What produces the Galaxy's 511 keV positron annihilation line?
In plain words
About $5 \times 10^{43}$ 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 $\sim 5 \times 10^{43}\,\mathrm{positrons}/\mathrm{s}$, 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: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
The COSI MeV mission, planned for launch in 2027, will map the line with better angular and energy resolution.
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