A team of astronomers have observed an explosion in the universe unlike any ever witnessed before.

The gamma-ray bursts from outside the Milky Way galaxy repeated several times over the course of a day. Gamma-ray bursts (GRBs) are the most powerful explosions in the universe, normally caused by the catastrophic destruction of stars, but no known scenario can completely explain this new GRB.

The discovery, which has just been published in the The Astrophysical Journal Letters, was made by Dr Antonio Martin-Carrillo (UCD School of Physics) and colleagues, who is co-lead author on the paper ‘The day long, repeating GRB 250702B: A unique extragalactic transient.’ The phenomenon was spotted through the European Southern Observatory (ESO) Very Large Telescope.

Dr Martin-Carrillo said: “This event is unlike any other seen in 50-years of GRB observations. GRBs are catastrophic events so they are expected to go off just once because the source that produced them does not survive the dramatic explosion. This event baffled us not only because it showed repeated powerful activity but also because it seemed to be periodic, which has never seen before.

“If a massive star – about 40 times the mass of the Sun – had died, like in typical GRBs, then it had to be a special type of death where some material kept powering the central engine. Alternatively, the periodicity of the flashes of gamma-ray radiation could be caused by a star being ripped apart by a black hole, a phenomenon known as a tidally disruption event (TDE). However, unlike more typical TDEs, to explain the properties of this explosion would require an unusual star being destroyed by an even more unusual black hole, likely the long-sought ‘intermediate mass black hole’. Either option would be a first, making this event extremely unique.”

In addition, GRBs have been known to last only milliseconds to minutes but this one – referred to as GRB 250702B – lasted about a day, which is “100 to 1000 times longer than most GRBs,” according to fellow co-lead author Prof Andrew Levan, astronomer at Radboud University in The Netherlands.

On 2 July, NASA’s Fermi Gamma-ray Space Telescope picked up the signal for the mystery GRB but could only give an approximate location of the activity. A day earlier, the Einstein Probe – an X-ray space telescope mission by the Chinese Academy of Sciences with the European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics – had similarly detected activity.

After these detections, the ESO team used the Very Large Telescope (VLT) to pinpoint the exact location of the explosion. Prof Levan said: “The event seemed to have originated from within our galaxy due to its proximity to the galactic plane. The VLT fundamentally changed that paradigm.”

Using the VLT’s HAWK-I camera, they found evidence that the source may actually reside in another galaxy. This was later confirmed by the NASA/ESA Hubble Space Telescope. “What we found was considerably more exciting – the fact that this object is extragalactic means that it is considerably more powerful,” said Dr Martin-Carrillo. The size and brightness of the host galaxy suggest it may be located a few billion light-years away, but more data are needed to refine this distance.

To learn more about this GRB, the team has been monitoring the aftermath of the explosion with different telescopes and instruments, including the VLT’s X-shooter spectrograph and the James Webb Space Telescope, a joint project of NASA, ESA and the Canadian Space Agency. Finding that this explosion took place in another galaxy will be key to deciphering what caused it.

Dr Martin-Carrillo said: “We keep collecting more data. Determining the true distance of this event will be key to measuring its true energy and will help us improve our physical modelling.”

He said: “We are still not sure what produced this or if we can ever really find out but, with this research, we have made a huge step forward towards understanding this extremely unusual and exciting object.”

For more, see the European Southern Observatory (ESO) article.