The SPI instrument onboard INTEGRAL has performed a search for 511 keV
emission (resulting from positron-electron annihilation) all over the sky.
The figure represents the results of this search: the all-sky map in galactic
co-ordinates shows that 511 keV emission is - so far - only seen towards the
center of our Galaxy. The SPI data are equally compatible with galactic bulge
or halo distributions, the combination of a bulge and a disk component, or
a combination of a number of point sources. Such distributions are expected
if positrons originate either from low-mass X-ray binaries, novae, Type Ia
supernovae, or possibly light dark matter.
