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For about forty years it has been known that something in the Galactic bulge is a prodigious source of positrons. The bulge outshines the disk in diffuse 511 keV positron annihilation radiation despite the fact that it is both less massive and has much less on-going star-formation. I will discuss -- and invite comments on -- our hypothesis that the positrons originate in the decay of radioactive 56Ni synthesised in a peculiar class of underluminous Type I supernovae. These supernovae likely result from collisions between ~0.6 solar white dwarfs in perturbed binaries inhabiting the bulge. These white dwarfs originate from >10 Gyr old ~solar mass stars which have only recently run through their main sequence lifetimes. In our scenario, then, the positrons are echoes of the last big burst of bulge star-formation at z ~ 2. Similar WD-WD collisions, occurring in the thick disk, ultimately supply much of the 511 keV radiation emission extending along the Galactic plane. |
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