Researchers at Caltech are investigating a cosmic mystery that may represent an entirely new type of stellar explosion. Their study in The Astrophysical Journal Letters describes evidence that AT2025ulz began with a supernova and quickly produced a neutron star merger. That merger may have triggered a second eruption known as a kilonova.
The proposed sequence is so unusual that the team calls the event a possible superkilonova. A rapidly spinning massive star may have collapsed and produced two unusually light neutron stars. Within hours, the pair spiraled together and collided. The first blast then caught up with the merger’s light and transformed what telescopes could see.
The event remains a candidate rather than a confirmed new class of explosion. Even so, it could reshape how astronomers search for kilonovae and interpret gravitational-wave alerts. Some neutron star mergers may be concealed inside the brighter debris of dying massive stars.
A gravitational-wave alert begins the chase
The chase began on August 18, 2025, when the twin LIGO detectors in Louisiana and Washington recorded a ripple in spacetime. The Virgo detector in Italy also participated in the observation. The signal appeared consistent with two compact objects moving together and merging.
Within minutes, an international gravitational-wave collaboration sent an alert to astronomers. The notification included a broad region of sky where the source was likely located. Observatories around the world began scanning that region for a burst of visible or infrared light.
A few hours later, the Zwicky Transient Facility at Caltech’s Palomar Observatory found a rapidly fading red object. It lay about 1.3 billion light-years from Earth and appeared to occupy the same region as the gravitational-wave source. The transient was initially cataloged as ZTF25abjmnps and later received the name AT2025ulz.
More than a dozen telescopes turned toward the object. These included the W. M. Keck Observatory in Hawaiʻi and the Fraunhofer telescope at Germany’s Wendelstein Observatory. The global response allowed astronomers to track changes in brightness, color and chemical fingerprints during the event’s first crucial days.
AT2025ulz changes color
During its first three days, AT2025ulz behaved much like the historic GW170817 kilonova observed in 2017. It faded rapidly and emitted a deep red glow. Those traits are expected when neutron-rich debris expands from the collision of two neutron stars.
“At first, for about three days, the eruption looked just like the first kilonova in 2017,” said Mansi Kasliwal, a Caltech professor of astronomy and the study’s lead author.
Kilonova debris can contain freshly forged heavy elements such as gold and platinum. These complex atoms absorb shorter wavelengths of light and allow more red light to escape. The changing glow gives astronomers clues about the composition, temperature and speed of the expanding material.
On roughly the fourth day, the story changed. AT2025ulz began brightening again and shifted toward blue wavelengths. Its spectrum also developed hydrogen emission lines. That combination is associated with a core-collapse supernova whose outer hydrogen layer has largely been stripped away.
The later observations made the transient look increasingly like a supernova. Yet the gravitational-wave signal still pointed toward a compact-object merger in the same area of sky. Connecting those two pieces produced the superkilonova hypothesis.
How one star could explode twice
A supernova begins when a massive star can no longer support itself through nuclear fusion. Its core collapses under gravity and becomes an extremely dense stellar remnant. The outer layers are thrown into space at enormous speed.
In the proposed AT2025ulz scenario, a rapidly rotating stellar core followed a more complicated path. It may have split into two small neutron stars during the collapse. Another possibility involves one compact remnant forming alongside a dense disk that later condensed into a second object.
Either route would place the newborn objects extremely close together. Their motion would release energy as gravitational waves, causing the orbit to shrink rapidly. The pair could then merge within hours of the supernova.
The collision would create a kilonova inside the expanding supernova debris. It would also eject neutron-rich material where some of the heaviest elements in nature can form. For a short period, telescopes could see the red merger glow through or ahead of the surrounding stellar wreckage.
As the supernova developed, its larger cloud of debris would dominate the view. The expanding material could conceal the kilonova and produce the later blue light and hydrogen signatures seen in AT2025ulz. Astronomers would then witness two explosions as one evolving point of light.
The sub-solar neutron star clue
The masses inferred from the gravitational-wave data provide one of the event’s most intriguing clues. At least one colliding object appeared to have less mass than the Sun. That would place it below the usual range measured for known neutron stars.
“We are continuing to analyze the data and it’s clear that at least one of the colliding objects is less massive than a typical neutron star,” said David Reitze, LIGO’s executive director and a research professor at Caltech.
Most measured neutron stars contain more mass than the Sun packed into a sphere roughly the size of a city. Their immense density comes from the collapse of a massive stellar core. A sub-solar neutron star would require an unusual formation history because ordinary stellar collapse generally produces a heavier remnant.
Rapid rotation offers a possible route. A collapsing core spinning at extreme speed might divide into two lower-mass objects. Those remnants would begin life in a tight orbit and could merge almost immediately on astronomical timescales.
“The only way theorists have come up with to birth sub-solar neutron stars is during the collapse of a very rapidly spinning star,” said Brian Metzger, an astrophysicist at Columbia University whose theoretical work helped shape the proposed explanation.
The signal was less confident than some previous LIGO alerts, so its inferred properties require continued analysis. Even so, the possibility of a sub-solar compact object fits the unusual birth mechanism needed for a superkilonova.
Why the evidence remains uncertain
A single event cannot establish a new category of cosmic explosion. AT2025ulz was distant and the gravitational-wave localization covered a broad patch of sky. Its apparent connection to the optical transient could reflect a chance alignment.
The team therefore presents AT2025ulz as a candidate superkilonova. The proposed sequence provides a coherent explanation for the red early light, the later supernova spectrum and the compact merger signal. Each observation also carries uncertainties that leave room for other interpretations.
One possibility involves an ordinary stripped-envelope supernova occurring near the gravitational-wave localization. Under that explanation, the two detections would come from separate sources. Statistical analysis can estimate the likelihood of such a coincidence, though one case cannot eliminate it.
The changing light also presents a challenge. GW170817 supplied astronomers with their clearest template for a kilonova. AT2025ulz evolved differently because any merger light may have traveled through supernova debris. That surrounding material could change the brightness and color in ways that are difficult to reconstruct from a distant observation.
Researchers will need detailed models of the explosion, neutron star formation and expanding ejecta. Those models must reproduce both the gravitational-wave properties and the full optical light curve. They must also explain the timing of the hydrogen features and the transition from red to blue.
How astronomers can test the idea
The strongest test will come from finding more events with the same sequence. Astronomers can search gravitational-wave alerts for optical transients that first resemble kilonovae and later develop the signatures of supernovae. Repeated examples would reveal whether AT2025ulz belongs to a broader population.
“Future kilonovae events may not look like GW170817 and may be mistaken for supernovae,” Kasliwal said. Her warning highlights a practical challenge for time-sensitive astronomy. Researchers may move telescopes away from a promising object when it begins to resemble a familiar stellar explosion.
The Vera C. Rubin Observatory could help by repeatedly surveying large areas of the southern sky. Its wide field and rapid cadence are suited to finding transients soon after they appear. Early measurements are essential because a superkilonova’s kilonova-like phase may last only a few days.
Future observatories could add information at wavelengths that reveal hidden debris. NASA’s Nancy Grace Roman Space Telescope will study the infrared universe with a wide field of view. NASA’s UVEX mission is designed to examine the ultraviolet sky and rapidly changing cosmic sources. Caltech projects such as the Deep Synoptic Array-2000 and Cryoscope could provide further ways to identify or characterize unusual explosions.
Continued work by LIGO, Virgo and KAGRA will remain central to the search. Gravitational waves reveal compact mergers even when surrounding debris obscures their visible light. Coordinating those signals with rapid telescope observations gives astronomers the best chance of catching every stage.
If additional cases appear, multi-messenger astronomy may uncover a hidden population of neutron star mergers born inside supernovae. Such events would connect the death of massive stars with the creation of some of the universe’s heaviest elements. They could also reveal how rapidly spinning cores produce compact objects with masses beyond the familiar range.
