energy bins is $\rm{\sim 0.01~keV}$ for both detectors (see response in Fig. \[fig:fakelinemerge\]). The de-redshifting was based on optical redshifts known from previous catalogues (see Tab.\[tab:clusterpars\]). We added all de-redshifted, unabsorbed spectra and averaged the response files with weights according to the fraction of the total counts of the merged spectrum (in the 2-5keV range). In the fitted range 2-5keV, the merged spectra contain $\rm{3.2\times10^6}$ counts (ACIS-I) and $\rm{2.7\times10^6}$ counts (ACIS-S). The average exposure times of each observation were added to obtain an average count rate in the final spectra. The background spectrum was renormalised to match the 10.0-12.5keV energy range count rate. We used the `XSPEC` models `apec+apec+zgauss` without absorption and z=0 to fit the spectra and estimate the additional flux allowed for a line at 3.55keV. The fit of the merged spectra was performed between 3-5keV because there is no strong variation in the detector response in that region and thus possible systematics in the calculated average response are minimal. Fig. \[fig:linemerge\] shows some residual structures due to the modelling of a complex stack of cluster spectra with a two-temperature `apec` model.

We tested fitting the emission features in the spectrum with separate Gaussian lines on top of 