What produces the Galaxy's 511 keV positron annihilation line?
In plain words
About 5e43 positrons per second annihilate in the Milky Way, mostly in the central bulge, and no known source class explains both the rate and the bulge-heavy pattern.
Precise statement
Explain the $511\,\mathrm{keV}$ line (annihilation rate $\sim 5e43\,\mathrm{positrons}/\mathrm{s}$, bulge-to-disk luminosity ratio $\sim 1$) with positrons injected below a few MeV (required by in-flight annihilation limits). Candidates: beta-plus decay of $\mathrm{Al}-26$, $\mathrm{Ti}-44$ and $\mathrm{Co}-56$ from supernovae and novae, microquasars and low-mass X-ray binaries, past activity of Sgr A*, MeV-mass dark matter. Answer: a source mixture reproducing rate, morphology and spectrum, including positron transport.
What would settle it
High-resolution imaging and spectroscopy of the line, resolving point sources or a profile that singles out one class.
Status in the literature
Unverified note
The COSI MeV mission, planned for launch in 2027, will map the line with better angular and energy resolution.
Related problems
- More general than How far do Galactic positrons travel before annihilating?