Astronomers may have found a strange new kind of cosmic explosion

So far, scientists have confirmed only one clear example of a kilonova. That event, called GW170817, occurred in 2017 when two neutron stars merged. The collision sent out both gravitational waves and light, allowing researchers to observe it in multiple ways. The gravitational waves were detected by the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and its European partner Virgo, while telescopes around the world captured the light from the explosion.

A New and Puzzling Cosmic Event

Astronomers now believe they may have found evidence of a second kilonova, although the situation is far from straightforward. The candidate event, named AT2025ulz, appears to be tied to a supernova that happened just hours earlier. That earlier explosion may have hidden key details, making the event much harder to interpret.

"At first, for about three days, the eruption looked just like the first kilonova in 2017," says Caltech's Mansi Kasliwal (PhD '11), professor of astronomy and director of Caltech's Palomar Observatory near San Diego. "Everybody was intensely trying to observe and analyze it, but then it started to look more like a supernova, and some astronomers lost interest. Not us."

Kasliwal led a study describing the findings in The Astrophysical Journal Letters. Her team suggests this unusual event could represent something entirely new, a superkilonova, meaning a kilonova triggered by a supernova. While scientists have proposed this idea before, it has never been observed.

Gravitational Waves Point to Something Unusual

The first sign of this rare event appeared on August 18, 2025. LIGO detectors in Louisiana and Washington, along with Virgo in Italy, recorded a new gravitational-wave signal. Within minutes, an alert went out to astronomers worldwide, noting that the signal likely came from two merging objects. At least one of those objects seemed unusually small. The alert also included a rough location in the sky.

"While not as highly confident as some of our alerts, this quickly got our attention as a potentially very intriguing event candidate," says David Reitze, the executive director of LIGO and a research professor at Caltech. "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."

A few hours later, the Zwicky Transient Facility (ZTF) at Palomar Observatory identified a fading red source about 1.3 billion light years away, located in the same region as the gravitational-wave signal. Initially named ZTF 25abjmnps, the object was later given the official designation AT2025ulz.

A Signal That Changed Over Time

Roughly a dozen telescopes around the world quickly began observing the event, including the W. M. Keck Observatory in Hawaiʻi, the Fraunhofer telescope in Germany, and facilities connected to the GROWTH (Global Relay of Observatories Watching Transients Happen) program led by Kasliwal.

Early observations showed the object fading rapidly and glowing red, similar to what was seen in the 2017 kilonova. In that earlier event, the red color came from heavy elements like gold, which absorb blue light and allow red wavelengths to pass through.

However, the behavior of AT2025ulz soon changed. A few days after the initial flash, it brightened again, shifted to bluer light, and showed hydrogen in its spectra. These features are typical of a supernova, specifically a "stripped-envelope core-collapse" supernova, not a kilonova. Because supernovae in distant galaxies usually do not produce detectable gravitational waves, some astronomers concluded that the event was likely an ordinary supernova unrelated to the earlier signal.

Clues Point to a Possible Superkilonova

Kasliwal and her team noticed several signs that the event did not fit neatly into either category. AT2025ulz did not fully match the characteristics of a classic kilonova or a typical supernova. At the same time, gravitational-wave data suggested that at least one of the merging objects had a mass smaller than the Sun, raising the possibility that two unusually small neutron stars were involved.

Neutron stars are the dense remnants left behind after massive stars explode. They are roughly the size of San Francisco (about 25 kilometers across) and typically have masses between 1.2 and three times that of our Sun. Some theories suggest that even smaller neutron stars could exist, but none have been directly observed.

Scientists have proposed two ways such tiny neutron stars might form. In one scenario, a rapidly spinning massive star explodes and splits into two smaller neutron stars through a process called fission. In another, known as fragmentation, the explosion creates a disk of material around the collapsing core, and clumps in that disk eventually form a small neutron star, similar to how planets form.

A Hidden Collision Inside a Supernova

According to co author Brian Metzger of Columbia University, it is possible that two newly formed neutron stars could spiral inward and collide, producing a kilonova that emits gravitational waves. As this happens, the explosion would initially appear red due to the formation of heavy elements, just as telescopes observed. Meanwhile, debris from the earlier supernova could obscure the view, hiding the kilonova within it.

In simple terms, a supernova may have created two newborn neutron stars that quickly merged, producing a second explosion.

"The only way theorists have come up with to birth sub-solar neutron stars is during the collapse of a very rapidly spinning star," Metzger says. "If these 'forbidden' stars pair up and merge by emitting gravitational waves, it is possible that such an event would be accompanied by a supernova rather than be seen as a bare kilonova."

More Evidence Needed

Although this explanation is compelling, the researchers emphasize that it is still uncertain. There is not yet enough evidence to confirm that AT2025ulz is truly a superkilonova.

To test this idea, astronomers will need to identify more events like it. "Future kilonovae events may not look like GW170817 and may be mistaken for supernovae," Kasliwal says. "We can look for new possibilities in data like this from ZTF as well as the Vera Rubin Observatory, and upcoming projects such as NASA's Nancy Roman Space Telescope, NASA's UVEX [led by Caltech's Fiona Harrison], Caltech's Deep Synoptic Array-2000, and Caltech's Cryoscope in the Antarctic. We do not know with certainty that we found a superkilonova, but the event nevertheless is eye opening."

Study Details and Funding

The study, titled "ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger," received funding from the Gordon and Betty Moore Foundation, the Knut and Alice Wallenberg Foundation, the National Science Foundation (NSF), the Simons Foundation, the US Department of Energy, a McWilliams Postdoctoral Fellowship, and the University of Ferrara in Italy. Other Caltech authors include Tomás Ahumada (now at NOIRLab, Chile), Viraj Karambelkar (now at Columbia University), Christoffer Fremling, Sam Rose, Kaustav Das, Tracy Chen, Nicholas Earley, Matthew Graham, George Helou, and Ashish Mahabal.

Caltech's ZTF is supported by the NSF and an international group of partners, with additional funding from the Heising-Simons Foundation and Caltech. ZTF data are processed and archived by IPAC, an astronomy center at Caltech.