Discovery of a 3.5 keV line in the Galactic Center and a Critical Look
  at the Origin of the Line Across Astronomical Targets
  We examine the claimed excess X-ray line emission near 3.5 keV including both
a new analysis of XMM-Newton observations of the Milky Way center and a
reanalysis of the data on M~31 and clusters. In no case do we find conclusive
evidence for an excess. In the case of the Galactic center we show that known
plasma lines, including in particular K XVIII lines at 3.48 and 3.52 keV,
provide a satisfactory fit to the XMM data. We estimate the expected flux of
the K XVIII lines and find that the measured line flux falls squarely within
the predicted range based on the brightness of other well-measured lines in the
energy range of interest and on detailed multi-temperature plasma models. We
then re-assess the evidence for excess emission from clusters of galaxies,
allowing for systematic uncertainty in the expected flux from known plasma
lines and additional uncertainty due to potential variation in the abundances
of different elements. We find that no conclusive excess line emission can be
advocated when considering systematic uncertainties in Perseus or in other
clusters. We also re-analyze the XMM data for M~31 and find no statistically
significant line emission near 3.5 keV to a level greater than one sigma.
Finally, we analyze the Tycho supernova remnant, which shows similar plasma
features to the sources above, but does not host any significant dark matter.
We detect a 3.55 keV line from Tycho, which points to possible systematic
effects in the flux determination of weak lines, or to relative elemental
abundances vastly different from theoretical expectations.
