A gamma-ray Monte Carlo study of the clumpy debris of SN 1987A

We have performed Monte Carlo calculations of gamma-ray transport in models of the clumpy debris cloud of the LMC supernova, SN 1987A, to study the influence of composition mixing and heterogeneity on its emergent gamma-ray and X-ray fluxes. In particular, we have focused on the problematic Ginga band (16-28 keV) flux at day 600, whose measured value was an order of magnitude higher than predicted by previous theory. We find that the hydrogen of the envelope could not have been intimately mixed with the heavy elements of the core and that the hydrogen/helium volume filling factor interior to 4000 km s^-1 must have been large (≥40%). Furthermore, we demonstrate that one cannot mimic the effects of clumping by artificially decreasing the photoelectric cross sections by some factor. A physical separation of the Compton scattering region and the regions occupied by the high-Z elements is required. The 600 day models that best fit both the line data at 847 keV and 1238 keV and the measured Ginga band fluxes suggest that as much as 50% of the explosively produced ⁵⁶Ni stayed interior to 1000 km s^-1 and 2 M_⊙. The ⁵⁶Ni may have been more centrally concentrated than in the benchmark models. ⁵⁶Ni filling factors greater than 60% are not preferred, since such models are too good at absorbing photons below 100 keV. Furthermore, a total envelope mass between 10 and 15 M_⊙ is favored.