The theory of gamma-ray emergence in supernova 1987A

Abstract

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 0.075M_ȧ of radioactive ⁵⁶Ni in the explosion⁴. It was anticipated that the decay of ⁵⁶Co to ⁵⁶Fe would give rise to detectable γ-ray line emission^5,6 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 detected^15–18, 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.