A jet of particles traveling at nearly the speed of light emerges from a massive star in this artist’s concept. The star’s core ran out of fuel and collapsed into a black hole. Some of the matter spinning toward the black hole was redirected into twin jets shooting in opposite directions. We see a burst of gamma rays when one of these jets heads directly for Earth. Credit: NASA Goddard Space Flight Center Concept Imaging Laboratory
Use of data from NASAWith the Fermi Space Gamma-ray Telescope, researchers detected a unique energy peak after the brightest gamma-ray burst ever observed, suggesting the annihilation of electrons and positrons. This discovery provides new insights into the behavior of cosmic jets and the extreme conditions following such outbursts.
In October 2022, astronomers were stunned by what was quickly dubbed BOAT—the brightest gamma-ray burst (GRB) of all time. Now, an international science team reports that data from NASA’s Fermi Space Gamma-ray Telescope reveals a feature never seen before.
An unprecedented spectral feature has been identified
“A few minutes after the BOAT eruption, the Fermi gamma-ray monitor registered an unusual energy peak that caught our attention,” said lead researcher Maria Edwige Ravazio of Radboud University in Nijmegen, the Netherlands, and associated with the Brera Observatory, part of INAF ( Italian National Institute of Astrophysics) in Merate, Italy. “When I first saw that signal, I got goosebumps. Our analysis has since shown that this is the first high-certainty emission line ever observed in 50 years of studying GRBs.
An article about the discovery appears in the July 26 issue of the journal Science.
The Fermi Space Gamma-ray Telescope observes the cosmos using the highest-energy form of light, providing an important window into the universe’s most extreme phenomena, from gamma rays and black hole jets to pulsars, supernova remnants and the origins of cosmic rays. Credit: © Daniëlle Futselaar/MPIfR (artsource.nl)
When matter interacts with light, the energy can be absorbed and re-emitted in characteristic ways. These interactions can brighten or darken certain colors (or energies), producing key features visible when light is spread, like a rainbow, into 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 matter and antimatter, that annihilate to produce gamma rays.
“While some previous studies have reported possible evidence of absorption and emission features in other GRBs, a follow-up review has revealed that these may all be mere statistical fluctuations. What we see in BOAT is different,” said co-author Om Sharan Salafia of the INAF-Brera Observatory in Milan, Italy. “We found that the probability that this feature is just a fluctuation in noise is less than one chance in half a billion.”
The nature and effects of gamma rays
GRBs are the most powerful explosions in space and emit 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 rotating Black hole. Matter falling into the black hole sets off counter-directed jets of particles that blast through the star’s outer layers at nearly the speed of light. We detect a GRB when one of these jets points almost directly at Earth.
The BOAT, officially known as GRB 221009A, erupted on October 9, 2022, and immediately saturated most of the gamma-ray detectors in orbit, including Fermi’s. This prevented them from measuring the most intense part of the blast. The reconstructed observations, combined with statistical arguments, suggest that BOAT, if part of the same population as previously detected GRBs, is likely to be the brightest burst to appear in Earth’s sky in 10,000 years.
The brightest gamma-ray burst ever recorded has given scientists a new high-energy feature to study. Learn what NASA’s Fermi mission saw and what this feature can tell us about the burst’s light-speed jets. Credit: NASA Goddard Space Flight Center
Insights into Cosmic Particle Interactions
The suspected emission line appeared almost 5 minutes after the burst was detected, and well after it had dimmed 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). In 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, producing a pair of gamma rays with an energy of 0.511 MeV,” said co-author Gore Hovhannesian of the Gran Sasso Science Institute and Gran Sasso National Laboratory in Aquila, Italy. “Because we’re looking into the jet, where the matter is moving at near the speed of light, this emission becomes strongly blue-shifted and pushed to much higher energies.”
Future research directions and collaborations
If this interpretation is correct, to produce an emission line with a peak at 12 MeV, the annihilating particles must have been moving towards us at about 99.9% of the speed of light.
“After decades of studying these amazing cosmic explosions, we still don’t understand the details of how these jets work,” noted Elizabeth Hayes, a Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Finding clues like this remarkable emission line will help scientists probe deeper into this extreme environment.”
The Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership operated by Goddard. Fermi was developed in collaboration with the US Department of Energy, with significant input from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.
