Astronomers have detected a cosmic explosion unlike anything ever seen before—an event that could point to a rare type of black hole long suspected but never confirmed.
The explosion, known as a gamma-ray burst (GRB), flashed repeatedly over the course of a full day, a phenomenon never observed in more than half a century of GRB research.
Gamma-ray bursts are the most powerful explosions in the universe. They are usually caused by the catastrophic collapse of massive stars or the violent mergers of stellar remnants, and they typically last anywhere from milliseconds to a few minutes.
Once a GRB occurs, the source is destroyed in the process, leaving no chance for it to happen again.
That is why the new discovery, reported in The Astrophysical Journal Letters, has left astronomers baffled.
“This event is unlike any other seen in 50 years of GRB observations,” said Dr. Antonio Martin-Carrillo of University College Dublin, who co-led the study.
“GRBs are catastrophic events, so they are expected to go off just once. This one not only showed repeated powerful activity but also seemed to follow a periodic rhythm, which has never been observed before.”
The burst, officially named GRB 250702B, was first detected on July 2 by NASA’s Fermi Gamma-ray Space Telescope.
The day before, the Einstein Probe, a new X-ray space mission led by the Chinese Academy of Sciences with support from the European Space Agency and the Max Planck Institute, had also picked up signs of unusual activity.
Together, these detections sparked a rapid follow-up effort by ground-based observatories.
Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, astronomers were able to pinpoint the exact location of the burst.
At first, the explosion appeared to come from within the Milky Way due to its position near the galactic plane. But closer analysis using the VLT’s HAWK-I camera, followed by confirmation from the NASA/ESA Hubble Space Telescope, revealed that the event actually originated far beyond our galaxy, in another galaxy billions of light-years away.
“What we found was considerably more exciting,” said Dr. Martin-Carrillo. “The fact that this object is extragalactic means it is far more powerful than we first imagined.”
So what could cause such a strange, long-lasting, repeating gamma-ray burst?
One possibility is that a massive star, perhaps 40 times heavier than our Sun, underwent a highly unusual type of collapse in which some of its material continued to fuel the explosion after the initial blast.
Another explanation is that a star was torn apart by the gravity of a black hole in what is known as a tidal disruption event. But in this case, the evidence points to something even rarer—an intermediate-mass black hole, a long-theorized “missing link” between stellar-mass black holes and the supermassive giants that anchor galaxies.
If confirmed, this would be the first time astronomers have found such a black hole. “Either explanation would be a first, making this event extremely unique,” said Martin-Carrillo.
The team continues to monitor the aftermath of the explosion with some of the world’s most powerful instruments, including the James Webb Space Telescope. Measuring the true distance of the event will allow them to determine exactly how much energy was released and narrow down the list of possible causes.
“We are still not sure what produced this or if we will ever fully find out,” said Martin-Carrillo. “But with this discovery, we’ve taken a huge step forward in understanding one of the most unusual and exciting cosmic events ever recorded.”
