  Recent work by Bulbul et al. and Boyarsky et al. has suggested that a line
feature at approx. 3.5 keV in the X-ray spectra of galaxy clusters and
individual galaxies seen with XMM-Newton is due to the decay of sterile
neutrinos, a dark matter candidate. This identification has been criticized by
Jeltema and Profumo on the grounds that model spectra suggest that atomic
transitions in helium-like potassium (K XVIII) and chlorine (Cl XVI) are more
likely to be the emitters. Here it is pointed out that the K XVIII lines have
been observed in numerous solar flare spectra at high spectral resolution with
the RESIK crystal spectrometer and also appear in Chandra HETG spectra of the
coronally active star sigma Gem. In addition, the solar flare spectra at least
indicate a mean coronal potassium abundance which is a factor of between 9 and
11 higher than the solar photospheric abundance. This fact, together with the
low statistical quality of the XMM-Newton spectra, completely accounts for the
approx. 3.5 keV feature and there is therefore no need to invoke a sterile
neutrino interpretation of the observed line feature at 3.5 keV.
