Gamma-Rays from Positron Annihilation
  SPI on INTEGRAL has provided spectra and a map of the sky in the emission
from annihilations of positrons in the interstellar medium of our Galaxy. From
high-resolution spectra we learned that a warm, partially-ionized medium is the
site where the observed gamma-rays originate. The gamma-ray emission map shows
a major puzzle for broader astrophysics topics, as it is dominated by a bright
and extended apparently spherical emission region centered in the Galaxy's
center. Only recently has the disk of the Galaxy been detected with SPI. This
may be regarded as confirmation of earlier expectations that positrons should
arise predominantly from sources of nucleosynthesis distributed throughout the
plane of the Galaxy, which produce proton-rich unstable isotopes. But there are
other plausible sources of positrons, among them pulsars and accreting binaries
such as microquasars. SPI results may be interpreted also as hints that these
are more significant as positron sources on the Galactic scale than thought
before, in the plane and therefore also in the bulge of the Galaxy. This is
part of the attempt to understand the surprisingly-bright emission from the
central region in the Galaxy, which otherwise also could be interpreted as a
first rather direct detection of dark matter annihilations in the Galaxy's
gravitational well. INTEGRAL has a unique potential to shed light on the
various aspects of positron astrophysics, through its capability for imaging
spectroscopy.
