Possible scenario explained in the second part of Sect. 5.3. Top panel: cluster as seen in projection, bottom panel: cluster as it appears in the plane of the sky. We show the shock front in two moments, when it first encounter the cloud of aged plasma (t₀) and in the current position (t₁). The colour encodes the radio spectral index of the plasma from red (flat) to blue (steep), as it is expected to be at t = t₁.