Mysterious deep space radio signal detected in 2025 has no reasonable explanation

NASA's Fermi Gamma-ray Space Telescope spotted an unusually long gamma-ray burst (GRB) on July 2, 2025. It was unlike anything ever since, even though GRBs are quite common. This one lasted seven hours on and off, releasing bursts every few seconds. Scientists were excited to see what was going on and so started observations using on-ground telescopes. They wanted to know its origin and why it was behaving so differently from most GRBs. They concluded that it was the longest gamma-ray burst ever recorded and likely came from an explosion that no one had recorded. Named GRB 250702B, the event travelled at least 99 per cent of the speed of light, sending cosmic material in our direction. The observation was published in The Astrophysical Journal Letters on November 26.

Figuring out what was going on in this distant point in space wasn't easy. The researchers turned to the most advanced telescope on Earth, including the Gemini telescopes in Chile and Hawaii, the Very Large Telescope in Chile, the Keck Observatory in Hawaii, and the Hubble Space Telescope. The GRB was observed in various wavelengths of light to learn about its true nature. Various analyses showed that GRB 250702B originated 8 billion light-years away in a dusty galaxy. This is astounding since the radio signal had to pierce through a thick blanket of material that blocks all visible light to reach our solar system.

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Where did the radio signal originate?

The telescopes could only detect infrared and high-energy X-ray wavelengths because of the cloud of dust, and the signal was invisible in ordinary light. The cause of the GRB remains a mystery, as the researchers have a few possible explanations, but are unsure which one is correct. "This was the longest gamma-ray burst that humans have observed — long enough that it does not fit into any of our existing models for what causes gamma-ray bursts," Jonathan Carney, lead author of the study, said in a statement.

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The scientists presented three possible events: the death of a massive star, a black hole gorging on a star, or the merger of a helium star and a black hole, which causes an explosion when the black hole moves into the core of the massive star. "But we can't yet tell which explanation is correct," Carney said, adding that GRB 250702B will now act as the primary event to which similar future occurrences will be compared.