GRB 250702B, the longest gamma-ray burst in history

[Astronuc]

Longest gamma-ray burst confounds astrophysicists, CERN Courier, 14 January 2026
https://cerncourier.com/longest-gamma-ray-burst-confounds-astrophysicists/

On 2 July 2025, NASA’s Fermi Gamma-ray Space Telescope observed a gamma-ray burst (GRB 250702B) of a record seven hours in duration. Intriguingly, high-resolution images from the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) revealed that the burst emerged nearly 1900 light-years from the centre of its host galaxy, near the edge of its disc. But its most unusual feature is that it was seen in X-rays a full day before any gamma rays arrived.

The high-energy transient sky is filled with a cacophony of exotic explosions produced by stellar death. Short GRBs of less than two seconds are produced by the merging of compact objects such as black holes and neutron stars. Longer GRBs are produced by the death of massive stars, with “ultralong” GRBs most often hypothesised to originate in the collapse of massive blue supergiants, as they would allow for accretion onto their central black-hole engines over a period from tens of minutes to hours.

Peculiar observations​

GRB 250702B lasted for at least 25,000 seconds (7 hours), superseding the previous longest GRB 111209A by over 10,000 seconds. However, the duration alone was not enough to identify this event as a different class of GRB or as an extreme outlier. Two other observations immediately marked GRB 250702B as peculiar: the multiple gamma-ray episodes seen by Fermi and other high-energy satellites; and the soft X-rays from 0.5 to 4 keV seen by China’s Einstein Probe over a period extending a full day before gamma rays were detected.

There is little consensus on the origin of GRB 250702B, beyond that it involved an accreting black hole

The National Optical Astronomy Observatory (NOIRLab) has reported on the results of its study of the longest gamma-ray burst in history. Thanks to observations from several telescopes, astronomers were able to obtain important information about its origin.

https://universemagazine.com/en/ast...et-of-the-longest-gamma-ray-burst-in-history/

Gamma-ray bursts are among the most powerful events in the Universe, second only to the Big Bang in terms of energy. Most of them are observed as flashes that fade away within a few seconds or minutes. But on July 2, 2025, astronomers recorded a very unusual gamma-ray burst, designated GRB 250702B. It lasted seven hours, which is currently a record for the entire history of observations.

GRB 250702B was first detected by the Fermi space telescope. Other observatories then joined in the observations. Data collected by ESO’s Very Large Telescope showed that its source was located outside the Milky Way.

Analysis of the data they collected showed that GRB 250702B was not detectable in visible light, partly due to interstellar dust scattered across the Milky Way, but more significantly due to dust within the galaxy where the burst source was located.

Of the approximately 15,000 gamma-ray bursts observed since the phenomenon was first detected in 1973, only half a dozen come close to GRB 250702B in duration. Their presumed sources range from the collapse of a blue supergiant star, tidal disruption of a star, or a newborn magnetar.
. . . .

However, GRB 250702B does not fit into any of the known categories. Based on the data obtained to date, scientists have put forward several hypotheses about possible scenarios for its origin: . . . .

 

[AlexB23]

Longest gamma-ray burst confounds astrophysicists, CERN Courier, 14 January 2026
https://cerncourier.com/longest-gamma-ray-burst-confounds-astrophysicists/



There is little consensus on the origin of GRB 250702B, beyond that it involved an accreting black hole


https://universemagazine.com/en/ast...et-of-the-longest-gamma-ray-burst-in-history/

Wow, 7 hours is longer than some commercial flights across the Atlantic. What would happen to the solar system, Earth's atmosphere, and civilization if this GRB happened say 5,000 light-years away from Earth?

Also, why did the X-rays arrive quicker than the gamma rays? Do high enough energy photons travel slower through space? As far as space is, the surface is smooth and not foamy.

Space is smooth:
https://www.science.org/content/article/einstein-trumps-quantum-gravity

 

[Ibix]

Also, why did the X-rays arrive quicker than the gamma rays?

I don't think the suggestion is that they arrived quicker, but rather they were emitted before the gamma burst. Whatever process produced the gamma rays spent some time beforehand producing x-rays. We don't know why.