behind the depth of the upper limit on the mixing-angle in the sterile neutrino scenario is the DM mass in the FOV where the spectra have been extracted divided by the luminosity distance squared. To maximize this property, homogeneous, deep coverage out to large radii of a massive nearby system is needed (higher $\rm{[10^{10}~M_\odot~Mpc^{-2}]}$ value in Tab. \[tab:clusterpars\]). In this sample the best candidate for such a study with *Chandra* was Abell1795 because of its very deep exposure (see Tab. \[tab:clusterpars\]).

As demonstrated by @2014ApJ...789...13B a 1Ms observation of the Perseus cluster with the *Hitomi (Astro-H)* SXS instrument [@2014arXiv1412.1176K] will allow to distinguish between a plasma emission line of the ICM, broadened by the turbulence in the cluster ($\rm{\sim 300~km/s}$) and a DM decay line, broadened by the virial velocity ($\rm{\sim 1300~km/s}$) of the DM halo. The large FOV of the *eROSITA* observatory [@2012arXiv1209.3114M] will allow for tight constraints on the line [see also @2015JCAP...09..060Z], homogeneously covering nearby X-ray bright clusters to large radii, even with lower effective area at 3.55keV compared to **XMM-Newton** and *Chandra*. Only deeper observations with current or future instruments will allow to finally decide the nature of the detection as summarised in a recent white 