An unidentified line in X-ray spectra of the Andromeda galaxy and
  Perseus galaxy cluster
  We report a weak line at 3.52+/-0.02 keV in X-ray spectra of M31 galaxy and
the Perseus galaxy cluster observed by MOS and PN cameras of XMM-Newton
telescope. This line is not known as an atomic line in the spectra of galaxies
or clusters. It becomes stronger towards the centers of the objects; is
stronger for Perseus than for M31; is absent in the spectrum of a deep "blank
sky'' dataset. Although for each object it is hard to exclude that the feature
is due to an instrumental effect or an atomic line, it is consistent with the
behavior of a dark matter decay line. Future (non-)detections of this line in
multiple objects may help to reveal its nature.
