ar X iv : a st ro - p h / 04 11 45 4 v 1 1 6 N ov 2 00 4 Hypernovae and light dark matter as possible Galactic positron sources

Recently INTEGRAL/SPI has observed a strong and extended emission resulting from electron-positron annihilation in the Galactic center region, consistent with the Galactic bulge geometry. The positron production rate, estimated to more than 10 per second, is very high and raises a challenging question about the nature of the Galactic positron source. Commonly considered astrophysical positron injectors, namely type Ia supernovae are rare events and fall short to explain the observed positron production rate. In this paper, we study the possibility of Galactic positron production by hypernovae events, exemplified by the recently observed SN2003dh/GRB030329, an asymmetric explosion of a Wolf-Rayet star associated with a gamma-ray burst. In this kind of events, the ejected material becomes quickly transparent to positrons, which spread out in the interstellar medium. Non radioactive processes, such as decays of heavy dark matter particles (neutralinos) predicted by most extensions of the standard model of particle physics, could also produce positrons as byproducts. However they are expected to be accompanied by a large flux of high-energy gamma-rays, which were not observed by EGRET and ground based Tcherenkov experiments. In this context we explore the possibility of direct positron production by annihilation of light dark matter particles.