The discovery focuses on GRB 221009A, the most luminous GRB ever observed. "A few minutes after the GRB started, Fermi’s Gamma-ray Burst Monitor recorded an unusual feature that caught our attention," said Maria Edvige Ravasio, a researcher at Radboud University in Nijmegen and affiliated with INAF - Brera Observatory in Merate, who led the team. This unusual feature is in the form of an excess of gamma-ray light at around 10 megaelectronvolts (MeV). "Our analysis shows that this peak is the first high-confidence emission line ever seen in 50 years of studying GRBs."
Energetic events
Gamma-ray bursts are among the most energetic events in the universe, capable of releasing more energy in a few seconds than the Sun will emit in its entire lifetime. Events like GRB 221009A occur when the core of a massive star exhausts its fuel, collapses, and forms a rapidly spinning black hole, which launches two oppositely directed jets. We detect GRBs when one of these jets points almost directly toward Earth. Despite their discovery over 50 years ago, the exact mechanisms driving GRBs and the composition of their jets have remained shrouded in mystery.Scientists have speculated about the presence of emission lines in GRBs for over half a century but have never confirmed their existence with high confidence. "Previous studies hinted at possible emission lines in GRBs, but they were dismissed as statistical fluctuations. What we see in this GRB is different," said coauthor Om Sharan Salafia at INAF - Brera Observatory in Milan, Italy. "We’ve determined that the odds this feature is just a noise fluctuation are less than one chance in half a billion."
GRB Jets
This finding offers a new window into the composition and dynamics of GRB jets, an area that has remained poorly understood despite decades of research. The team proposes three potential explanations for this emission line, with the leading hypothesis being the annihilation of electron-positron pairs within the GRB jet. "When an electron and a positron collide, they annihilate, producing a pair of gamma rays with an energy of 0.511 MeV," said coauthor Gor Oganesyan at Gran Sasso Science Institute and Gran Sasso National Laboratory in L’Aquila, Italy. "Because we’re looking into the jet, where matter is moving at near light speed, this emission becomes greatly blue-shifted and pushed toward much higher energies."If this interpretation is correct, to produce an emission line peaking at 12 MeV, the annihilating particles had to have been moving toward us at about 99.9% the speed of light.
Literature reference A mega-electron volt emission line in the spectrum of a gamma-ray burst. Door: Maria Edvige Ravasio, Om Sharan Salafia, Gor Oganesyan, Alessio Mei, Giancarlo Ghirlanda, Stefano Ascenzi, Biswajit Banerjee, Samanta Macera, Marica Branchesi, Peter G. Jonker, Andrew J. Levan, Daniele B. Malesani, Katharine B. Mulrey, Andrea Giuliani, Annalisa Celotti, & Gabriele Ghisellini. In: Science , 26 juli 2024. [ origineel
