Relation between apparent SZ signal (Y₅₀₀) and the corresponding normalised Y_X parameter for single systems confirmed with XMM-Newton (green and red points). Black points show clusters in the Planck-ESZ sample with XMM-Newton archival data as presented in Planck Collaboration (2011c). The blue lines denote the Y₅₀₀ scaling relations predicted from the REXCESS X-ray observations (Arnaud et al. 2010). The grey area corresponds to median Y₅₀₀ values in Y_X bins with  ± 1σ standard deviation.