A supernova replay observed by the Hubble Space Telescope

Image showing the observations of the supernova SN Refsdal. In the uppermost circle there's the possible observation happened in 1998, in the middle circe the 2015 observation and in the lowermost circle the 2014 observation
Image showing the observations of the supernova SN Refsdal. In the uppermost circle there’s the possible observation happened in 1998, in the middle circe the 2015 observation and in the lowermost circle the 2014 observation

The Hubble Space Telescope allowed us to observe a supernova just during the explosion. This is due to the fact that its appearance was foretold. For the first time, the use of complex calculations related to the theory of relativity made it possible to capture the supernova nicknamed Refsdal when it exploded. It’s the first time such a feat was achieved by exploiting the gravitational lensing of that galaxy cluster MACS J1149.5 + 2223, which bent the light from that star showing the explosion several times in different areas of the sky.

The nickname Refsdal was given to the supernova after the Norwegian astrophysicist Sjur Refsdal (1935-2009), famous in astronomy exactly because of his pioneering work on the phenomenon of gravitational lensing. In the ’60s he published various articles on the subject, particularly on the use of this effect to measure the universe’s expansion.

The supernova SN Refsdal was discovered in November 2014 and immediately showed the influence of the effect of gravitational lensing. In fact, the scientists saw four images of the supernova arranged in the shape called Einstein Cross around one of the galaxies that are part of the cluster MACS J1149.5 + 2223.

This multiple image showed that the astronomers were observing a phenomenon caused by a gravitational lens so there was the possibility that the supernova SN Refsdal appeared again. The explosion is estimated to have taken place almost ten billion years ago and the its light’s convoluted route allows it to reach the Earth at different times. Seven models of the galaxy cluster MACS J1149.5 + 2223 were used to calculate the time and the area where the new image would appear but it was far from easy.

The calculations had to take into account the gravitational effects of the various galaxies that are part of the cluster MACS J1149.5 + 2223. To collect the data needed to understand how the various galaxies would bend the light from the supernova SN Refsdal the Hubble Space Telescope wasn’t enough. There was an international collaboration using the MUSE (Multi Unit Spectroscopic Explorer) spectrograph installed on ESO’s VLT (Very Large Telescope) and the Keck Observatory.

Nevertheless, a degree of approximation remained because the researchers were studying the effects of a galactic cluster located about 5.3 billion light years away. It’s a good thing that the cluster MACS J1149.5 + 2223 is well known because it’s one of those used in the observations within the Hubble Frontier Fields program. That’s a 3-year survey that aims to get the deepest observations of the universe using the gravitational lensing effects of six galaxy clusters to explore more distant regions.

Because of the approximations in the calculations, the Hubble Space Telescope started observing the cluster MACS J1149.5 + 2223 at the end of October. On December 11, the supernova SN Refsdal appeared, showing that the models were correct. It was a great opportunity to test models of the distribution of matter within this galaxy cluster, including dark matter. It was also another demonstration of the possible synergies between the Hubble Space Telescope and ground-based telescopes.



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