The longest gamma-ray burst ever observed has University of North Carolina at Chapel Hill astronomers questioning the very nature of these cosmic explosions, after research involving some of the best data ever recorded on such events.
GRB 250702B persisted for nearly seven hours, an astonishing length as gamma-ray bursts are usually measured in just minutes and seconds. Due to the event’s extended duration, researchers were able to follow up on initial observations from space-based platforms with subsequent ground-based observations, which provided the UNC researchers with data for their new paper published in The Astrophysical Journal Letters.
Gamma Ray Bursts
As the brightest and most powerful type of explosion in the universe, gamma ray bursts represent the immediate aftermath of the most catastrophic variety of cosmic events. For context, they are so energetic that only the Big Bang itself was more powerful than gamma-ray bursts.
These extremely high-energy events give researchers rare opportunities to probe physics at the edge cases of reality. During such explosions, matter can approach the speed of light, gravity becomes strong enough to warp spacetime, and material can be compressed to densities greater than atomic nuclei. They also enrich the universe with heavy elements that are pushed outward and scattered—some of which are essential to life.
Viewing GRB 250702B
“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 lead author Jonathan Carney, a PhD student in physics and astronomy at UNC-Chapel Hill.
In GRB 250702B’s case, ground-based observatories captured its fading glow as the light reached Earth from a distant, dusty galaxy. Normally, the rush to capture data from these leaves little time to capture these brief events. However, this time, the extended duration allowed researchers to capture the event’s environment as it occurred and afterward.
The UNC team leading the new paper coordinated observations using telescopes across the United States, later combining their results with data from the European Southern Observatory’s Very Large Telescope, NASA’s Hubble Space Telescope, and additional X-ray instruments to build a more complete picture. Unfortunately, the high dust content in the host galaxy blocked much of the visible light, leaving only infrared and high-energy emissions for scientists to analyze.
Explaining Cosmic Violence
The data the team obtained provided clues that enabled the researchers to narrow down the list of possible types of events that may have produced the explosion. Possible explanations include a star collapsing into a black hole, colliding exotic stellar remnants, or the death of a massive star.
“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.”
“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,” Carney added, noting that “when astronomers discover similar explosions, they’ll ask whether they match GRB 250702B’s properties or represent something different entirely.”
The paper, “Optical/Infrared Observations of the Extraordinary GRB 250702B: A Highly Obscured Afterglow in a Massive Galaxy Consistent with Multiple Possible Progenitors,” appeared in The Astrophysical Journal Letters on December 1, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.