When the most massive stars in our universe reach the end of their lives after a few billion years, they swell excessively and then explode, shedding the outer layers of their surrounding atmosphere. This sudden and violent phenomenon is called a supernova. It causes a flash of light so intense that it can be seen from other galaxies. The bright burst lasts a few days and then fades. But where the star died, things continue to develop: the ejected gas layers form a kind of shell that slowly spreads out into space. And at the heart of this envelope, the star's body compresses, forming a dark and ultra-dense star – sometimes a neutron star and sometimes a black hole.
This is what we have learned in all astronomy books and courses for decades. This knowledge was made possible through the work of astrophysicists Walter Baade and Fritz Zwicky, who recognized in the 1930s that supernovae must mark the transition between a star collapsing on itself and a neutron star. Then a little later we discovered the alternative possibility of a black hole forming when, in the 1960s, we began describing these stars as so dense that not even light can escape.
But all of these conclusions have so far remained theoretical works and are based on Einstein's general theory of relativity and mathematical-logical considerations. Thanks to modern telescopes, astronomers have observed the occurrence of supernovae in other galaxies. Astronomers have also observed and even photographed the existence of neutron stars and black holes. But no one had yet seen a live neutron star or a black hole formed from a supernova.
Repeated periodic oscillations
It is this missing link that was just filled by two independent teams of astrophysicists after they heard in May 2022 about a supernova that had just appeared in our Milky Way's neighboring galaxy NGC 157, 75 million light-years away. Researchers quickly managed to point the best telescopes at the supernova to study its light and discovered that its “behavior” was unusual. Instead of falling linearly over time, the brightness curve fell in a twelve-day cycle with up and down oscillations. “In the data we see a repeating sequence of brightening and fading,” explains Thomas Moore, a doctoral student at Queen's University in Belfast, Northern Ireland, who led one of the two studies. “This is the first time that repeated periodic oscillations over many cycles have been detected in the light curve of a supernova,” says this study published in The Astrophysical Journal Letters.
This can be explained if there were not one but two stars at the heart of the supernova: the star that died and a companion star that orbited it. The companion would have survived the supernova explosion and would continue to orbit the stellar body (neutron star or black hole). Each time its companion passes close enough, the gravitational pull of the stellar body would tear a little matter (gas) out of its atmosphere.
New gamma ray source
This hypothesis is supported and expanded upon by another study led by astrophysicist Ping Chen of the Weizmann Institute of Science in Israel, published Thursday, January 11, in Nature. Ping Chen's team observed the same supernova (SN 2022jli) in 2022 and noticed the same light oscillations. She also noticed periodic changes in the amount of hydrogen measured in this supernova. For researchers, this makes no difference: fluctuations in luminosity every twelve days + fluctuations in hydrogen every twelve days = a companion star rotates around the stellar body in twelve days and a little hydrogen is “stolen” with each pass. “This periodic process of flight and accumulation of matter generates a lot of energy, which manifests itself in observations in the form of regular changes in luminosity,” summarizes the European Southern Observatory, whose telescopes in the Atacama Desert (Chile) made these discoveries possible.
Ping Chen and his colleagues even discovered a new source of gamma rays at the site where the star died. All the more reason to believe that there is now a neutron star or a black hole there that produces gamma ray bursts by swallowing the atmosphere of the companion star.
In short, the two teams of astronomers seem to have seen live for the first time the connection between a supernova and the appearance of a neutron star or a black hole. This confirmation will perhaps shape the history of astronomy. But the study of SN 2022jli is far from over: new observation campaigns will undoubtedly help to understand whether we are dealing with a neutron star or a black hole, for example, or what sauce the companion star will be eaten with in the coming years.