Researchers using data from NASA’s Fermi Space Telescope have discovered a unique energy peak after the brightest gamma-ray burst ever observed, suggesting the annihilation of electrons and positrons. The discovery provides new insight into the behavior of cosmic jets and the extreme conditions that occur after these explosions.

In October 2022, astronomers were stunned by the brightest gamma-ray burst (GRB) of all time, which they quickly named BOAT. Now an international scientific team has reported that data from NASA’s Fermi space gamma-ray telescope has revealed a never-before-seen feature.

An unprecedented spectral function was discovered

“A few minutes after the BOAT explosion, the Fermi gamma-ray burst monitor recorded an unusual energy peak that caught our attention,” said lead researcher Maria Edwige Ravazio of Radbaud University in Nijmegen, the Netherlands, which is part of the Brera Observatory. Merate is part of INAF (Italian National Institute for Astrophysics) in Italy. “When I first saw this signal, I got goosebumps. Our analysis since then shows that this is the first high-confidence emission line ever seen in 50 years of gamma-ray burst research.”

An article about the discovery was published in an issue of the magazine Science For July 26th.

When matter interacts with light, energy can be absorbed and re-emitted in characteristic patterns. These interactions can brighten or darken certain colors (or energies), creating the basic features that become visible as the light spreads across the spectrum like a rainbow. These features can reveal a huge amount of information, such as the chemical elements involved in the interaction. At higher energies, spectral features can reveal specific particle processes, such as the annihilation of matter and antimatter to produce gamma rays.

“While some previous studies have reported possible evidence of absorption and emission features in other gamma-ray bursts, further investigations have shown that these could all be just statistical fluctuations. What we see in BOAT is different,” said co-author Om Sharan Salafia of the INAF-Brera Observatory in Milan, Italy. “We determined that the probability of this feature being just a fluctuation in the noise is less than one in half a billion.”

The nature and impact of gamma-ray bursts

GRBs are the most powerful explosions in space, emitting large amounts of gamma rays, the highest-energy form of light. The most common type occurs when the core of a massive star runs out of fuel, collapses, and forms a rapidly spinning black hole. The matter falling into the black hole sets off jets of particles that shoot out of the star’s outer layers at nearly the speed of light. We detect GRBs when one of these jets is directed almost directly at Earth.

BOAT, officially known as GRB 221009A, exploded on October 9, 2022, instantly saturating most orbiting gamma-ray detectors, including those at Fermi, preventing them from measuring the most intense part of the burst. The reconstructed observations, combined with statistical arguments, suggest that BOAT was likely the brightest explosion in Earth’s sky in 10,000 years, if it was part of the same population as previously detected gamma-ray bursts.

Ideas about cosmic interactions of particles

The predicted emission line appears almost 5 minutes after the flare was detected, and long after the saturation effect for Fermi had become weak enough to cease. The line persisted for at least 40 seconds, and the radiation reached a peak energy of about 12 MeV (millions of electron volts). For comparison, the energy of visible light ranges from 2 to 3 electron volts.

So what caused this spectral feature? The team believes the most likely source is the annihilation of electrons and their antimatter counterparts, positrons.

“When an electron and a positron collide, they annihilate and produce a pair of gamma rays with an energy of 0.511 MeV,” said co-author Horo Hovhannesian of the Gran Sasso Institute of Science and the Gran Sasso National Laboratory in L’Aquila, Italy. “Because we are looking at a jet where matter is moving at close to the speed of light, this radiation is strongly blueshifted and pushed towards much higher energies.”

Future directions of research and collaboration

If this interpretation is correct, to obtain an emission line with a peak of 12 MeV, the annihilator particles would have to be moving towards us at about 99.9% of the speed of light.

“After decades of studying these incredible cosmic explosions, we still don’t understand the details of how these jets work,” said Fermi Project Scientist Elizabeth Hayes of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Finding clues like this striking emission line will help scientists probe this extreme environment more deeply.”

The Fermi Space Gamma-ray Telescope is a partnership in astrophysics and particle physics directed by Goddard. Fermi was developed in collaboration with the U.S. Department of Energy, with participation from scientific institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.