The XMM Cluster Survey: new evidence for the 3.5 keV feature in clusters is inconsistent with a dark matter origin

There have been several reports of a detection of an unexplained excess of X-ray emission at ≃ 3.5 keV in astrophysical systems. One interpretation of this excess is the decay of sterile neutrino dark matter. The most influential study to date analysed 73 clusters observed by the XMM-Newton satellite. We explore evidence for a ≃ 3.5 keV excess in the XMM-PN spectra of 117 redMaPPer galaxy clusters (0.1 < z < 0.6). In our analysis of individual spectra, we identify three systems with an excess of flux at ≃ 3.5 keV. In one case (XCS J0003.3+0204) this excess may result from a discrete emission line. None of these systems are the most dark matter dominated in our sample. We group the remaining 114 clusters into four temperature (T_ X) bins to search for an increase in ≃ 3.5 keV flux excess with T_ X - a reliable tracer of halo mass. However, we do not find evidence of a significant excess in flux at ≃ 3.5 keV in any T_ X bins. To maximise sensitivity to a potentially weak dark matter decay feature at ≃ 3.5 keV, we jointly fit 114 clusters. Again, no significant excess is found at ≃ 3.5 keV. We estimate the upper limit of an undetected emission line at ≃ 3.5 keV to be 2.41 × 10^-6 photons cm^-2 s^-1, corresponding to a mixing angle of sin^2(2θ)=4.4 × 10^-11, lower than previous estimates from cluster studies. We conclude that a flux excess at ≃ 3.5 keV is not a ubiquitous feature in clusters and therefore unlikely to originate from sterile neutrino dark matter decay.