A perfectly spherical cosmic explosion surprises astrophysicists

Artistic illustration of explosion perfectly spherical associated with a kilonova.

Kilonovas, those gigantic explosions that occur when neutron stars merge, form a perfectly spherical ball during the early stages of their development, show the work of astrophysicists Danish published in the journal Nature.

Kilonovas are recently described phenomena that occur during the merger of two neutron stars, these remnants of large stars that have a tendency to revolve around each other, explains astrophysicist Olivier Hernandez , director of the Planétarium Rio Tinto Alcan, who did not participate in the study.

These neutron stars are usually only about twenty kilometers in diameter, but they can weigh one and a half times to twice as much as the Sun. A teaspoon of neutron star material would weigh about as much as Mount Everest.

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When two such stars merge, they cause a gigantic, luminous explosion called a kilonova.

“C is an astronomical phenomenon whose luminosity is about 1000 times brighter than that of a typical nova, but only 1% to 10% of that of a supernova. »

— Olivier Hernandez, Director of the Rio Tinto Alcan Planetarium

• In Latin, the term nova means new. In astronomy, it refers to the appearance of a bright star in the sky.
• Several astronomical phenomena can explain their appearance.
• A nova occurs in a binary star system composed of a white dwarf and a star close enough for the white dwarf to attract material from its companion.
• A kilonova occurs when two compact objects, such as neutron stars binaries or a neutron star and a black hole, collide.
• A supernova forms when a massive star at the end of its life exhausts its nuclear fuel, causing its core to gravitationally collapse and cause an explosion.

The existence of kilonovas was theoretically predicted in 1974, but it was not clearly confirmed until 2013. It was not until 2017 that detailed data of a kilonova was obtained for the first time.

In the present work, astrophysicists Albert Sneppen and Darach Watson from the University of Copenhagen and their colleagues focused on the 2017 data collected during the observation of the kilonova, nicely called AT2017gfo, detected using the LIGO and VIRGO instruments.

Their analyzes show that this kilonova, located 140 million light-years from Earth, took the form of a globular explosion, a reality that goes against the theory that the merger of two neutron stars take the form of an oblate, asymmetrical disk rather than a perfectly spherical system, Hernandez notes.

< p>“Observations show, over very short periods of a day after the two neutron stars merged, that we are dealing with a perfect sphere. No one expected the explosion to look like this. »

— Olivier Hernandez, Director of the Rio Tinto Alcan Planetarium

But a few days later, the flattening coefficient became much more visible, which means it's a lot less spherical than when they first started, adds Hernandez.

The spherical nature of the early stages of kilonova development remains unexplained at this time.

This probably means that the theories and simulations of kilonovas carried out in recent years are inaccurate, explains in a press release Albert Sneppen, first author of the study.

One ​​of the hypotheses advances involves an enormous amount of energy, a real energy bomb, which would have escaped from the center of the explosion to create its strangely round shape.

It takes a very strong source of energy to bring about such a sphere, says Hernandez, who believes the presence of a black hole could be part of the equation.

Thus, the enormous magnetic field energy released by neutron star mergers could collapse into a black hole.

The authors of this work believe that neutrinos, these enigmatic elementary particles of which we still do not know much, could also play an important role in the spherical nature of the phenomenon.

There are currently two methods for measuring the speed of expansion of the Universe which indicate, among other things, its age. However, estimates made with these techniques diverge by about a billion years.

With kilonovas, we may have a third method which would complete the other measurements, explains Albert Sneppen.

According to Olivier Hernandez, the interest of kilonovas for measuring cosmic distances comes from its form.

Anything spherical is fantastic in physics, notes Olivier Hernandez, who explains that a spherical structure is easier to model.

“We may be able to use kilonovas as cosmic candles whose distance we can measure independently.

— Olivier Hernandez

A cosmic candle is a celestial object whose luminosity is known, which makes it possible to use it to determine distances in the distant Universe.

The perfect shape of a kilonova gives it gives an advantage. An object that is not spherical emits different luminosity depending on the angle of view, while a spherical explosion always offers the same luminosity.

Kilonovas could possibly serve new "cosmic rule", says Darach Watson.

We will eventually be able to more accurately determine the distances of these objects, which will help us to more accurately determine the age of the Universe, note the researchers, who point out that collecting data from a larger number of kilonovas will allow the technique to be refined.

About 50 have been detected to date, but the presence of most of them remains to be confirmed.