Analysis of X-ray observations of the SN1987A supernova remnant, obtained by the XMM-Newton satellite, provides new details on the interaction between the shock wave generated by the supernova and the circumstellar material, as well as on the abundance of oxygen present in the supernova remnant.

The SN1987A supernova remnant is undoubtedly one of the most iconic objects for the study of supernovae, their remnants and massive stars at the end of their evolution. Exploding in 1987 in the Large Magellanic Cloud, generated by a blue supergiant star, it is the closest case to us of a supernova exploding in modern times. It is therefore the only case in which we have multi-band and multi-messenger observations (including neutrino detection) of the progenitor star, the supernova and the supernova remnant. The latter is constantly monitored in all bands of the electromagnetic spectrum to follow its evolution and understand the complex physical processes at work.

In particular, X-ray observations allow us to study the high-energy processes occurring within the remnant and to determine the physical properties of the plasma at millions of degrees and of the chemical elements in high states of ionisation.

The X-ray emission from SN1987A has also been monitored over the years by the European Space Agency's XMM-Newton satellite. The analysis of this data is the subject of two publications led by astrophysicist Sun Lei of Nanjing University.

The study entitled ‘Evolution of X-Ray Gas in SN 1987A from 2007 to 2021: Ring Fading and Ejecta Brightening Unveiled through Differential Emission Measure Analysis’ presents a detailed analysis of observations made between 2007 and 2021. In particular, the authors studied the variation over time of the X-ray emission measurement, a quantity linked to the amount of high-temperature plasma responsible for the emission. A peak emission was detected from plasma with temperatures between 5 and 12 million degrees, with a contribution from even hotter plasma, above 58 million degrees. This peak shifted towards increasingly higher temperatures between 2011 and 2014, when the supernova shock wave hit a ring of circumstellar material previously ejected by the progenitor star, subsequently losing intensity as the shock wave left the ring-shaped cloud. Meanwhile, a second emission peak was observed, originating from plasma at temperatures of 30-60 million degrees. This secondary emission peak is actually of great importance, as it comes from fragments of the star ejected during the supernova explosion (the ejecta), compressed and heated by reverse shock waves generated by the interaction between the main wave and the circumstellar material. This emission was predicted by models but had never been observed until now.

The second study, entitled ‘Unusual X-Ray Oxygen Line Ratios of SN 1987A Arising from the Absorption of Galactic Hot Interstellar Medium’, presents a spectroscopic analysis of the X-ray emission of ionised oxygen atoms, which have retained one or two electrons (i.e. in states similar to hydrogen or helium). The observed signals appear to be influenced by the absorption of X-rays by the hot, diffuse plasma that populates the Milky Way halo. Taking this absorption into account, the abundance of oxygen in the supernova remnant is about 20% greater than previous estimates.
Both articles were published in The Astrophysical Journal, with the participation of astrophysicists S. Orlando and E. Greco from INAF – Palermo Astronomical Observatory and M. Miceli from the University of Palermo.

The cover image (click here to view it in full) shows the portion of the spectrum containing the oxygen lines analysed in the study, highlighted in the boxes. The crosses indicate the observed values, while the lines represent the models used for the analysis of the lines and for determining the chemical-physical properties of the plasma.