X-ray photons from late-decaying majoron dark matter

An attractive way to generate neutrino masses as required to account for current neutrino oscillation data involves the spontaneous breaking of lepton number. The resulting majoron may pick up a mass due to gravity. If its mass lies in the kilovolt scale, the majoron can play the role of late-decaying dark matter (LDDM), decaying mainly to neutrinos. In general the majoron has also a sub-dominant decay to two photons leading to a mono-energetic emission line which can be used as a test of the LDDM scenario. We compare expected photon emission rates with observations in order to obtain model-independent restrictions on the relevant parameters. We also illustrate the resulting sensitivities within an explicit seesaw realization, where the majoron couples to photons due to the presence of a Higgs triplet.