Hypernovae/gamma-ray bursts in the Galactic center as possible sources of galactic positrons

The observation of a strong and extended positron-electron line annihilation emission in the central regions of the Galax), by the SPectrometer on the International Gamma-Ray Astrophysical Laboratory (INTEGRAL/SPI), consistent with the Galactic bulge geometry and without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV Al-26 decay line, is challenging. Leaving aside the geometrical question, we address the problem of the adequate positron sources, showing the potentiality of a new category of Type Ic supernovae (SNe Ic), exemplified by SN 2003dh, that is associated with a gamma-ray burst (GRB). This kind of supernova/ hypernova/GRB event is interpreted as the result of a bipolar Wolf-Rayet explosion, which produces a large amount of Ni-56 and ejects it at high velocity along the rotation axis. The bulk of positions resulting from Co-56 decay escapes in the surrounding medium as a result of the rapid thinning of the ejecta in the polar direction. We show that a rate of about 0.02 SN 2003dh-like events per century in the central region of the Galaxy is sufficient to explain the positron flux detected by INTEGRAL/SPI. In order to explain this flux by SN la events alone. a rate of 0.5 per century is necessary, much higher than indicated by Galactic evolutionary models applied to the bulge. Further observations of late light curves of SNe la and SNe Ic in the bulge of spiral galaxies, together with three-dimensional hydrodynamic calculations of anisotropic ejections of Ni-56 in SN Ic/GRB events, will allow us to estimate the separate contributions of SNe la and SNe Ic to positron injection.