SuperModel Analysis of Abell 1246 and J255: on the Evolution of Galaxy
  Clusters from High to Low Entropy States
  We present an analysis of high-quality X-ray data out to the virial radius
for the two galaxy clusters Abell 1246 and GMBCG J255.34805+64.23661 (J255) by
means of our entropy-based SuperModel. For Abell 1246 we find that the
spherically-averaged entropy profile of the intracluster medium (ICM)
progressively flattens outwards, and that a nonthermal pressure component
amounting to ~20% of the total is required to support hydrostatic equilibrium
in the outskirts; there we also estimate a modest value C~1.6 of the ICM
clumping factor. These findings agree with previous analyses on other
cool-core, relaxed clusters, and lend further support to the picture by Lapi et
al. (2010) that relates the entropy flattening, the development of nonthermal
pressure component, and the azimuthal variation of ICM properties to weakening
boundary shocks. In this scenario clusters are born in a high-entropy state
throughout, and are expected to develop on similar timescales a low entropy
state both at the center due to cooling, and in the outskirts due to weakening
shocks. However, the analysis of J255 testifies how such a typical evolutionary
course can be interrupted or even reversed by merging especially at
intermediate redshift, as predicted by Cavaliere et al. (2011b). In fact, a
merger has rejuvenated the ICM of this cluster at z~0.45 by reestablishing a
high entropy state in the outskirts, while leaving intact or erasing only
partially the low-entropy, cool core at the center.
