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?
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Annihilation lines are emission spectral lines caused by the radiation emitted during an annihilation process. – lemon Sep 29 '14 at 10:45
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1Have look at this article for a nice summary of the status of electron-positron annihilation line astronomy. – John Rennie Sep 29 '14 at 11:12
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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.
Kyle Kanos
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Could Hawking radiation from the supermassive black hole in the center be the cause of this hot spot? – John Dvorak Dec 20 '14 at 20:28
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@JanDvorak: It's been proposed, but I don't believe that it's been widely accepted. – Kyle Kanos Dec 20 '14 at 20:39