cluster with the NuSTAR satellite to constrain possible decay lines up to higher energies but due to lower sensitivity at 3.5keV do not exclude previous detections.

The main caveat of our mixing-angle calculations is the uncertainty of the DM masses in the FOV which are derived from scaling relations with the temperature of the hot ICM. The main caveats of the flux constraints are possible uncertainties in the calibration of the effective area and detector response and in the assumed background spectrum (derived from *Chandra* blank sky observations). Alternative explanations for the potential line could be unexpectedly strong plasma emission lines or charge exchange processes around 3.55keV . Candidate plasma lines are KXVIII (3.476, 3.496 and 3.515keV), ClXVII (3.510 and 3.509keV) as well as ArXVII (3.617 and 3.618keV) [^1].

The merged spectra in Fig. \[fig:linemerge\] show an additional weak ($\rm{\gtrsim1\sigma}$) residual around 4.7keV (mainly in ACIS-S) which is not visible in the simulations (see Fig. \[fig:fakelinemerge\]). This could be explained by an unexpectedly high abundance (higher than assumed in `apec` metallicity model) of Titanium (TiXXI, multiple lines between 4.70-4.76keV). The residual at 4.7keV disappears for 1-3 times solar abundance but in this case the line of TiXXII at about 4.9keV 