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This artist’s illustration, which shows a high-speed jet of
material being launched from a source that is embedded in a very dusty galaxy,
depicts GRB 250702B — the longest gamma-ray burst that astronomers have ever
observed. This powerful, extragalactic explosion was first detected on 2 July
2025. It exhibited repeated bursts that lasted over seven hours. Credit:
NOIRLab/NSF/AURA/M. Garlick |
A record-breaking cosmic explosion has forced astronomers to rethink what they know about gamma-ray bursts.
Researchers at the University of North Carolina at Chapel
Hill have contributed to new insights into the most long-lived cosmic explosion
ever recorded. The event was a gamma-ray burst that remained active for nearly
seven hours. Known as GRB 250702B, it pushes beyond long-standing theories
about the processes that create these powerful outbursts.
Gamma-ray bursts are brief but extreme surges of high-energy
radiation triggered by violent events in space and typically last only seconds
or minutes. GRB 250702B defied that pattern, setting a new duration record.
Following its discovery by space-based instruments, astronomers turned to some
of the world’s largest ground-based telescopes to observe the fading afterglow.
These observations traced the burst to a large, dust-filled galaxy.
The observations were part of a coordinated international
campaign led in part by the UNC team, which relied on major ground-based
telescopes in the United States. Their findings were combined with data from
the European Southern Observatory’s Very Large Telescope, NASA’s Hubble Space
Telescope and X-ray observations. Together, the evidence points to several
possible explanations for the explosion, ranging from the collapse of a massive
star to the merger of unusual stellar remnants, or even a star being shredded
by a black hole. For now, the available data are not sufficient to determine
which of these possibilities is responsible.
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Left: The stellar field around the host galaxy of GRB
250702B — the longest gamma-ray burst that astronomers have ever observed.
Right: Close-up view of the host galaxy taken with the Gemini North telescope.
This image is the result of over two hours of observation, yet the host galaxy
appears extremely faint due to the large amount of dust surrounding it. Credit:
International Gemini Observatory/CTIO/NOIRLab/DOE/NSF/AURA Image processing: M.
Zamani & D. de Martin (NSF NOIRLab) |
“This was the longest gamma-ray burst that humans have
observed—long enough that it does not fit into any of our existing models for
what causes gamma-ray bursts,” said Jonathan Carney, lead author of the study
and PhD student in physics and astronomy at UNC-Chapel Hill.
A Rare Opportunity to Study Extreme Physics
Gamma-ray bursts are among the largest explosions in the
universe, and astronomers have to rush to capture data before their light
fades. Because this event was more prolonged and unusual, it offered scientists
a rare opportunity to study the environment with both information from the
explosion itself and later imaging of the host galaxy. The researchers found
the explosion came from a distant, massive galaxy full of dust that blocked
visible light—allowing only infrared and high-energy emissions to be detected.
“We’re not sure what caused this record-breaking event,”
said Igor Andreoni, co-author and assistant professor of physics and astronomy
at UNC-Chapel Hill. “We know it occurred billions of light-years away in a very
complex galaxy. Our data revealed that an energetic phenomenon launched a
narrow jet of material in our direction that traveled at least 99% the speed of
light, piercing through thick layers of cosmic dust.”
Understanding these massive explosions helps scientists
probe some of the universe’s most extreme environments where matter moves at
nearly the speed of light, densities exceed atomic nuclei, and gravity is
strong enough to warp spacetime itself. They also play a key role in scattering
heavy elements, including those essential for life, across the cosmos.
“Our analysis shows this event could have several different
causes—including the death of a massive star, the collision of a helium star,
or even a star being ripped apart by a black hole,” Carney said. “But we can’t
yet tell which explanation is correct. In the future, this event will serve as
a unique benchmark—when astronomers discover similar explosions, they’ll ask
whether they match GRB 250702B’s properties or represent something different
entirely.”