Since June 1987 we have had little cause to doubt the presence of radioactivity in supernova 1987A. The optical light curve has tracked the 77.1-day half-life of 56Co to better than 1% (refs 1-3), at least through November, demonstrating conclusively the synthesis of 0.075Ṁ of radioactive 56Ni in the explosion4. It was anticipated that the decay of 56Co to 56Fe would give rise to detectable γ-ray line emission5,6 at 847 and 1,238 keV with a peak flux7 of 10-3 photons cm-2 s-1 about one year after the explosion. Many calculations of the light curves for these lines were made for the particular case of SN1987A (refs 8-14), and in August, both lines were detected15-18, with a strength within a factor of two of 10-3 photons cm-2 s-1, but about six months earlier than predicted (see ref. 8 for example). Here we show that the early emergence of γ-rays can be accounted for in a 'mixed' model, in which an approximately isotropic process destroys chemical segregation with respect to radial mass coordinate and velocity.
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