Redshift determination from XMM-Newton spectroscopy for the highest quality data (top row, PLCK G234.2 − 20.5) and lowest quality data (bottom row, PLCK G268.5-28) in the sample. Left panels: variation of χ² with z when fitting the EPIC spectra, all other parameters being let free. The dashed and full lines correspond to the 68% and 90% error range, respectively. Right panels: EPIC spectra (data points with errors), together with the best-fitting model thermal model (solid lines) with the position of the redshifted Fe K line marked. Only the data points above 2 keV are shown for clarity, but data down to 0.3 keV are used in the spectral fitting. For PLCK G234.2-20.5 (top right panel), the Fe-K line complex is clearly detected in the EPIC MOS1&2 (red and black points) and pn (green points) spectra. For PLCK G268.5-28 (bottom right panel), only MOS data can be used (see Sect. 3.1) and the spectra are of poor statistical quality. The redshift estimate is ambiguous and the χ² distribution (bottom left panel) shows several minima. The MOS1&2 spectra, summed for clarity, are compared to the best fitting model for z = 0.47 (red line) and z = 0.86 (green line), corresponding to the two lowest minima.