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| This illustration shows the ingredients of a long gamma-ray burst, the most common type. The core of a massive star (left) has collapsed, forming a black hole that sends a jet of particles moving through the collapsing star and out into space at nearly the speed of light. Radiation across the spectrum arises from hot ionized gas (plasma) in the vicinity of the newborn black hole, collisions among shells of fast-moving gas within the jet (internal shock waves), and from the leading edge of the jet as it sweeps up and interacts with its surroundings (external shock). Credit: NASA's Goddard Space Flight Center |
On October 9, 2022, an intense pulse of gamma-ray radiation swept through
our solar system, overwhelming gamma-ray detectors on numerous orbiting
satellites, and sending astronomers on a chase to study the event using the
most powerful telescopes in the world.
The new source, dubbed GRB 221009A for its discovery date, turned out to be
the brightest gamma-ray burst (GRB) ever recorded.
In a new study that appears today in Astrophysical Journal Letters,
observations of GRB 221009A spanning from radio waves to gamma-rays,
including critical millimeter-wave observations with the Center for
Astrophysics | Harvard & Smithsonian's Submillimeter Array (SMA) in
Hawaii, shed new light on the decades-long quest to understand the origin of
these extreme cosmic explosions. This study is part of a series of
discoveries that are to be published as a collection in Astrophysical
Journal Letters.
The gamma-ray emission from GRB 221009A lasted over 300 seconds. Astronomers
think that such "long-duration" GRBs are the birth cry of a black hole,
formed as the core of a massive and rapidly spinning star collapses under
its own weight. The newborn black hole launches powerful jets of plasma at
near the speed of light, which pierce through the collapsing star and shine
in gamma-rays.
With GRB 221009A being the brightest burst ever recorded, a real mystery lay
in what would come after the initial burst of gamma-rays. "As the jets slam
into gas surrounding the dying star, they produce a bright 'afterglow' of
light across the entire spectrum," says Tanmoy Laskar, assistant professor
of physics and astronomy at the University of Utah, and lead author of the
study. "The afterglow fades quite rapidly, which means we have to be quick
and nimble in capturing the light before it disappears, taking its secrets
with it."
As part of a campaign to use the world's best radio and millimeter
telescopes to study the afterglow of GRB 221009A, astronomers Edo Berger and
Yvette Cendes of the Center for Astrophysics (CfA) rapidly gathered data
with the SMA.
"This burst, being so bright, provided a unique opportunity to explore the
detailed behavior and evolution of an afterglow with unprecedented detail—we
did not want to miss it," says Edo Berger, professor of astronomy at Harvard
University and the CfA. "I have been studying these events for more than
twenty years, and this one was as exciting as the first GRB I ever
observed."
"Thanks to its rapid-response capability, we were able to quickly turn the
SMA to the location of GRB 221009A," says SMA project scientist and CfA
researcher Garrett Keating. "The team was excited to see just how bright the
afterglow of this GRB was, which we were able to continue to monitor for
more than 10 days as it faded."
After analyzing and combining the data from the SMA and other telescopes all
over the world, the astronomers were flummoxed: The millimeter and radio
wave measurements were much brighter than expected based on the visible and
X-ray light.
"This is one of the most detailed datasets we have ever collected, and it is
clear that the millimeter and radio data just don't behave as expected,"
says CfA research associate Yvette Cendes. "A few GRBs in the past have
shown a brief excess of millimeter and radio emission that is thought to be
the signature of a shockwave in the jet itself, but in GRB 221009A the
excess emission behaves quite differently than in these past cases."
She adds, "It is likely that we have discovered a completely new mechanism
to produce excess millimeter and radio waves."
One possibility, says Cendes, is that the powerful jet produced by GRB
221009A is more complex than in most GRBs. "It is possible that the visible
and X-ray light are produced by one portion of the jet, while the early
millimeter and radio waves are produced by a different component."
"Luckily, this afterglow is so bright that we will continue to study its
radio emission for months and maybe years to come," adds Berger. "With this
much longer time span, we hope to decipher the mysterious origin of the
early excess emission."
Independent of the exact details of this particular GRB, the ability to
respond rapidly to GRBs and similar events with millimeter-wave telescopes
is an essential new capability for astronomers.
"A key lesson from this GRB is that without fast-acting radio and millimeter
telescopes, such as the SMA, we would miss out on potential discoveries
about the most extreme explosions in the universe," says Berger. "We never
know in advance when such events will occur, so we have to be as responsive
as possible if we're going to take advantage of these gifts from the
cosmos."
Reference:
Maia A. Williams et al, GRB 221009A: Discovery of an Exceptionally Rare
Nearby and Energetic Gamma-Ray Burst, Astrophysical Journal Letters (2023).
DOI: 10.3847/2041-8213/acbcd1.
Joe S. Bright et al, Precise Measurements of Self-absorbed Rising Reverse
Shock Emission from Gamma-ray Burst 221009A, arXiv (2023).
DOI: 10.48550/arxiv.2303.13583
Zheng-Hua An et al, Insight-HXMT and GECAM-C observations of the
brightest-of-all-time GRB 221009A, arXiv (2023).
DOI: 10.48550/arxiv.2303.01203
Tags:
Space & Astrophysics