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We report the results of a search for an emission line from radiativelydecaying dark matter in the Chandra X-ray Observatory spectrum of theultra-faint dwarf spheroidal galaxy Willman 1. 99% confidence line flux upperlimits over the 0.4-7 keV Chandra bandpass are derived and mapped to an allowedregion in the sterile neutrino mass-mixing angle plane that is consistent withrecent constraints from Suzaku X-ray Observatory and Chandra observations ofthe Ursa Minor and Draco dwarf spheroidals. A significant excess to thecontinuum, detected by fitting the particle-background-subtracted sourcespectrum, 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 at68% (90%) confidence. Interpreting this as an emission line from sterileneutrino radiative decay, we derive the corresponding allowed range of sterileneutrino mass and mixing angle using two approaches. The first assumes thatdark matter is solely composed of sterile neutrinos, and the second relaxesthat requirement. The feature is consistent with the sterile neutrino mass of5.0 +/- 0.2 keV and a mixing angle in a narrow range for which neutrinooscillations can produce all of the dark matter and for which sterile neutrinoemission from the cooling neutron stars can explain pulsar kicks, thusbolstering both the statistical and physical significance of our measurement.

Dark Matter Search Using Chandra Observations of Willman 1, and a Spectral Feature Consistent with a Decay Line of a 5 keV Sterile Neutrino