In October 2022, astronomers were amazed by the discovery of what is now known as the BOAT—the brightest gamma-ray burst (GRB) ever recorded. An international team of scientists has now reported a groundbreaking feature revealed by NASA’s Fermi Gamma-ray Space Telescope.
“A few minutes after the BOAT’s eruption, Fermi’s Gamma-ray Burst Monitor detected an unusual energy peak that immediately drew our attention,” said lead researcher Maria Edvige Ravasio from Radboud University in Nijmegen, Netherlands, and Brera Observatory, part of INAF (the Italian National Institute of Astrophysics) in Merate, Italy. “Seeing that signal gave me goosebumps. Our analysis has since shown it to be the first high-confidence emission line detected in 50 years of GRB studies.”
The discovery is detailed in a paper published in the journal Science. When matter interacts with light, it can absorb and reemit energy in characteristic ways, creating features visible in a spectrum. These features provide valuable information about the chemical elements involved and, at higher energies, can reveal specific particle processes like matter-antimatter annihilation that produce gamma rays.
“Previous studies have suggested possible absorption and emission features in other GRBs, but these were often dismissed as statistical anomalies. What we observe in the BOAT is different,” explained co-author Om Sharan Salafia from INAF-Brera Observatory in Milan, Italy. “We’ve determined that the likelihood of this feature being a statistical fluctuation is less than one in half a billion.”
Gamma-ray bursts are the most powerful explosions in the universe, emitting vast amounts of gamma rays, the highest-energy form of light. They often occur when the core of a massive star collapses to form a rapidly spinning black hole, which then produces high-speed particle jets. GRBs are detected when these jets point almost directly toward Earth.
The BOAT, formally designated GRB 221009A, erupted on October 9, 2022, and was so intense that it saturated most gamma-ray detectors in orbit, including those on Fermi. This prevented measurement of the blast’s most intense phase. Reconstructed observations suggest that the BOAT was likely the brightest GRB to appear in Earth’s skies in 10,000 years.
The detected emission line appeared about 5 minutes after the burst and persisted for at least 40 seconds, reaching a peak energy of approximately 12 MeV (million electron volts). For comparison, visible light has energies ranging from 2 to 3 electron volts. The team speculates that this spectral feature may be due to the annihilation of electrons and positrons, which, when moving at nearly the speed of light, produce gamma rays that are blueshifted to higher energies.
“If our interpretation is correct, the annihilating particles must have been moving towards us at around 99.9% the speed of light,” said co-author Gor Oganesyan from Gran Sasso Science Institute and Gran Sasso National Laboratory in L’Aquila, Italy.
Elizabeth Hays, the Fermi project scientist at NASA’s Goddard Space Flight Center, highlighted the significance of the discovery: “After decades of studying these cosmic explosions, we still lack a complete understanding of jet dynamics. Finding clues like this emission line helps scientists probe these extreme environments more deeply.”
