Webb and global telescopes probe seven-hour gamma-ray burst, nearly doubling previous record

An extraordinary cosmic blast that kept shining for hours has left astronomers rethinking what powers the universe’s most energetic explosions. Researchers using the James Webb Space Telescope and a worldwide network of observatories are scrutinizing GRB 250702B, a long gamma-ray burst whose emission continued for at least seven hours—nearly twice as long as the previous record holder.

NASA announced that Webb examined the event, which was first detected by the Fermi Gamma-ray Space Telescope on July 2. Long gamma-ray bursts are usually linked to the collapse of a massive star into a black hole, producing a brief, intense flash. This one did not fit the mold.

“This object shows extreme properties that are difficult to explain,” said Huei Sears, a postdoctoral researcher in Rutgers University’s Department of Physics and Astronomy who is studying the explosion.

“Usually, these bursts are over in less than a minute, but GRB 250702B lasted for hours and even showed signs of X-ray activity a day prior.” Observatories around the world, including teams working with China’s Einstein Probe and the National Science Foundation’s Very Large Array, are analyzing data across the spectrum—gamma rays, X-rays, infrared and radio—because no single instrument could capture the full picture.

“Only through the combined power of instruments on multiple spacecraft could we understand this event,” said Eric Burns, an astrophysicist at Louisiana State University. Eliza Neights of NASA’s Goddard Space Flight Center called it “an outburst unlike any other we have seen in the past 50 years.” Scientists are weighing several explanations.

One possibility is that this was an unusually extreme gamma-ray burst. Another is a tidal disruption event, in which a black hole thousands of times the Sun’s mass shreds a star that strays too close. A more unusual idea suggests a smaller black hole merged with a stripped helium star and consumed it from within.

NASA also released an animation illustrating one potential scenario: a black hole about three times the mass of the Sun, with an event horizon roughly 11 miles (18 kilometers) wide, orbiting and merging with a companion star. Whatever the mechanism, the event appears to have launched powerful jets of energy into space.

Hubble Space Telescope images initially showed an odd-looking galaxy at the burst’s location—possibly two galaxies in the process of merging or a single system bisected by a dark dust lane. Later Webb observations placed the host galaxy about 8 billion light-years away, meaning the explosion occurred long before Earth formed.

To probe the site, Sears led follow-up imaging with Webb’s NIRCam several months after the outburst. “In such vibrant and unprecedented detail, we see just one very large galaxy with a dust lane,” Sears said.

“The galaxy has such complex structure that it’s not 100% clear if there’s anything left to see of the explosion, but if there is, it’s really faint.” That view supports the idea that GRB 250702B was a gamma-ray burst rather than a tidal disruption event. For now, the cause remains unsettled.

Teams continue to sift through the multiwavelength data to determine how an explosion of this length and power could occur, and what it means for long-standing models of the most extreme blasts in the cosmos.