XMM-Newton observations of the binary cluster system Abell 399/401

Both Abell 399 and Abell 401 are rich clusters of galaxies, with global temperatures of 7.2 (Abell 399) and 8.5 keV (Abell 401), respectively. They lie at a projected separation of ~3 Mpc, forming a close pair. We have observed the system with the XMM-Newton satellite. The data of each cluster show significant departures from our idealized picture of relaxed rich clusters. Neither of the two contains a cooling flow, and we find that their central regions are nearly isothermal, with some small-scale inhomogeneities. The image analysis derives β values that are smaller than the canonical value of 0.65, and the surface brightness distribution is not symmetric around the central dominant galaxies: there are irregularities in the central ~200 kpc, and asymmetries on larger scales, in that the intracluster gas in each cluster is more extended towards the other member of the system. Both clusters host extended radio haloes and a plethora of tailed radio galaxies. The halo in Abell 399 appears to be correlated with a sharp edge apparent in the XMM-Newton images, and a region of harder X-ray emission. There is also evidence for enhanced X-ray flux in the region between the two clusters, where the temperature is higher than our expectations.

Although tidal or compression effects might affect the large-scale structure of the two clusters, we show that these cannot account for the distortions seen in the inner regions. We argue that the reasonably relaxed morphology of the clusters, and the absence of major temperature anomalies, argues against models in which the two have already experienced a close encounter. The properties of the intermediate region suggests that they are at an early stage of merging, and are currently interacting mildly, because their separation is still too large for more dramatic effects. The substructure we find in their inner regions seems to point to their individual merging histories. It seems likely that in the Abell 399/401 system, we are witnessing two merger remnants, just before they merge together to form a single rich cluster of galaxies. This picture is consistent with recent numerical simulations of cluster formation.