It appeared every 18.18 minutes (18 minutes and 16 seconds).

It was a very intense radio wave flash that repeated itself at a specific location in the Milky Way and which today "intrigues astronomers", assures Natasha Hurley-Walker, an astrophysicist at Curtin University, member of the research team that discovered it.

Their observations, published on Wednesday January 26 in the scientific journal Nature, quickly sparked a wave of articles on this new "space mystery" embodied by a light signal of unknown origin, visible with the punctuality of a Swiss cuckoo for three months. in 2018 before disappearing completely.

On the trail of massive stars that exploded

These radio flashes were emitted by an as yet unidentified space object, dubbed GLEAM-X J162759.5-523504 and located 4,000 light years from Earth. When it lit up every 18.18 minutes, it was simply the "brightest point visible from Earth", say the authors of the study. Even more than "the radio emissions of supermassive black holes which are nevertheless the brightest we know in this wave spectrum", assures Natasha Hurley-Walker. A feature already intriguing enough in itself.

But there was a seemingly much bigger mystery, which has to do with the rate at which GLEAM-X J162759.5-523504 "turned on".

It is certainly not the only space object to send out flashes of radio waves at regular intervals.

It is even one of the trademarks of pulsars or neutron stars, which are what remains "after the explosion of a large star about eight to ten times the size of our Sun", explains Ismaël Cognard , astrophysicist at the Laboratory of Physics and Chemistry of the Environment and Space in Orléans and specialist in pulsars, contacted by France 24.

Once formed, these pulsars begin to turn on themselves;

first very quickly, then gradually more slowly (over periods of several million years).

Their poles emit those famous flashes of radio waves, which can be picked up by telescopes when they come into their field of view.

Hence the impression that they turn on and off.

But none of the 3,000 currently identified pulsars complete their revolution in 18.18 minutes… “Generally they spin much faster, around a second. The slowest ever observed to date takes 23 seconds to complete its rotation” , says Ismaël Cognard.

GLEAM-X J162759.5-523504 can't be the naughty little pulsar either.

"When these neutron stars rotate too slowly, they no longer have the energy to emit their radio flashes and go out completely," says this astrophysicist.

An energy released comparable to 1010 Hiroshima bombs

“Realizing that we had never seen anything like this before, I had a cold sweat. After all, there is just a large community that is looking for exactly that: space radio signals that repeat themselves and could be the sign of the presence of extraterrestrial life," wrote Natasha Hurley-Walker.

In other words, would Earth have missed its Third Kind Encounter in 2018?

Everything indicates, however, that these emissions are of natural origin. “The energy released every 18 minutes by these flashes corresponds to that sent by the sun to Earth for 100 days or even equivalent to that of 1010 Hiroshima bomb explosions. , able to do that", underlines Diego Gotz, astrophysicist at the Commissariat for Atomic Energy and Alternative Energies (CEA), contacted by France 24.

Pulsars or, potentially, little green men are not the only ones capable of emitting radio signals, however.

"The most likely responsible is the magnetar", assures Ismaël Cognard.

When a massive star explodes, it is not doomed to become a pulsar.

It can also transform into a magnetar, which is another form of neutron star.

"The big difference is that the magnetic field around them is 100 to 1,000 times stronger than that of pulsars," says Diego Gotz.

A crucial characteristic: it allows the magnetar to continue to emit radio flashes much longer "by drawing its energy from this magnetic field that the pulsar does not have", explains the CEA expert.

The fact remains that until now "the longest rotation ever observed for a magnetar was just over a minute", underlines Ismaël Cognard.

Most flash every 2 to 12 seconds.

But, for him, it is theoretically quite possible.

GLEAM-X J162759.5-523504 would simply be the oldest magnetar ever observed.

Its rotation of 18.18 minutes suggests that it is "about the age of our galaxy", concludes Diego Gotz.

The track of the very old magnetar is also the one favored by the Australian researchers who discovered this strange phenomenon.

But "it can also be a type of space object that has never been observed before, simply because we have not yet looked for radio signals that repeat themselves over such long periods" , notes Natasha Hurley-Walker.

In which case, this would only be the beginning of the adventure.

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