Gamma-ray line emission from SN1987A

Abstract

The gamma-ray spectrometer (GRS) on NASA's Solar Maximum Mission satellite (SMM) has observed a significant (>5δ) net line flux at ˜847 keV in the background-subtracted spectrum of SN1987A—accumulated between 1 August and 31 October 1987. This is the energy of the strong gamma-ray line from decay of ⁵⁶Co, which was predicted^1,3 to be seen from supernovae. The inferred average line flux during this period is ˜(1.0±0.25) x 10⁻³ photons cm⁻² s⁻¹ at an energy of 843 ± 5 keV. This feature cannot be explained by any statistical or systematic fluctuations observed in the seven previous years of GRS data. There is also evidence for the 1,238-keV line from ⁵⁶Co decay, with an average flux of ∼(6±2)xlO⁻⁴ photons cm⁻² s⁻¹ The quoted photon fluxes for both lines are preliminary. This observation confirms that ⁵⁶Co is present in the supernova ejecta (as implied by the light curve^4,5) and that nucleosynthesis took place during the explosion. The first appearance of the gamma-ray lines was roughly coincident with the first detections of X-rays by Ginga and MIR^6,7.