In October 2022, astronomers were stunned by what was quickly dubbed the BOAT — the brightest-of-all-time gamma-ray burst (GRB). Now an international science team reports that data from NASA’s Fermi Gamma-ray Space Telescope reveals a feature never seen before.
The brightest gamma-ray burst yet recorded gave scientists a new
high-energy feature to study. Learn what NASA’s Fermi mission saw, and what
this feature may be telling us about the burst’s light-speed jets. Credit:
NASA's Goddard Space Flight Center
“A few minutes after the BOAT erupted,
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, Netherlands, and affiliated with Brera Observatory, part of INAF (the Italian National Institute of Astrophysics) in Merate, Italy.
“When I first saw that signal, it gave me goosebumps. Our analysis since then
shows it to be the first high-confidence emission line ever seen in 50
years of studying GRBs.”
A paper about the discovery appears in the July 26 edition of the journal
Science.
When matter interacts with light, the energy can be absorbed and
reemitted in characteristic ways. These interactions can brighten or dim
particular colors (or energies), producing key features visible when the light
is spread out, rainbow-like, in 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 uncover specific particle processes,
such as matter and antimatter annihilating to produce gamma rays.
“While some previous studies have reported possible evidence for absorption and emission features in other GRBs, subsequent scrutiny revealed that all of these could just be statistical fluctuations. What we see in the BOAT 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.”
A jet of particles moving at nearly light speed
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 swirling toward the
black hole was redirected into dual jets firing in opposite directions. We see
a gamma-ray burst when one of these jets happens to point directly at Earth.
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 oppositely directed particle jets that blast through the
star’s outer layers at nearly the speed of light. We detect GRBs when one of
these jets points almost directly toward Earth.
The BOAT, formally known as GRB 221009A, erupted Oct. 9, 2022, and
promptly saturated most of the gamma-ray detectors in orbit, including those on
Fermi. This prevented them from measuring the most intense part of the blast.
Reconstructed observations, coupled with statistical arguments, suggest the
BOAT, if part of the same population as previously detected GRBs, was likely
the brightest burst to appear in Earth’s skies in 10,000 years.
The putative emission line appears 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). 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 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 blueshifted
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.
“After decades of studying these incredible cosmic explosions, we still
don’t understand the details of how these jets work,” noted Elizabeth Hays, the
Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Finding clues like this remarkable
emission line will help scientists investigate this extreme environment more
deeply.”
The Fermi Gamma-ray Space Telescope is an astrophysics and particle
physics partnership managed by Goddard. Fermi was developed in collaboration
with the U.S. Department of Energy, with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden, and the
United States.
By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Download
high-resolution video and images from NASA's Scientific Visualization Studio
Source: NASA’s Fermi Finds New Feature in Brightest Gamma-Ray Burst Yet Seen - NASA Science
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