detections.

Fig. \[fig:linemerge\] shows the additional residuals at $\rm{\sim3.85~keV}$ and $\rm{\sim4.7~keV}$. The shape of spectral features in the CCD spectra can change with temperature because they are blends of several neighbouring emission lines and the ratio between those is temperature dependent.

We used simulations to verify the line-detection efficiency of our stacking and modelling method (see Appendix \[sec:simspec\]) and show that small additional residuals from modelling the many averaged temperatures of the ICM with a two temperature model does not affect the limits derived for the 3.55keV line (see Appendix \[sec:residuals\]).

Estimating cluster masses {#sec:mass}
=========================

To constrain the expected strength of the DM emission line in the cluster spectra we need to estimate the DM mass in the FOV. With the measured average temperatures of the clusters we used temperature-mass scaling relations from @2009ApJ...692.1033V to estimate the overall cluster mass $\rm{M_{500}}$ and the related overdensity radius $\rm{r_{500}}$ .

We assumed the cluster density follows an NFW profile [@1997ApJ...490..493N] and used a fixed concentration (C=4), as the expected average value of the sample [compare e.g. @2006ApJ...640..691V]. By integrating $\rm{\rho(r)}$ along the line of sight we obtained the DM mass surface density profiles [see e.g. @2000ApJ...534...34W; @2015MNRAS.451.2447U]. We estimated the DM 