In this artist’s rendering, a jet of particles traveling nearly the speed of light erupts from a massive star. The star’s core ran out of fuel and collapsed into a black hole. Some of the material spiraling toward the black hole was redirected into two jets shooting in opposite directions. If one of these jets were aimed directly at Earth, it would create a gamma-ray burst. Credit: NASA Goddard Space Flight Center Conceptual Imagery Lab
Leveraging Data National Aeronautics and Space Administration (NASA)Using observations from the Fermi Gamma-ray Space Telescope, researchers have discovered an unusual energy peak suggesting the annihilation of electrons and positrons after the most luminous gamma-ray burst ever observed, providing new insight into the behavior of cosmic jets and the extreme conditions that follow such bursts.
In October 2022, astronomers were astonished by what was soon to be called BOAT. The brightest gamma-ray burst (GRB) in historyThe international scientific team NASA’s Fermi Gamma-ray Space Telescope Revealing features never before seen.
Unprecedented spectral features identified
“A few minutes after BOAT exploded, the Fermi Gamma-Ray Burst Monitor recorded an unusual energy peak that caught our attention,” said lead investigator Maria Edvige Ravasio of Radboud University in Nijmegen, the Netherlands, and the Brera Observatory of INAF (Italian National Institute for Astrophysics) in Merate, Italy. “When I first saw the signal, it gave me goosebumps. Our analysis since then has shown that this is the first reliable emission line observed on Earth. 50 Years of GRB Research.”
A paper about the findings appears in the July 26 issue of the journal Neuroscience. Science.
The Fermi Gamma-ray Space Telescope observes the Universe using the highest energy light, providing an important window into the Universe’s most extreme phenomena, from gamma-ray bursts 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, energy is absorbed and re-emitted in characteristic ways. These interactions cause certain colors (or energies) to become brighter or dimmer, producing important signatures that are visible when light is spread across a rainbow in the spectrum. These signatures 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.
“Previous studies have reported evidence of absorption and emission signatures in other GRBs, but subsequent scrutiny has revealed that all of these may simply be statistical fluctuations. That is not what we are seeing with BOAT,” said co-author Om Sharan Salafia of the INAF-Brera Observatory in Milan, Italy. “We have determined that there is less than one chance in 500 million that this signature is simply a noise fluctuation.”
The nature and effects of gamma-ray bursts
GRBs are the most powerful explosions in the universe, emitting huge amounts of gamma rays (the most energetic light). The most common type occurs when the core of a massive star runs out of fuel and collapses, forming a rapidly rotating galaxy. Black HoleMatter falling into the black hole produces opposite-directed particle jets that shoot through the star’s outer layers at nearly the speed of light. A GRB is detected when one of these jets is pointed almost toward Earth.
BOAT (officially named GRB 221009A) exploded on October 9, 2022, quickly saturating most of the orbiting gamma-ray detectors, including Fermi’s, preventing the most violent parts of the explosion from being measured. Reconstructed observations, combined with statistical arguments, suggest that if BOAT was part of the same population of previously detected GRBs, it was likely the brightest burst to appear in Earth’s sky in the past 10,000 years.
The most luminous gamma-ray burst ever recorded has given scientists new high-energy properties to study. Learn what NASA’s Fermi mission saw and what the properties tell us about the burst’s faster-than-light jet. Credit: NASA Goddard Space Flight Center
Insights into cosmic particle interactions
The putative emission lines appeared about 5 minutes after the burst was detected, long after it had become dim enough for Fermi saturation effects to cease. The emission lines persisted for at least 40 seconds, and the emission reached a peak energy of about 12 MeV (million electron volts). By comparison, visible light has energies in the range of 2 to 3 electron volts.
So what gives rise to this spectral feature? The research team believes that the most likely cause is the annihilation of an electron and its antimatter counterpart, a positron.
“When an electron and a positron collide, they annihilate and produce a gamma-ray pair with an energy of 0.511 MeV,” said co-author Gor Oganessian of the Gran Sasso Scientific Institute and Gran Sasso National Laboratory in L’Aquila, Italy. “Because we are observing a jet of material moving close to the speed of light, this radiation is strongly blueshifted and pushed to much higher energies.”
Future research directions and collaborations
If this interpretation is correct, then to produce an emission line that peaks at 12 MeV, the annihilation particle would have had to be traveling towards us at about 99.9% of the speed of light.
“After decades of studying these amazing cosmic explosions, we still don’t know the details of how these jets work,” said Fermi project scientist Elizabeth Hayes of NASA’s Goddard Space Flight Center in Greenbelt, Md. “Finding clues like this remarkable emission line can help scientists probe this extreme environment more deeply.”
The Fermi Gamma-ray Space Telescope is a partnership of astrophysics and particle physics managed by the Goddard Space Flight Center. Fermi was developed in collaboration with the U.S. Department of Energy, with significant contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.
