X-rays expected from supernova 1987A compared with the source discovered by the Ginga satellite

Abstract

The X-ray astronomy satellite Ginga has detected X-rays from the region of supernova 1987 A¹. The location and hard spectrum of this X-ray source make it a strong candidate for SN1987A^1,2. Among the possible mechanisms of X-ray emission from SN1987A^3,4, including the collision of the ejecta with circumstellar matter, non-thermal radiation from a pulsar, is Compton scattering of monoenergetic γ-rays from the radioactive decay ⁵⁶Ni→⁵⁶Co→ ⁵⁶Fe (refs 3, 5–7). It is clear that radioactive decays are powering the optical emission of SN1987A because its luminosity is now declining exponentially with the ⁵⁶Co decay rate^8,9. We have constructed hydrodynamic models of SN1987A and calculated the optical light curve and the X-ray light curve and spectrum from ⁵⁶Co decay. For the optical light curve, the model agrees better with the observations if some ⁵⁶Co is mixed into outer layers rather than being confined to the deepest layer of the ejecta. Such mixing has much larger effects on the X-ray light curve. Without the mixing of ⁵⁶Co, X-rays would not be observable by Ginga until ˜250 d after the explosion. With mixing the X-rays emerge much earlier. X-ray emission from radioactive decay may therefore qualitatively account for the hard component of X-rays from SN1987A observed by Ginga, if ⁵⁶Co has been mixed into the outer layers. The soft component of the spectrum cannot be explained by the radioactive model and must be due to another mechanism.