Continuum gamma-ray emission from light dark matter positrons and electrons
Abstract
The annihilation of light dark matter was recently advocated as a possible explanation of the large positron injection rate at the Galactic center deduced from observations by the SPI spectrometer aboard INTEGRAL. The modelling of internal Bremsstrahlung and in-flight annihilation radiations associated to this process drastically reduced the mass range of this light dark matter particle. We estimate critically the various energy losses and radiations involved in the propagation of the positron before its annihilation --in-flight or at rest. Using a simple model with mono-energetic positrons injected and confined to the Galactic bulge, we compute energy losses and gamma-ray radiations caused by ionization, Bremsstrahlung interactions as well as in-flight and at rest annihilation and compare these predictions to the available observations, for various injection energies. Confronting the predictions with observations by the EGRET, COMPTEL, SPI and IBIS/ISGRI instruments, we deduce a new mass upper bound of 25-30 MeV for the hypothetical light dark matter particle. We stress out how the precise limit depends on the degree of ionization of the propagation medium and how crucial morphology studies will be to set more stringent constraints, as simple flux considerations suffer from our poor knowledge of the radiation from unresolved sources and cosmic-ray interactions.Keywords
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