster is also confirmed by our discovery of a radio relic in its South-East region.