Signal buried in biggest explosion ever seen suggests total annihilation

Every now and then the night sky lights up with a spectacular explosion that shines with the most energetic light our Universe can produce. Known as gamma ray bursts, they can release in a few seconds what our Sun will emit during its lifetime.

Now scientists have found a never-before-seen signal buried in the spectrum of electromagnetic radiation emitted by the brightest gamma-ray burst ever recorded.

That signal is an emission light – a bright region in the gamma-ray spectrum produced by something that emits light in that specific wavelength range. And that something, scientists have determined, is complete and utter destruction: the mutual annihilation of electron-positron pairs, matter and its antimatter counterpart.

Nicknamed ‘the BOAT’ (short for Brightest Of All Time), the explosion was detected in October 2022. It was the result of a huge star exploding as its core collapsed into a black holescattering so many energetic photons that we had a difficulty taking accurate measurements.

But in fact, measurements were taken and scientists knew very early on that they were looking at something special.

“A few minutes after the BOAT eruption, Fermi’s Gamma-ray Burst Monitor recorded an unusual energy spike that caught our attention,” it says astrophysicist Maria Edvige Ravasio, from Radboud University, in the Netherlands.

“When I first saw this signal, I got goosebumps. Our analysis since then shows that it is the first high-confidence emission line ever seen in 50 years of studying gamma-ray bursts.”

Appearing as bright bars in the spectrum of light emitted by a source, emission lines indicate that something is causing light of certain wavelengths to shine more brightly – an extra emission source. Likewise, darker bars known as absorption lines can appear where wavelengths of light are absorbed.

Emission and absorption characteristics can tell us a lot about the constituent chemistry of various astrophysical processes, from metallicity of stars for molecules in composition of distant planetary atmospheres.

There are different scenarios that could produce a gamma ray burstan explosion of the most energetic light in the electromagnetic spectrum, but the one that produced the BOAT – officially named GRB 221009A – was the birth of a Black Hole. The star’s outer material was violently ejected into space, while the core collapsed under the influence of gravity to form the densest type of object in the Universe.

This explosion was accompanied by jets or particles exploding in opposite directions. This is what we see in a gamma ray burst, with the jet tilted toward our line of sight.

When GRB 221009A first exploded, the brightness was so extreme that it obscured any details. Over the course of 80 seconds, the explosion evolved from a peak energy of 12 million electron volts to about 6 million electron volts. Just five minutes later, the brightness dimmed enough for the Fermi gamma-ray telescope to discern an emission line that lasted at least 40 seconds.

Researchers believe in a phenomenon called annihilation was responsible for the improved brightness. When an electron and its antimatter counterpart, a positron, collide, they produce a photon with an energy of 0.511 million electron volts.

The emission characteristic observed in BOAT reflects much higher energy, but researchers have an explanation for this. The annihilation occurred in a jet that was moving toward us at about 99.9% the speed of light. This made the energy appear much louder than it was, an illusion based on our viewing angle that can be seen in other astrophysical jets.

“As we are looking at the jet, where matter is moving at almost the speed of light, this emission becomes very blueshifted and pushed to much higher energies,” explains astrophysicist Gor Oganesyan from the Gran Sasso Institute of Sciences, in Italy.

Although it appeared that GRB 221009A was an extreme example of a gamma-ray burst, subsequent observations and analyzes have since revealed that BOAT was actually quite normal – its brightness was merely the result of its orientation, bursting right towards us.

In fact, this is good news. This means we can use BOAT to understand how gamma-ray bursts typically evolve over time and the processes that occur within them.

“After decades of studying these incredible cosmic explosions, we still don’t understand the details of how these jets work,” says astrophysicist Elizabeth Hays from NASA’s Goddard Space Flight Center.

“Finding clues like this remarkable emission line will help scientists investigate this extreme environment more deeply.”

The findings were published in Science.

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