Fermi Telescope Discovers New Feature in Brightest Gamma-Ray Burst Ever Observed

In October 2022, astronomers were stunned by what was quickly dubbed the BOAT, the brightest gamma-ray burst (GRB) ever. Now, an international science team reports that data from NASA’s Fermi Gamma-ray Space Telescope reveals a feature never seen before.

“A few minutes after the BOAT flare, the Fermi gamma-ray burst monitor recorded an unusual energy spike that caught our attention,” said Maria Edvige Ravasio, principal investigator at Radboud University Nijmegen, the Netherlands, and affiliated with the Brera Observatory, part of INAF (the Italian National Institute of Astrophysics) in Merate, 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 observed in 50 years of studying gamma-ray bursts.”

An article about the discovery appears in the journal Science.

When matter interacts with light, energy can be absorbed and re-emitted in characteristic ways. These interactions can brighten or dampen certain colors (or energies), producing key features that are visible when light is scattered, like a rainbow, across a spectrum. These features can reveal a wealth 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.

“Some previous studies have already reported possible signs of absorption and emission in other gamma-ray bursts, but a closer look revealed that these could all be just statistical fluctuations. What we see in the BOAT is different,” said study co-author Om Sharan Salafia of the INAF-Brera observatory in Milan, Italy. “We determined that the probability that this feature is just a noise fluctuation is less than a one in half a billion chance.”

Gamma-ray bursts are the most powerful explosions in the cosmos, emitting large amounts of gamma rays, the most energetic form of light. The most common type occurs when the core of a massive star exhausts its fuel, collapses, and forms a rapidly rotating black hole. The material falling into the black hole fuels jets of particles in opposite directions that travel through the star’s outer layers at nearly the speed of light. We detect gamma-ray bursts when one of these jets points almost directly toward Earth.

BOAT, officially known as GRB 221009A, erupted on October 9, 2022, and quickly overwhelmed most orbiting gamma-ray detectors, including those on Fermi. This prevented them from measuring the most intense part of the explosion. The reconstructed observations, combined with statistical arguments, suggest that BOAT, if part of the same population as previously detected GRBs, was likely the brightest explosion to appear in Earth’s sky in 10,000 years.

The suspected emission line appeared about 5 minutes after the explosion was detected and well after it had faded enough to end saturation effects for Fermi. The line persisted for at least 40 seconds, and the emission reached a peak energy of about 12 MeV (million electron volts). For comparison, the energy of visible light ranges from 2 to 3 electron volts.

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

“When an electron and a positron collide, they annihilate, producing a pair of gamma rays with an energy of 0.511 MeV,” said study co-author Gor Oganesyan of the Gran Sasso Institute of Science and the Gran Sasso National Laboratory in L’Aquila, Italy. “Since we are observing the jet, where matter is moving at close to the speed of light, this emission undergoes a significant blueshift and is pushed to much higher energies.”

If this interpretation is correct, to produce an emission line peaking at 12 MeV, the annihilating particles would have had to be moving toward 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 Elizabeth Hays, a Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Finding clues like this remarkable emission line will help scientists study this extreme environment more deeply.”