This 7-hour cosmic explosion is the longest gamma-ray burst ever seen. Could it be from an elusive class of black hole?

Astronomers have spotted the longest gamma-ray burst ever seen, a cosmic explosion that lasted seven hours — and they determined it could be the work of a black hole destroying a star.

Gamma-ray bursts (GRBs) are considered to be the most powerful cosmic explosions since the Big Bang, but when GRB 250702B was first detected on July 2, 2025 by NASA's Fermi Gamma-ray Space Telescope, it quickly stood out among the around 15,000 GRBs detected thus far. The blast could be the result of an elusive intermediate-mass black hole devouring a star.

The record-breaking GRB is the longest ever seen, with none of its contemporaries even coming close to its seven-hour duration. Not only that, but GRB 250702B also exhibited repeating explosive bursts, stretching its duration over days. Because most GRBs last for just a few minutes, this meant GRB 250702B offered astronomers a unique opportunity to study such an event. They also believe this is an entirely new form of cosmic explosion.

"The initial wave of gamma rays lasted at least 7 hours, nearly twice the duration of the longest GRB seen previously, and we detected other unusual properties," team member Eliza Neights of George Washington University and NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "This is certainly an outburst unlike any other we've seen in the past 50 years."

Following up on Fermi's initial detection, astronomers turned to the Very Large Telescope (VLT) located on Cerro Paranal in the Atacama desert of northern Chile. This revealed the location from which the GRB erupted, a galaxy billions of light-years away from the Milky Way.

Investigating the afterglow of GRB 250702B with three of the world's most powerful ground-based telescopes, the Víctor M. Blanco 4-meter Telescope and the twin 8.1-meter International Gemini Observatory telescopes, a team of researchers led by Jonathan Carney, of the University of North Carolina at Chapel Hill, found clues as to what caused this record-breaking explosion.

"The ability to rapidly point the Blanco and Gemini telescopes on short notice is crucial to capturing transient events such as gamma-ray bursts," Carney said in a statement. "Without this ability, we would be limited in our understanding of distant events in the dynamic night sky."

What caused the longest gamma-ray burst ever recorded?

This research suggested that the initial gamma-ray signal from GRB 250702B emerged from a narrow, near-light-speed jet of plasma slamming into surrounding gas and dust. This indicates that the galaxy that is home to this event is packed with a vast amount of dust around the point of emission. The team also found that the host galaxy of GRB 250702B is more massive than those of other GRBs.

One current theory about the creation of GRBs suggests they happen when massive blue supergiant stars collapse at the end of their lives, when an extreme neutron star called a magnetar is born, or when a black hole rips apart a star in a so-called "tidal disruption event" (TDE). However, GRB 250702B doesn't seem to completely fall in line with any of these scenarios.

Researchers currently have three ideas of what event could have launched this record-breaking GRB. The first would involve a black hole colliding with a star that has had its outer layers of hydrogen stripped and is now composed mostly of helium.

The second scenario suggests a star or a smaller stellar body like a brown dwarf or even a planet could have encountered the immense gravitational influence of a black hole or a neutron star, resulting in a smaller, less powerful TDE-type event called a micro-tidal disruption event.

The third potential launch mechanism for GRB 250702B involves an elusive class of black hole called an "intermediate mass black hole," with a mass ranging from 100 times that of the sun, to 100,000 times the mass of our star. Despite the fact that scientists believe the cosmos is packed with these middle-of-the-road black holes, they are rarely detected. If this scenario is the right fit for GRB 250702B, this would represent the first time astronomers have spotted an intermediate-mass black hole producing a plasma jet after ripping up a star.

"This work presents a fascinating cosmic archaeology problem in which we're reconstructing the details of an event that occurred billions of light-years away," Carney said. "The uncovering of these cosmic mysteries demonstrates how much we are still learning about the universe's most extreme events and reminds us to keep imagining what might be happening out there."

The team's research was published in November in The Astrophysical Journal Letters.