Electron/positron annihilation lines in astrophysics I have a reasonable understanding of electron/positron annihilation, in that it is a collision between a pair of particles, one matter and one antimatter, that generally produces gamma radiation. However, I do not understand how this concept can be applied in astrophysics. In particular, what are annihilation lines?
 A: Annihilation lines are spectral lines caused by the collision of particle-antiparticle pairs. In the case of $e^-e^+$, the emission is at 511 keV. 

(source (arXiv link))
However, because it is caused by a collision of particles, rather than an absorption-emission of a photon, the peak is Doppler broadened. This means that the peak is spread out over a few keV, rather than a very sharp feature only at 511 keV, as an emission spectra would be. This effect is true for all annihilation lines, not just $e^-e^+$.
The galactic plane is a hotspot for the 511 keV line, but we're not really sure (as far as I know, at least) what is the source for the positrons.

(source)
Some heavy elements can produce positrons in supernova nucleosynthesis (e.g., the ${}^{56}{\rm Ni}$ chain). It's also possible that low-mass X-ray binaries can produce positrons via pair-production in outflows. Another possible source could be dark matter annihilation. As far as I know, no single source has declared as "the" source for galactic positrons.
