Dark Matter Search Using Chandra Observations of Willman 1, and a
  Spectral Feature Consistent with a Decay Line of a 5 keV Sterile Neutrino
  We report the results of a search for an emission line from radiatively
decaying dark matter in the Chandra X-ray Observatory spectrum of the
ultra-faint dwarf spheroidal galaxy Willman 1. 99% confidence line flux upper
limits over the 0.4-7 keV Chandra bandpass are derived and mapped to an allowed
region in the sterile neutrino mass-mixing angle plane that is consistent with
recent constraints from Suzaku X-ray Observatory and Chandra observations of
the Ursa Minor and Draco dwarf spheroidals. A significant excess to the
continuum, detected by fitting the particle-background-subtracted source
spectrum, indicates the presence of a narrow emission feature with energy 2.51
+/- 0.07 (0.11) keV and flux [3.53 +/- 1.95 (2.77)] X 10^(-6) photons/cm^2/s at
68% (90%) confidence. Interpreting this as an emission line from sterile
neutrino radiative decay, we derive the corresponding allowed range of sterile
neutrino mass and mixing angle using two approaches. The first assumes that
dark matter is solely composed of sterile neutrinos, and the second relaxes
that requirement. The feature is consistent with the sterile neutrino mass of
5.0 +/- 0.2 keV and a mixing angle in a narrow range for which neutrino
oscillations can produce all of the dark matter and for which sterile neutrino
emission from the cooling neutron stars can explain pulsar kicks, thus
bolstering both the statistical and physical significance of our measurement.
