Infrared and optical observations from NOIRLab and Gemini observatories indicate a relativistic jet formed in an extremely dense and dusty environment in a massive galaxy
“The ability to quickly adjust the Blanco and Gemini telescopes on short notice is critical for capturing transient events like gamma-ray bursts,” says Jonathan Carney, a graduate student at the University of North Carolina at Chapel Hill. That quote sums up the hunt for GRB 250702B—an unusual gamma-ray burst that broke records: Instead of a flash lasting seconds to minutes, the signal burst repeatedly and continued to emit high-energy radiation for more than seven hours.
Gamma-ray bursts (GRBs) are some of the most extreme explosions in the universe. Most of them fade away very quickly, but on July 2, 2025, NASA’s Fermi Space Telescope detected an unusual signal. After space-based instruments narrowed down the location using X-ray observations, follow-up observations began from around the world. An early clue came from infrared data from ESO’s Very Large Telescope (VLT), which confirmed that the event was outside the Milky Way.
Tracking the "Later Glow"
Karney's team focused on the "afterglow" — the light that gradually faded after the initial gamma-ray burst. The way the brightness changed over time allowed them to learn about the explosion and the environment in which it occurred. To do this, the researchers used three of the world's most powerful ground-based telescopes: the 4-meter Victor M. Blanco Telescope in Chile, and two 8.1-meter Gemini Observatory telescopes — Gemini North in Hawaii and Gemini South in Chile. The trio monitored the event from about 15 hours after the detection to about 18 days later.
The data were collected using several key instruments: the NEWFIRM infrared camera and the 570-megapixel DECam camera, mounted on Blanco, as well as the GMOS spectrographs on Gemini North and Gemini South. The findings were presented in a paper in The Astrophysical Journal Letters (26.11.2025; DOI: 10.3847/2041-8213/ae1d67).
Hiding behind cosmic dust
The analysis showed that the event is almost invisible in visible light. Some of the obscuration is due to interstellar dust in the Milky Way, but most of it comes from the host galaxy, which is particularly dusty. Gemini North, which provided the closest visible-wavelength detection of the galaxy, needed nearly two hours of observations to extract a faint signal that is obscured by layers of dust.
The team combined the measurements with new observations from the Keck I telescope, and publicly available data from the VLT, the Hubble Space Telescope, and X-ray and radio observatories. They then compared the data set with theoretical models to see what scenarios could explain a burst that lasted hours.
What happened there anyway?
According to the analysis, the initial signal was likely created by a very narrow, fast jet of material, striking the material surrounding the source—a relativistic jet. The data depict a dense, dust-rich environment around the burst, and a host galaxy that is particularly massive compared to most known GRB galaxies. The source may be behind a thick “dust lane” in the galaxy, right in the line of sight between us. This could explain why the infrared and long exposure times are required.
Of the 15 or so gamma-ray bursts observed since 1973, only about half a dozen come close to the unusual length of GRB 250702B. However, it doesn’t fit comfortably into any of the familiar categories. So several possible explanations have been proposed: a black hole engulfing a star that has been stripped of its hydrogen and is now almost entirely helium, a “micro” tidal disruption in which a star or substellar body (such as a planet or brown dwarf) is torn apart by a close encounter with a stellar black hole or neutron star, or the tidal disruption of a star by an intermediate-mass black hole—a black hole with a mass between 100 and 100,000 solar masses, which is considered common but difficult to detect.
If the latter scenario is correct, it could be the first time humans have observed a relativistic jet from an intermediate-mass black hole as it “eats” a star. At this point, more observations are needed to be conclusive, but the data collected so far are consistent with the new scenarios. “This is a cosmic archaeology problem,” Karni concludes, “We are reconstructing an event that happened billions of light-years away.”
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One response
If we lived there, my wife would go crazy from all this dust, especially before Passover…
With all due respect, and I'm squirming, is there no way to get a brief explanation in spoken Hebrew? I really couldn't understand what the incident was.