Astrophysicists from the University of Copenhagen have discovered a perfect spherical shape in a kilonova, formed by the collision of two orbiting neutron stars. Kilonovae give rise to the most extreme physical conditions in the universe, and it is in these extreme conditions that the universe creates the heaviest elements on the periodic table, such as gold, platinum, and uranium.

But there is still a lot we don’t know about this violent phenomenon. When a kilonova was detected 140 million light-years away in 2017, it was the first time scientists were able to collect detailed data. Scientists around the world continue to interpret the data from this colossal explosion, including Albert Sneppen and Darach Watson of the University of Copenhagen, who have made a startling discovery, which they have published in Nature.

Their analyzes have been carried out using data from the 2017 kilonova AT2017gfo. These data are ultraviolet, optical and infrared light from the X-shooter spectrograph of the Very Large Telescope of the European Southern Observatory, combined with previous analyzes of gravitational waves, waves radio and data from the Hubble Space Telescope.

“These are two super-compact stars that orbit each other 100 times per second before collapsing. Our intuition, and all previous models, say that the explosive cloud created by the collision must have a flattened and rather asymmetrical shape,” says Albert Sneppen. , a doctoral student at the Niels Bohr Institute and first author of the study.

So he and his research colleagues are surprised to find that this is not the case at all with the 2017 kilonova. It is completely symmetrical and close in shape to a perfect sphere.

“Nobody expected the explosion to look like this. It doesn’t make sense for it to be spherical, like a ball. But our calculations clearly show that it is. This probably means that the kilonovae theories and simulations we’ve been considering for the last 25 years are lacking. of important physics,” says Darach Watson, associate professor at the Niels Bohr Institute and second author of the study.

But how the kilonova can be spherical is a real mystery. According to the researchers, there must be some unexpected physics at play: “The most likely way for the explosion to be spherical is that a huge amount of energy comes out of the center of the explosion and smooths out an otherwise asymmetrical shape. So the spherical shape tells us that there is probably a lot of energy in the core of the collision, which was unexpected,” says Albert Sneppen.

When neutron stars collide, they briefly coalesce into a single hypermassive neutron star, which then collapses into a black hole. The researchers speculate whether it is in this collapse that much of the secret is hidden: “Perhaps a kind of ‘magnetic bomb’ is created at the moment when the energy of the enormous magnetic field of the hypermassive neutron star is released when the star collapses into a black hole.The release of magnetic energy could cause the matter in the explosion to become more spherically distributed. In that case, the birth of the black hole could be very energetic,” says Darach Watson.

However, this theory does not explain another aspect of the researchers’ discovery. According to previous models, although all elements produced are heavier than iron, extremely heavy elements, such as gold or uranium, should be created in different places in the kilonova than lighter elements, such as strontium or krypton, and they should be expelled in different directions. The researchers, on the other hand, only detect the lightest elements, and these are evenly distributed in space. 

Therefore, they believe that the enigmatic elementary particles, neutrinos, about which much is still unknown, also play a key role in the phenomenon. “An alternative idea is that in the milliseconds of a hypermassive neutron star’s lifetime, it emits a large number of neutrinos.