Astronomers have made a groundbreaking discovery by spotting a cosmic explosion of high-energy gamma-rays, designated as GRB 250702B, that defies all previous observations. This gamma-ray burst stands out from typical bursts due to its unique characteristic of exploding multiple times within a single day. Such behavior is particularly perplexing, as gamma-ray bursts (GRBs) are generally believed to result from the catastrophic deaths of massive stars, with no known scenarios explaining repeated explosions over an extended period.
Co-lead researcher and astronomer from University College Dublin, Antonio Martin-Carrillo, emphasized the rarity of this event, stating that GRB 250702B is unlike any other gamma-ray burst observed in the past 50 years. Initially detected by NASA's Fermi Gamma-ray Space Telescope on July 2, 2025, the location of this gamma-ray burst was initially uncertain. However, the subsequent investigation by the Very Large Telescope (VLT) the following day utilized its HAWK-I infrared camera to accurately pinpoint the source of GRB 250702B, confirming it was located outside the Milky Way, a finding later validated by the Hubble Space Telescope.
Gamma-ray bursts are believed to occur during the final moments of massive stars, which may collapse into black holes or neutron stars. Alternatively, they can result from a star being torn apart by a black hole in a phenomenon known as a tidal disruption event. These cosmic explosions are among the most energetic events in the universe, releasing energy equivalent to what the Sun will emit over approximately 10 billion years in a matter of milliseconds to minutes. In stark contrast, GRB 250702B lasted around a day, making it 100 to 1,000 times longer than typical gamma-ray bursts, according to Andrew Levan, co-team leader and researcher from Radboud University.
Typically, gamma-ray bursts are single catastrophic events, and Martin-Carrillo noted that the repeated nature of GRB 250702B is unprecedented. When Fermi first observed GRB 250702B, it detected the burst occurring three times within a few hours. Further data from the Einstein Probe X-ray Space Telescope revealed that the same source had erupted a day earlier, classifying GRB 250702B as a long-period repeating gamma-ray burst unlike anything previously documented.
Although the Fermi and Einstein Probe were unable to pinpoint the exact source of GRB 250702B, initial observations suggested it originated from the Milky Way. However, the VLT's findings drastically changed this perspective; observations using the HAWK-I camera showed that GRB 250702B actually erupted from another galaxy, well beyond the confines of our own. While the precise distance remains uncertain, the team believes the size and brightness of the host galaxy suggest it is located billions of light-years away. Martin-Carrillo stated, "What we found was considerably more exciting: the fact that this object is extragalactic means that it is considerably more powerful."
To deepen our understanding of GRB 250702B, further investigation is crucial to accurately determine its location and the cause behind this long-lasting repeating gamma-ray burst. One possibility includes the collapse of a massive star, which typically results in a gamma-ray burst lasting only seconds. Alternatively, a tidal disruption event could produce a day-long GRB, but this theory does not fully align with the unique characteristics of GRB 250702B, suggesting an extraordinary star is being destroyed by an unusual black hole.
The research team is actively monitoring the site of this remarkable explosion using the VLT and the James Webb Space Telescope, hoping to capture the aftermath and gain further insights into its nature. Martin-Carrillo concluded, "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."
