X-ray Constraints on Late Decaying Dark Matter Majorons (or Other Soft X-ray Emitting Candi- dates)*

An attractive way to generate neutrino masses is through the spontaneous breaking of lepton number. The resulting majoron can acquire a mass and thereby be the dark matter. Structure formation requires the dark matter mass to be ≈ 0.15 keV. The majorons can decay into two photons providing a mono-energetic emission line. Observing 0.1 keV photons is challenging, but we have obtained constraints in the 0.07-4.0 keV interval, which have been combined with earlier results and compared to realistic particle physics models for the majoron mass and interactions. The constraints applies to a wide range of dark matter candidates with radiative decays.