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The most powerful explosion astronomers have ever seen contains a mysterious signal thought impossible to exist. That signal gives us our first detailed look inside a gamma ray burst and suggests that they involve the annihilation of matter and antimatter.

Gamma ray bursts (GRBs) are the most powerful blasts of radiation in the universe, and are generated in cosmic explosions and collisions. Physicists suspect that the highest energy GRBs come from stars collapsing and forming a black hole. The black hole then produces a jet of material, moving at close to the speed of light, that pierces through the failing star and sends out blasts of radiation that we can observe on Earth. But exactly how this radiation is produced, or what the jet might contain, remains unknown.

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Much of this mystery comes from the spectrum of light that we can see. Unlike the light that we observe from other objects in the universe, which contains distinctive spikes that can tell us about the specific atoms or other matter that produced this burst of energy, the spectrum of light from gamma ray bursts always appears to be smooth and featureless.

In the 1990s, researchers became excited at the prospect that some GRBs appeared to show distinct lines, but after careful analyses they found these were statistical errors and concluded that GRB spectra couldn’t be spiky.

Now, at Radboud University in the Netherlands and her colleagues have discovered that GRB221009A, discovered in 2022 and the brightest explosion ever observed, in fact contains an energetic peak at about 10 megaelectronvolts.

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“The first time I saw the line, I thought I did something wrong,” says Ravasio. But after performing a detailed statistical analysis and ruling out problems with the observation instrument – the – Ravasio and her colleagues concluded that the spike in the spectrum was genuine. “When I realised it was not an error, I got goosebumps because I realised that it was something huge.”

Because almost all GRBs show a similar distribution of energies, astronomers analyse new GRB detections using data analysis methods that work best with this pattern. But Ravasio and her team instead used a method that allows for peaks, and they found that this fit the data better. “That part of the GRB spectrum has been the same for years, and nobody was looking into it,” says Ravasio. “The energy of [GRB221009A] allowed us to see that part of the spectrum much better.”

This peak points towards a specific physical process behind GRBs that is missing from our best models of them.

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To focus in on what this might be, Ravasio and her colleague worked under the assumption that there were no complete atoms in the jet, due to how energetic it must have been. This left one plausible explanation: the annihilation of electrons with their antimatter counterparts, positrons. Such annihilation would produce gamma rays at a distinct peak of 511 kiloelectronvolts. “This is already telling you the composition of the jet, which is something that we haven’t understood since the first GRBs,” says Ravasio.

The higher 10 MeV peak that the researchers observed was because the energy spectrum was shifted by the fast-moving jet that produced the radiation, similar to how the siren of an ambulance moving towards you sounds higher pitched.

This difference meant they could calculate the speed of the jet that produced the burst, which was travelling at 99.99 per cent of the speed of light.

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Finding a GRB with a distinctive line is “one of the biggest surprises in our field in more than a decade”, says at Louisiana State University.

Burns, who had helped analyse the original data that led to the discovery of GRB221009A, was presenting results at a conference with colleagues when he heard of Ravasio’s discovery. “None of us believed the paper could be correct,” says Burns. “We read the title, and every single one of us was like, this is wrong, there’s no way that this is correct.”

But the analysis that Ravasio and her colleagues performed appears to be correct, he says. “It’s rather astounding. We totally missed this because we didn’t even look for it, because we were absolutely convinced that gamma ray bursts don’t have lines,” says Burns.

It is possible that other GRBs also have spectral peaks like this, which could be worth looking for, but it is likely we could only see this one because it came from the brightest GRB of all time, says Burns.