2008ApJ...673..752B

We derived constraints on parameters of a radiatively decaying warm dark matter particle, e.g., the mass and mixing angle for a sterile neutrino, using Chandra X-ray spectra of galaxy cluster 1E 0657-56 (the ``bullet'' cluster). The constraints are based on nondetection of the sterile neutrino decay emission line. This cluster exhibits spatial separation between the hot intergalactic gas and the dark matter, which helps to disentangle their X-ray signals. It also has a very long X-ray observation and a total mass measured via gravitational lensing. This makes the resulting constraints on a sterile neutrino complementary to earlier results that used different cluster mass estimates. Our limits are comparable to the existing cluster constraints (although they are weaker than the best constraints derived from M31 and the Milky Way halo).