Blind sample stack of all the background quasar spectra available at the cluster redshift, showing the normalised flux in log in velocity space. This sample comprises a total of 16 224 spectra and leads to an S/N of the stack of 195. The detection has a rest EW of EW₂₇₉₆ = 0.056 ± 0.015 Å (3.7σ significance), corresponding to a column density of log [N(MgII)/cm⁻²] = 12.12 ± 0.1. The green line shows the fitted continuum used for the EW measurement.