55Fe in Supernova 1987A

Abstract

Observations of newly synthesized radioactive nuclei have provided a wealth of information about the extensively observed supernova 1987A. We add another such study, searching Chandra X-ray spectra of SN 1987A for evidence of 5.9 keV Mn Kα line emission due to the decay of radioactive 55Fe. This isotope is synthesized in explosive silicon burning and in normal freezeout of nuclear statistical equilibrium, mostly as 55Co, in the inner ejecta of core-collapse supernovae. The X-rays from the decay in these regions escape only after 10-20 yr of expansion, depending on the extent of mixing of core material outward in mass. Otherwise generally successful models of SN 1987A predict fluxes at 5.9 keV detectable by Chandra. However, in 400 ks of Chandra ACIS data, we find no evidence for this line emission, with the mean line flux ≤3 × 10-7 cm-2 s-1. We consider the implications for atomic mass A = 55 nucleosynthesis and for the kinematics and mixing of SN 1987A.