Gamma-ray bursts (GRBs) are the universe’s ultimate boom, brief, blazing explosions from dying stars or black holes feasting on unlucky neighbors. They usually flash and vanish in seconds.
But one recent GRB – GRB 250702B – broke the rules: it pulsed repeatedly for a whole day, like a cosmic heartbeat. And it wasn’t even from our galaxy. Astronomers, stunned by this celestial encore, used the Very Large Telescope to trace it to a distant galaxy.
Antonio Martin-Carrillo, astronomer at University College Dublin, Ireland, and co-lead author of a study, said, “This GRB is ‘unlike any other seen in 50 years of GRB observations.”
On July 2, NASA’s Fermi telescope spotted not one, but three gamma-ray bursts from the same cosmic source, hours apart. Turns out, the Einstein Probe had already seen signs of activity a day earlier. That’s unheard of: GRBs don’t usually repeat, and definitely not for this long.
But the source was hiding in a star-packed part of our galaxy, making it tricky to locate. So astronomers called in the big guns, ESO’s Very Large Telescope, to zoom in and pinpoint the exact spot.
Andrew Levan, astronomer at Radboud University, The Netherlands, and co-lead author of the study, said, “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.”
Astronomers used the VLT’s HAWK-I infrared camera to zoom in on the mysterious GRB and spotted a faint glow that didn’t match anything in our Milky Way. Suspicious, they called in the Hubble Space Telescope for backup, and boom, confirmation: the source lives in a different galaxy.
“What we found was considerably more exciting: the fact that this object is extragalactic means that it is considerably more powerful,” says Martin-Carrillo.
The galaxy hosting this strange GRB appears to be billions of light-years from Earth, but scientists need more clues to nail down the exact distance.
As for what caused it? No one’s sure yet. One idea: a giant star collapsed in on itself, blasting out a massive burst of energy as it died.
“If this is a massive star, it is a collapse unlike anything we have ever witnessed before,” says Levan, as in that case the GRB would have lasted just a few seconds. Alternatively, a star being ripped apart by a black hole could produce a day-long GRB, but explaining other properties of the explosion would require an unusual star being destroyed by an even more unusual black hole.
To crack the mystery of this bizarre, day-long gamma-ray burst (GRB), astronomers are watching its fading glow with some of the sharpest eyes in space. They’ve brought in the VLT’s X-shooter spectrograph to analyze the light in detail, and the James Webb Space Telescope to peer deeper into its host galaxy, a collaboration between NASA, ESA, and the Canadian Space Agency.
“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.
Journal Reference:
- Andrew J. Levan, Antonio Martin-Carrillo, Tanmoy Laskar, Rob A. J. Eyles-Ferris, Albert Sneppen et al. The Day-long, Repeating GRB 250702B: A Unique Extragalactic Transient. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/adf8e1