This is how we know that the initial gamma-ray flash is not caused by positron-electron recombination: > The dance of light and matter continues when positrons

This is how we know that the initial gamma-ray flash is not caused by positron-electron recombination: > The dance of light and matter continues when positrons encounter electrons again; they recombine and produce a flash of light of a precise and characteristic colour. So the spectrum of the initial flash is different from the spectrum of the light produced when the particles recombine. Or so I assume from reading the article. :)