Hypernovae/GRB in the Galactic Center as possible sources of Galactic Positrons
Abstract
The observation of a strong and extended positron-electron annihilation emission in the central region of the Galaxy by INTEGRAL-SPI, consistent with the bulge geometry, without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV $^{26}$Al 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 SNIc, exemplified by SN2003dh, which, incidentally, is associated to a gamma-ray burst. This kind of supernova/hypernova/GRB event is interpreted as the result of a non axi-symmetric Wolf-Rayet explosion, which produces a large amount of $^{56}$Ni and ejects it at high velocity along the rotation axis. The bulk of positrons resulting from $^{56}$Co decay escapes in the surrounding medium due to the rapid thinning of the ejecta in the polar direction. We show that a rate of 0.02 SN2003dh-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 SNIa events alone, a rate of 0.6 per century is necessary. Further observations of late light curves of SNIa and SNIc in the bulge of spiral galaxies, together with 3D hydrodynamic calculations of anisotropic ejections of $^{56}$Ni in SNIc/GRB events, will allow to estimate the separate contributions of SNIa and SNIc to positron injection.Keywords
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