Astronomers have identified a cosmic explosion so strange that it could become the first recorded superkilonova in history. This potential discovery combines in a single phenomenon the violence of a supernova and the extreme rarity of a kilonova, and if confirmed, it will force a re-evaluation of how we understand the birth, life, and death of the most massive stars.
The signal, named AT2025ulzDetected in August 2025, this event has sparked intense debate within the international scientific community. Designated AT2025ulz, it immediately triggered an alert for observatories worldwide, urging them to point their telescopes toward the designated region. enigmatic signs from the universe.
What is a superkilonova and why is it so special?
Under normal conditions, the death of a massive star culminates in a supernova: a thermonuclear explosion which strips away the outer layers of the star and leaves behind a compact remnant: a neutron star or, if the mass is very high, a black hole. Supernovae are relatively frequent on a cosmic scale, and astronomers catalog thousands of them every year in different galaxies (such as the famous star Betelgeuse).
In contrast, Kilonovae are much rarerThey occur when two neutron stars—the ultradense cores left over from some supernovae—collide and merge. These collisions generate flashes that are less bright than many supernovae in visible light, but very distinctive in gravitational waves and infrared light, and are candidates for being the main source of heavy elements such as gold, platinum, and uranium.
An A superkilonova would essentially be the combination of both phenomena: a supernova that triggers the formation of two very light neutron stars, which would collide shortly afterward to give rise to a kilonova in the same region of space. All of this in a very short time interval, something that until now had only been considered in theoretical models and computer simulations.
The beauty of it is that this type of event is not only spectacular, but also functions as a natural laboratory for study how are heavy chemical elements synthesized which then end up in rocky planets, in the Earth's crust, or even in our own bodies. Each explosion of this kind is a clue about the origin of the material we are made of.
The AT2025ulz event: from a kilonova to a supernova… or both
On August 18, 2025, the interferometers of LIGO (Laser Interferometer Gravitational-Wave Observatory) They recorded a gravitational wave signal that was remarkably similar to the first confirmed kilonova, observed in 2017. This new event was given the designation AT2025ulz and an alert was immediately issued to observatories around the world to point their telescopes towards the designated area.
The response was swift: cameras sensitive to visible light, infrared, X-rays and radio They began monitoring the region. The first three daysThe brightness observed at red wavelengths, mimicking the 2017 kilonova, fit well with what was expected from the merger of two neutron stars, including the signature of newly forged heavy elements.
However, what came next baffled many teams. Over time, the glow didn't fade like in a conventional kilonova, but instead It intensified and turned blueThis is a signal more typical of supernovae. In addition, signs of hydrogen gas and other characteristics more typical of a classic stellar explosion than a simple neutron star merger began to be detected.
Mansi Kasliwal, head of the Palomar Observatory at the California Institute of Technology (Caltech) and lead author of the study, explained that during the first few days "the eruption looked exactly like the 2017 kilonova," which is why numerous groups pointed their instruments towards AT2025ulz. When the signal began to resemble a supernovaSome teams lost interest, thinking it was just another case of an atypical supernova. Kasliwal's group, however, continued observing because something didn't quite add up.
The gravitational wave data pointed to the fusion of two compact objectsOne of them had an unusually low mass for a neutron star. This detail, combined with the strange evolution of its brightness at different wavelengths, raised all the alarms and opened the door to the superkilonova hypothesis.
How a supernova could split its core into two neutron stars
To explain what was observed in AT2025ulz, the international team of researchers has proposed several theoretical scenarios that have one thing in common: the The original star must have been spinning very fast before exploding as a supernova, similar to models of double detonation in the explosion proposed for some stellar collapses.
In one of the proposed models, after the supernova explosion the collapsed core would undergo a process of gravitational fissionIn the first scenario, the supernova would literally split into two fragments that would stabilize as low-mass neutron stars. In the other scenario, the supernova would initially form a single neutron star surrounded by a dense disk of material; over time, that disk would fragment and give rise to a second neutron star, again with a mass lower than that of the Sun.
Whatever the exact mechanism, in both cases the two newborn neutron stars would be trapped in a spiral of approach due to the emission of gravitational waves, until they collide and produce the kilonova. This sequence—supernova first, kilonova later—would fit with the evolution in color and brightness that has been observed in AT2025ulz.
One of the most striking aspects of the analysis is the presence, inferred from the data, of a neutron star with a mass less than that of the SunUntil now, such an object had never been observed and was considered highly improbable in theory. Theoretical physicist Brian Metzger of Columbia University, a co-author of the study, noted that the detection of a “subsolar” neutron star would pose a serious challenge to current models of stellar structure.
For the astrophysics community, It opens up a range of questionsHow many times can this process occur in the universe? What impact does it have on the production of heavy elements? Could superkilonovae have been mistaken in the past for exotic supernovae or for incompletely observed kilonovae?
A scientific puzzle that is still not fully solved
Despite the strength of the data and the suggestive nature of the superkilonova scenario, researchers insist that it is a hypothesis not yet confirmedIt cannot be completely ruled out that the gravitational wave signal and the explosion seen in light come from two different but close sources in the sky, which would have led to the mistaken association of both phenomena.
Furthermore, neither the existence of neutron stars so light Neither the exact process by which a supernova gives rise to two compact nuclei nor the exact process by which a supernova gives rise to two compact nuclei has been directly validated. These are plausible models supported by numerical simulations, but they require more observational examples to move from hypothesis to certainty.
Kasliwal summarized the situation by pointing out that it cannot yet be definitively stated that AT2025ulz is a superkilonova, but that the event is "revealing" in any case. The fact that it has shown almost overlapping characteristics of both kilonova and supernova is, in itself, a significant piece of information. It forces a review of the classic categories with which stellar explosions were ordered.
The only way to settle the debate will be to detect new similar events in the coming years. With gravitational wave interferometers improving their sensitivity and a global network of telescopes, including projects like the Vera Rubin ObservatoryWith increasing coordination, the astronomical community hopes to locate more candidates that will allow them to verify whether AT2025ulz is an isolated case or the tip of the iceberg of a type of stellar explosion that is more common than previously thought.
In this context, astronomers warn that future kilonovae may not resemble the now-famous GW170817 of 2017. Some could disguise themselves as atypical supernovaeAnd only a detailed analysis combining gravitational waves, visible and infrared light, X-rays and radio waves will allow us to identify them with certainty.
The role of Europe and Spain in the hunt for super-kilonovae
This type of phenomenon is not studied from a single perspective, but rather through a genuine collaboration. global network of observatoriesat space telescopes to ground-based facilities. In Europe, facilities such as Virgo (the gravitational wave interferometer near Pisa) and European Space Agency projects are coordinated with LIGO and telescopes spread across the five continents to track these ephemeral signals that vanish in a matter of days.
Spain plays a significant role within this network. Observatories located in Canary Islands, Sierra Nevada or Calar Alto They provide key optical and infrared observations, especially valuable when a rapid response to a gravitational wave alert is needed. The quality of the sky and extensive experience in monitoring stellar explosions make Spanish teams regular partners in international campaigns.
In addition to direct observations, research groups at universities and national centers participate in the data analysis and the generation of theoretical models They are trying to explain cases like AT2025ulz. Part of the work focuses on understanding how elements like gold or platinum are distributed in the universe, and what fraction of them can be attributed to kilonovae or superkilonovae versus other stellar processes.
This collaborative effort is not limited to academic circles. Initiatives of science communication in Spain and Europe They are using the example of the possible superkilonova to illustrate how "multi-messenger" astronomy works, in which light, gravitational waves and other signals are combined to reconstruct what happened in a very specific fraction of the cosmos millions of years ago.
Although the AT2025ulz phenomenon occurred far from Earth, its study has a direct impact on how European societies conceive of basic science, international cooperation, and the need to maintain cutting-edge scientific infrastructures capable of capturing these exceptional events when the universe decides to offer such a unique spectacle.
Everything points to AT2025ulz will mark a turning point In the study of stellar explosions: whether confirmed as a superkilonova or reinterpreted in the future, it has shown that the sky still holds surprises capable of testing our most established theories and of prompting observatories in Spain, Europe and the rest of the world to look more closely at each new signal that comes from deep space.
