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 $\simeq$ 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 $\simeq$ 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
$\simeq$ 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 ($T_{\rm X}$) bins to search for an increase in $\simeq$ 3.5 keV
flux excess with $T_{\rm X}$ - a reliable tracer of halo mass. However, we do
not find evidence of a significant excess in flux at $\simeq$ 3.5 keV in any
$T_{\rm X}$ bins. To maximise sensitivity to a potentially weak dark matter
decay feature at $\simeq$ 3.5 keV, we jointly fit 114 clusters. Again, no
significant excess is found at $\simeq$ 3.5 keV. We estimate the upper limit of
an undetected emission line at $\simeq$ 3.5 keV to be $2.41 \times 10^{-6}$
photons cm$^{-2}$ s$^{-1}$, corresponding to a mixing angle of
$\sin^2(2\theta)=4.4 \times 10^{-11}$, lower than previous estimates from
cluster studies. We conclude that a flux excess at $\simeq$ 3.5 keV is not a
ubiquitous feature in clusters and therefore unlikely to originate from sterile
neutrino dark matter decay.
