Upper-left panel: comparison between the radial profiles of the knot1 surface brightness (blue crosses) and that of a simulated point-like source (red crosses, including the contribution of the background) for the XMM-Newton data. Upper-right panel: same as left panel, but for the Chandra 2003 observation. Lower-left panel: same as upper-right panel, but for the Chandra 2012 observation. Lower-right panel: same as lower-left panel, but for a simulated Gaussian profile with σ = 3″ (red crosses).