NASA’s Fermi discovers new feature in brightest burst in range

In October 2022, astronomers were dazzled by what quickly became known as BOAT – the brightest gamma-ray burst (GRB) of all time. Now an international science team reports that data from NASA’s Fermi Gamma-ray Space Telescope reveals a never-before-seen feature.

“A few minutes after the spacecraft exploded, Fermi’s gamma-ray burst monitor recorded an unusual energy peak that caught our attention,” said lead researcher Maria Edvige Ravasio at 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, it blew my mind. Our analysis since then shows that it is the first high-confidence emission line ever seen in 50 years of studying GRBs.”

An article about the discovery appears in the July 26 edition of the journal Science.

When matter interacts with light, the energy can be absorbed and re-emitted in characteristic ways. These interactions can brighten or dim particular 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 the annihilation of matter and antimatter to produce gamma rays.

“While some previous studies have reported possible evidence for absorption and emission features in other GRBs, subsequent scrutiny revealed that these may all be mere statistical fluctuations. What we see in BOAT is different,” said co-author Om Sharan Salafia at INAF-Brera. Observatory in Milan, Italy. “We’ve determined that the odds that this feature is just a noise fluctuation are less than a chance in half a billion.”

GRBs are the most powerful explosions in the cosmos and emit copious amounts of gamma rays, the highest energy form of light. The most common type occurs when the core of a massive star exhausts its fuel, collapses and forms a rapidly spinning black hole. Matter falling into the black hole powers jets of counter-moving particles that blast through the star’s outer layers at nearly the speed of light. We detect GRBs when one of these jets heads almost directly toward Earth.

SHIP, officially known as GRB 221009A, exploded on October 9, 2022 and immediately saturated most of the gamma-ray detectors in orbit, including those at Fermi. This prevented them from measuring the most intense part of the eruption. The reconstructed observations, along with statistical arguments, suggest that SHIP, if part of the same population as previously detected GRBs, was likely the brightest burst to appear in Earth’s sky in 10,000 years.

The putative emission line appears almost 5 minutes after the burst was detected, and well after it had faded enough to end saturation effects for Fermi. The line continued for at least 40 seconds and the emission reached a maximum energy of about 12 MeV (million electron volts). By 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 co-author Gor Oganesyan at the Gran Sasso Science Institute and Gran Sasso National Laboratory in L’Aquila, Italy. “Because we’re looking at the jet, where matter is moving at near-light speeds, this emission becomes very blue and is pushed to much higher energies.”

If this interpretation is correct, to produce an emission line peaking at 12 MeV, the annihilation particles would have had to have moved 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,” noted Elizabeth Hays, Fermi project scientist at NASA’s Goddard Space 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 managed by Goddard. Fermi was developed in collaboration with the US Department of Energy, with significant contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.