Astronomers at the University of North Carolina at Chapel Hill have helped uncover new clues about the longest-lasting cosmic explosion ever observed, a gamma-ray burst that lasted nearly seven hours. The event, known as GRB 250702B, challenges decades of understanding about how and why these bursts occur.
Gamma-ray bursts are intense flashes of high-energy light produced by catastrophic cosmic events, usually lasting just a few seconds or minutes. But GRB 250702B broke all known records. After its initial detection by space-based observatories, researchers used some of the world's largest ground-based telescopes to capture images of a fading glow located in a massive, dusty galaxy.
As part of a coordinated international effort, the UNC team led observations with some of the United States' largest ground-based telescopes. The team's data, combined with observations from the European Southern Observatory's Very Large Telescope, NASA's Hubble Space Telescope and X-ray data suggests that this explosion could have several possible origins including the collapse of a massive star, the collision of exotic stellar remnants, or even a star being torn apart by a black hole, but current observations can't yet reveal which scenario is correct.
"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.
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."
GRB 250702B has set a new standard for how astronomers study the universe's most powerful and mysterious explosions. The research paper is available online in the The Astrophysical Journal Letters at: https://iopscience.iop.org/article/10.3847/2041-8213/ae1d67
