Detection of An Unidentified Emission Line in the Stacked X-ray spectrum
  of Galaxy Clusters
  We detect a weak unidentified emission line at E=(3.55-3.57)+/-0.03 keV in a
stacked XMM spectrum of 73 galaxy clusters spanning a redshift range 0.01-0.35.
MOS and PN observations independently show the presence of the line at
consistent energies. When the full sample is divided into three subsamples
(Perseus, Centaurus+Ophiuchus+Coma, and all others), the line is significantly
detected in all three independent MOS spectra and the PN "all others" spectrum.
It is also detected in the Chandra spectra of Perseus with the flux consistent
with XMM (though it is not seen in Virgo). However, it is very weak and located
within 50-110eV of several known faint lines, and so is subject to significant
modeling uncertainties. On the origin of this line, we argue that there should
be no atomic transitions in thermal plasma at this energy. An intriguing
possibility is the decay of sterile neutrino, a long-sought dark matter
particle candidate. Assuming that all dark matter is in sterile neutrinos with
m_s=2E=7.1 keV, our detection in the full sample corresponds to a neutrino
decay mixing angle sin^2(2theta)=7e-11, below the previous upper limits.
However, based on the cluster masses and distances, the line in Perseus is much
brighter than expected in this model. This appears to be because of an
anomalously bright line at E=3.62 keV in Perseus, possibly an Ar XVII
dielectronic recombination line, although its flux would be 30 times the
expected value and physically difficult to understand. In principle, such an
anomaly might explain our line detection in other subsamples as well, though it
would stretch the line energy uncertainties. Another alternative is the above
anomaly in the Ar line combined with the nearby 3.51 keV K line also exceeding
expectation by factor 10-20. Confirmation with Chandra and Suzaku, and
eventually Astro-H, are required to determine the nature of this new
line.(ABRIDGED)
