No known scenario is capable of satisfactorily explaining the violent gamma ray burst recently detected outside our galaxy. Its true nature remains a mystery.
Gamma ray bursts (GRBs) rank first among the most energetic events in the universe.
For a few seconds, the sky suddenly illuminates in the most energetic light of the electromagnetic spectrum, gamma rays, capable of damaging human tissues and DNA. Fortunately for life on Earth, these violent explosions occur in galaxies very distant from our own.
Alert for a totally unknown explosion
On July 2, 2025, NASA’s Fermi gamma-ray space telescope warned of an outburst that was repeated several times during the day and which turned out to be completely unknown to date. Given the magnitude of the finding, up to four different observatories, including Hubble, participated in the study of the newly named GRB 250702B. The results have been published in the prestigious magazine The Astrophysical Journal.
Furthermore, it was possible to locate the source of this powerful radiation in a galaxy far from the Milky Way.
But what general characteristics do these true cosmic cataclysms have, so unpredictable for astronomers who observe the universe in its most energetic light?
They come from any direction in the cosmos
GRBs are not located in a specific region of the universe and can occur in any direction and at any time. Unable to predict when and where the next burst will take place, active space telescopes such as NASA’s Fermi, and others in design such as the European Agency’s e-ASTROGRAM, constantly monitor the cosmos in search of these highly energetic events.
In fact, scientists at the Los Alamos National Laboratory in the US were studying data collected on July 2, 1967 by North American satellites specialized in gamma rays (looking for a possible Soviet nuclear explosion in space) when they found something that didn’t fit. The graphs indicated a peak in intensity, a drastic drop, another peak of lower intensity and a final much gentler drop, until fading completely. And all this in a few seconds.
The first burst of gamma rays in space, unrelated to human activity, had been discovered exactly 58 years before the unusual GRB 250702B that now causes confusion among scientists.
Since then, thousands of GRBs have been detected by space observatories in all directions of the cosmos, with NASA’s BATSE experiment being a pioneer in this search for sudden bursts of gamma rays.
Location in the cosmos of each of the 2704 GRBs detected until the year 2000 by the BATSE experiment during its 9-year mission. The plane of the Milky Way is directed on the horizontal axis and each burst is represented with a characteristic hue: the long and intense ones colored in red, and the short and weaker ones, in violet. Credits: NASA/MSFC. NASA, CC BY
They release a huge amount of energy in just a few seconds.
We are talking about the most energetic events in the universe.
To give us an idea of its magnitude, the brightest gamma-ray burst to date, called GRB 221009A, released 1,000 times more energy during its 290 seconds than the Sun did in the last 4.5 billion years.
Thus, in the initial moments, GRBs emit a large amount of energy in the range of gamma rays, the most energetic radiation in the electromagnetic spectrum. Subsequently, the intensity of the burst is attenuated in other wavelengths, progressively passing through X-rays, ultraviolet light, visible light, infrared and radio waves.
Electromagnetic spectrum ranging from the most energetic radiation (gamma rays, shorter wavelength) to radio waves. Credits: NASA. NASA, CC BY
This final part of the burst or afterglow –afterglowin English – can last up to months and is caused by the interaction of the highly energetic beam with the surrounding interstellar gas. The afterglows reveal the extragalactic origin of the GRBs, although gamma ray bursts have been identified without this final stretch of decreasing intensity.
No two gamma ray bursts are identical.
Using data collected by astronomical observatories, researchers are able to study GRBs using characteristic graphs called ‘light curves’. They represent the energy of the burst (vertical axis) as a function of time (horizontal axis).
No two explosions are the same, as can be seen in this succession of light curves. Some are short-lived, others longer, some weak, others more intense, some have more peaks of intensity, others have none, each one different from the other.
Light curves associated with a dozen GRBs recorded by astronomical observatories. No two gamma ray bursts are the same. Credits: JT Bonnell (NASA/GSFC). NASA, CC BY
Associated with the most catastrophic events in the universe
It is difficult to know in detail the causes of these violent explosions, mainly due to the enormous distance that separates us. However, depending on the duration of the GRB, there are different explanations that are quite accepted today.
While those of long duration (greater than 2 seconds) would be associated with the final moments of a dying massive star or supernovae, those of shorter duration or short-GRBs They would have their origin in the merger of two compact objects such as two neutron stars or a black hole and a neutron star.
In this sense, a hypothetical interaction between a primordial black hole of atomic size and a neutron star would generate a unique GRB of very short duration and without afterglow, according to its own publication on the matter.
So what is so unprecedented about the newly discovered gamma ray burst?
The exceptional GRB 250702B
Initially, the Fermi gamma ray observatory sounded the alarm on July 2, 2025, and the Einstein X-ray probe of the Chinese Academy of Sciences and the European Space Agency (ESA) later joined the study of this peculiar event.
After these first observations, the ESA research team used the Very Large Telescope (VLT) array in Chile, in collaboration with the Hubble Space Telescope, to locate the exact position of GRB 250702B and monitor its position for several days. afterglow.
Evolution of the gamma-ray burst GRB 250702B from July 3 to 15, 2025. The captures were taken by the Very Large Telescope in Chile (VLT, in yellow) and the Hubble Space Telescope (HST, in blue). Credits: ESO/A. Levan, A. Martin-Carrillo et al./NASA/ESA. ESA, CC BY
What was really surprising about this phenomenon was the detection of several gamma-ray bursts in the course of a day, something that had never been observed before in any GRB. That is, its duration was exceptionally long and the associated light curve does not have a clear explanation by the researchers.
Light curve from the GRB 250702B gamma ray burst. In three different tones, the three intensity peaks detected in a time interval of more than three hours, something totally unusual in long-duration GRBs. Credits: arxiv.org/pdf/2507.14286. arxiv.org, CC BY
The possible causes of this unusual event
In view of the above, the origin of GRB 250702B is totally unknown.
Although two different scenarios have been proposed to explain its long duration and periodicity, such as a new type of collapse of a dying massive star or the result of a star being torn apart by a black hole (tidal disruption phenomenon, TDE), neither of them can provide a complete explanation for it.
In the words of Antonio Martín-Carrillo, co-author of the study and astronomer at University College Dublin (Ireland), explaining the properties of this explosion would require that an unusual star be destroyed by an even more unusual black hole, probably one of intermediate mass.
Does this mean that the discovery of GRB 250702B would imply the existence of new species of stars and black holes? It is not possible to categorically state the above. What is clear is that the origin of the unusual GRB 250702B still remains uncertain almost six decades after humanity began studying the universe in its most energetic range.
This article was originally published on The Conversation.