GRB 250702B emerged from the top edge of its host galaxy’s dark dust lane (see inset). Credit: NASA, ESA, CSA, H Sears (Rutgers). Credit: STScI
Gamma-ray bursts are the universe’s ultimate flash in the pan. In a fraction of a second, they can violently release more energy than our Sun will emit over its entire ten-billion-year lifespan. Normally, these extreme flashes vanish almost as soon as they appear. They are one-and-done events.
But on July 2, 2025, the cosmos delivered something that completely shattered our expectations. A gamma ray event known as GRB 250702B arrived on the detectors of NASA’s Fermi Gamma-ray Space Telescope. It did not fade in seconds. Instead, it kept going for seven hours, firing three distinct bursts over an entire day. It even left behind a glowing aftermath that lasted for months.
Astronomers immediately recognized that they had caught a cosmic rule-breaker.
“This is certainly an outburst unlike any other we have seen in the past 50 years,” said Eliza Neights, an astronomer at NASA’s Goddard Space Flight Center.
So, what happened? Two major independent studies are now racing to explain this cosmic anomaly: one reveals a massive, distorted galaxy that appears to be in the middle of a violent merger, while the other concludes we have finally caught a rare middleweight black hole in the act of shredding a sun-like star.
A Signal from the Deep Universe
Initially, researchers assumed this bizarre, repeating signal had to come from our own cosmic backyard. The burst appeared to originate in the crowded central plane of the Milky Way.
“Before these observations, the general feeling in the community was that this GRB must have originated from within our galaxy. The VLT fundamentally changed that paradigm,” says Andrew Levan, an astronomer at Radboud University in the Netherlands.
Using the Very Large Telescope (VLT) in Chile, astronomers peered through the glare of nearby stars and realized the source lived far beyond our galaxy. Later, NASA’s James Webb Space Telescope measured the precise distance. The host galaxy sits a staggering eight billion light-years away. The light we see today left its source billions of years before the Earth even formed.
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“What we found was considerably more exciting: the fact that this object is extragalactic means that it is considerably more powerful,” says Antonio Martin-Carrillo, an astronomer at University College Dublin.
“This is 100-1000 times longer than most GRBs,” says Levan.
Even more baffling, the burst repeated.
“More importantly, gamma-ray bursts never repeat since the event that produces them is catastrophic,” says Martin-Carrillo.
Piercing Through the Dust
This artist’s concept depicts GRB 250702B (left of center) erupting within its host galaxy. This powerful explosion, first detected on July 2, blasted out narrow jets of particles at nearly the speed of light and exhibited repeated outbursts that lasted over seven hours. Astronomers conducting rapid follow-up observations with multiple telescopes around the world found that the burst occurred within a large, extremely dusty galaxy. Credit: NOIRLab/NSF/AURA/M. Garlick
To figure out what caused this massive, repeating explosion, scientists aimed a global armada of telescopes at the sky. They captured X-rays, radio waves, and infrared light to build a comprehensive profile of the anomaly.
“Only through the combined power of instruments on multiple spacecraft could we understand this event,” said Eric Burns, an astrophysicist at Louisiana State University.
A recent study led by astronomer Jonathan Carney and colleagues details how the team used infrared observations from the Magellan and Keck telescopes to pierce through a thick veil of cosmic dust. They discovered that the host galaxy is incredibly massive — weighing in at over 40 billion times the mass of our Sun — and deeply obscured.
The James Webb Space Telescope, which can peer through gas and dust thanks to its powerful infrared sensors, revealed this galaxy in unprecedented clarity. It appears highly distorted and asymmetrical, heavily suggesting it might actually be two distinct galaxies violently crashing into each other.
“In such vibrant and unprecedented detail, we see just one very large galaxy with a dust lane,” said Huei Sears, a postdoctoral researcher at Rutgers University.
This chaotic, dusty environment hid the explosion from optical telescopes entirely. But in the infrared, the brilliant afterglow shone through.
“The galaxy has such a 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,” Sears said.
In this chaotic environment, the researcher suggest the explosion could be caused by several exotic progenitors, ranging from a star merger with a black hole to a micro-TDE where a stellar-mass compact object shreds a star. Essentially, they conclude that the messy merger of two galaxies creates the perfect conditions for a rare, ultra-long explosion that doesn’t fit the standard mold.
Suspects in a Cosmic Crime
GRB 250702B (circled in red), taken on 3 July with the VLT’s HAWK-I infrared camera. Credit: NASA/ESA Hubble Space Telescope
So, what exactly blew up? The extreme length and repeating nature of GRB 250702B leave researchers debating several exotic suspects.
“This object shows extreme properties that are difficult to explain,” said Sears. “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.”
Standard gamma-ray bursts happen when a massive star collapses into a black hole, or when two dead neutron stars collide. Neither scenario fits perfectly here.
“If this is a massive star, it is a collapse unlike anything we have ever witnessed before,” says Levan.
Another idea suggests a stellar-mass black hole merged with a stripped-down star, essentially eating it from the inside out. But a third, highly compelling theory points to a phenomenon known as a tidal disruption event. This happens when an unlucky star wanders too close to a black hole and gets shredded by intense gravity.
Now, a separate research team, led by astrophysicist Jonathan Granot, recently published a study arguing that this specific explosion bears the undeniable fingerprints of a highly sought-after cosmic object: an intermediate-mass black hole.
The Missing Link of Black Holes
Black holes usually come in two flavors. We have small, stellar-mass black holes formed from dying stars. Then we have supermassive giants anchoring the centers of galaxies.
Between them lies a hypothetical missing link known as intermediate-mass black holes. These middleweights range from a few hundred to a hundred thousand times the mass of our Sun. While our models say they should be everywhere, finding them has been challenging.
Granot and his team calculate that GRB 250702B was caused by an intermediate-mass black hole roughly 6,500 times heavier than our Sun. The explosion did not happen in the galaxy’s center. It erupted about 18,000 light-years away from the galactic core. That is exactly where a wandering middleweight black hole might lurk, especially in a galaxy undergoing a messy merger.
According to this model, an ordinary star drifted too close. The black hole’s gravity stretched and ripped the star apart. The star did not die instantly. Instead, it made multiple agonizing orbits. On each pass, the black hole stripped away more stellar material, generating a fresh burst of energy. This perfectly explains the repeating signals Fermi detected.
Some early theories suggested the victim might have been a dense white dwarf star. However, Granot’s team argues that a white dwarf would be consumed far too quickly to power a seven-hour light show. Only a larger, Sun-like star provides enough fuel to sustain an explosion of this magnitude and duration.
A Window into Extreme Evolution
If the intermediate-mass black hole theory holds up, GRB 250702B represents a monumental milestone in astronomy. It would be the first time astronomers have ever watched a medium-sized black hole actively devour a star while firing a jet of particles at nearly the speed of light.
However, the scientific community thrives on healthy skepticism, and the debate is far from settled. The immense distance and the thick dust obscuring the host galaxy make it difficult to completely rule out other bizarre phenomena.
“We have only seen a few tidal disruption events of this type, so we don’t know for sure how they’re supposed to evolve,” Sears said. “A lot of the studies on this explosion provide different, and sometimes contradictory, explanations. It’s still early in our understanding of what really happened.”
Astronomers will continue to monitor the fading embers of this explosion using radio and infrared telescopes. They hope to catch a glimpse of an underlying supernova, or a telltale shift in the light that might definitively prove one theory over the others.
In the grand story of the cosmos, events that break our models are the ones that teach us the most. We are standing on the edge of a new frontier in high-energy astrophysics, driven by an explosion that refused to fade away quietly.
“We are still not sure what produced this, but with this research we have made a huge step forward towards understanding this extremely unusual and exciting object,” says Martin-Carrillo.
For scientists, often the mystery itself is the prize.
“This gives us a unique chance to study the extremes of how stars and black holes evolve,” Sears said. “GRB 250702B could even be the discovery of something unexpected and new.”