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The possible cosmic explosion received the designation AT2025ulz and drew attention for starting similar to a kilonova, changing behavior in a few days, and raising the hypothesis of a “superkilonova,” a phenomenon in which a collision of neutron stars would be hidden inside a supernova.
Astronomers may have identified a **cosmic explosion** of a type not yet observed, after detecting an event that started similar to a kilonova and, a few days later, began to behave like a supernova. The phenomenon, called **AT2025ulz**, may indicate a “superkilonova,” a hypothesis in which a collision of neutron stars occurs within a supernova.
The discovery involves gravitational wave signals, telescope observations, and an unexpected change in the object’s brightness. The possibility has not yet been confirmed, but the case led researchers to consider that future kilonovae may not resemble the classic event recorded in 2017.
**Cosmic explosion** started similar to a kilonova
The first signal of the event appeared on **August 18, 2025**, when detectors from **LIGO**, in Louisiana and Washington, and **Virgo**, in Italy, registered a gravitational wave signal. Minutes later, an alert was sent to astronomers around the world, indicating that the signal likely came from the merger of two objects.
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The alert also indicated that at least one of the involved objects appeared to have an unusually small mass. Hours later, the **Zwicky Transient Facility**, at the **Palomar Observatory**, identified a fading red source about **1.3 billion light-years** away, in the same region of the sky associated with the gravitational signal.
The object was initially named **ZTF 25abjmnps** and later received the official designation **AT2025ulz**. For approximately three days, the emission resembled the kilonova observed in 2017, when two neutron stars merged and produced gravitational waves and light.
Signal changed color and confused researchers
The first observations showed a rapidly fading red object. This behavior resembled the 2017 kilonova, in which the red color was linked to heavy elements, such as gold, which absorb blue light and allow reddish wavelengths to pass through.
The cosmic explosion, however, changed behavior a few days after the initial flash. The object became brighter again, started emitting bluer light, and showed hydrogen in its spectra, characteristics associated with a stripped-envelope core-collapse supernova.
This turn led some astronomers to consider that the event could be just a common supernova unrelated to the gravitational wave signal. Supernovae in distant galaxies typically do not produce detectable gravitational waves, which increased the difficulty of interpretation.
Hypothesis points to a “superkilonova”
The team led by **Mansi Kasliwal**, professor of astronomy at **Caltech** and director of the **Palomar Observatory**, identified signals that did not fully fit either a classic kilonova or a typical supernova. The study with the results was published in **The Astrophysical Journal Letters**.
The hypothesis presented is that the **cosmic explosion** represents a “superkilonova,” a kilonova triggered by a supernova. The idea had already been proposed by scientists but had never been observed.
In this scenario, a massive star would have exploded and created two newly formed neutron stars. These stars would have spiraled toward each other and merged rapidly, producing a kilonova within the supernova remnants.
Small neutron stars enter the debate
Neutron stars are dense remnants left after the explosion of massive stars. They are approximately the size of **San Francisco**, about **25 kilometers** in diameter, and typically have masses between **1.2 and three times** the mass of the Sun.
Gravitational wave data suggested that at least one of the colliding objects had a mass smaller than that of the Sun. Some theories admit the existence of even smaller neutron stars, but none have been directly observed.
Scientists have proposed two paths for the formation of these small neutron stars. In one, a rapidly rotating massive star explodes and splits into two smaller stars by fission; in another, the explosion creates a disk of material around the collapsing core, and clumps in this disk form a small neutron star.
More events will be needed to confirm discovery
The explanation still requires new evidence. Researchers emphasize that there is not yet enough data to confirm that AT2025ulz is truly a “superkilonova,” despite the event having attracted attention due to its unusual behavior.
New searches may use data from ZTF, the Vera Rubin Observatory, and future or developing projects. The list includes NASA’s Nancy Roman Space Telescope, UVEX, led by Fiona Harrison of Caltech, the Deep Synoptic Array-2000, and Cryoscope, in Antarctica.
The investigation also reinforces the importance of observing events that could be mistaken for common supernovae. If new similar occurrences are identified, the possible cosmic explosion AT2025ulz could become the first concrete clue of an unprecedented class of stellar cataclysm.
With information from ScienceDaily
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