Scientists have an unprecedented explanation for the cause of the phenomenon, but they insist on the imperative of continuing research to clarify the matter.

The event, cataloged AT2021lwx, is not the brightest on record. This distinction goes to the gamma-ray burst (a colossal jet of energy in the collapse of a star)GRB221009A, detected in October 2022 and supposed to be "the brightest of all time".

But the explosion described in the journal Monthly Notices of the British Royal Astronomical Society can be described as the "largest", because it released in three years infinitely more energy than the gamma-ray burst, according to the study's lead author, Philip Wiseman, an astrophysicist at the British University of Southampton.

AT2021lwx is the result of an "accidental discovery", he told AFP.

The explosion had certainly been detected in 2020, automatically, by the American observatory Zwicky Transient Facility in California. But that detection "went unused in the database," Wiseman said. Before scientists noticed it the following year.

Direct observation of the phenomenon has changed the situation. Analysis of the light established that it took eight billion years to reach the telescope.

"A real enigma"

Astronomers are still wondering about the cause of the phenomenon. It could be a supernova, the explosion of a massive star at the end of its life, but the luminosity here is ten times greater than expected.

Another possibility is a tidal rupture event, in which a star is torn apart by the attractive forces of a black hole to which it has come too close. But then again, AT2021lwx is three times too bright to validate such a scenario.

The measured brightness has no equivalent known except with that of quasars, these galaxies sheltering in their heart a supermassive black hole that gorges itself with matter by emitting a phenomenal amount of light.

But the light of quasars is twinkle, whereas in this case, it suddenly became more pronounced three years ago. "We have never observed such a thing (...). It appeared out of nowhere," says the scientist.

His team does have an idea, set out in the study. His theory is that a gigantic cloud of gas, the size of 5,000 suns, is being devoured by a supermassive black hole.

As the principle of science is that "there are never certainties", the team is working on new simulations – using the data – to test the "unavoidable plausibility" of their theory.

The problem is that supermassive black holes are thought to be at the center of galaxies. And that the one of the AT2021lwx event should be a size equivalent to our Milky Way.

However, no one has yet detected a galaxy in the vicinity of the observed event. "It's a real enigma," says Philip Wiseman.

It remains to search the heavens, and in the databases of celestial observations, for similar events that could help lift the veil on the explosion.

© 2023 AFP