A puzzling cosmic blast detected in both light and gravitational waves may hint at a previously unseen type of explosion, challenging astronomers to rethink how neutron stars are born and collide. Credit: Shutterstock

A double explosion may have generated both gravitational waves and light.

When the largest stars exhaust their fuel, they end their lives in powerful supernova explosions. These blasts scatter heavy elements such as carbon and iron into space, helping enrich the universe. A different and far rarer kind of cosmic explosion, known as a kilonova, happens when two neutron stars collide.

Neutron stars are the dense remnants of dead stars, and when they merge, they can create even heavier elements, including gold and uranium. Materials like these later become part of new stars, planets, and other cosmic structures.

To date, astronomers have confirmed only one clear example of a kilonova. This landmark event, called GW170817, was observed in 2017. It involved the merger of two neutron stars and produced both gravitational waves, which are ripples in space-time, and visible light that traveled across the universe.

The gravitational-wave signal was recorded by the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) along with its European counterpart, the Virgo gravitational-wave detector. At the same time, dozens of telescopes on Earth and in space captured the burst of light from the explosion.

A Possible Second Kilonova, With a Twist

Astronomers are now examining evidence that could point to a second kilonova, though the interpretation remains uncertain. The newly identified candidate, known as AT2025ulz, presents an unusual challenge. Researchers believe it may be linked to a supernova that erupted just hours earlier, a sequence of events that likely interfered with and partially concealed the view of the suspected kilonova.

“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.”

This artist’s concepts shows a hypothesized event known as a superkilonova. A massive star explodes in a supernova (left), which generates elements like carbon and iron. In the aftermath, two neutron stars are born (middle), at least one of which is believed to be less massive than our Sun. The neutron stars spiral together, sending gravitational waves rippling through the cosmos, before merging in a dramatic kilonova (right). Kilonovae seed the universe with the heaviest elements, such as gold at platinum, which glow with red light. Credit: Caltech/K. Miller and R. Hurt (IPAC)

Kasliwal is lead author of a new study describing the findings in The Astrophysical Journal Letters. In the report, she and her colleagues describe evidence that this oddball event may be a first-of-its-kind superkilonova, or a kilonova spurred by a supernova. Such an event has been hypothesized but never seen.

A Signal From Space-Time

Evidence for the possible rarity first came on August 18, 2025, when the twin detectors of LIGO in Louisiana and Washington, as well as Virgo in Italy, picked up a new gravitational-wave signal. Within minutes, the team that operates the gravitational-wave detectors (an international collaboration that also includes the organization that runs the KAGRA detector in Japan) sent an alert to the astronomical community letting them know that gravitational waves had been registered from what appeared to be a merger between two objects, with at least one of them being unusually tiny. The alert included a rough map of the source’s location.

“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), a survey camera at Palomar Observatory, was the first to pinpoint a rapidly fading red object 1.3 billion light-years away, which is thought to have originated in the same location as the source of gravitational waves. The event, initially called ZTF 25abjmnps, was later renamed AT2025ulz by the International Astronomical Union Transient Name Server.

This artist’s animation shows a hypothesized event known as a superkilonova. A massive star explodes in a supernova, collapsing into a stellar core that forms two neutron stars. The neutron stars spiral together and merge, sending gravitational waves rippling through the cosmos and seeding the universe with heavy elements, such as gold and platinum. Credit: Caltech/K. Miller and R. Hurt (IPAC)

About a dozen other telescopes set their sights on the target to learn more, including the W. M. Keck Observatory in Hawaiʻi, the Fraunhofer telescope at the Wendelstein Observatory in Germany, and a suite of telescopes around the world that were previously part of the GROWTH (Global Relay of Observatories Watching Transients Happen) program, led by Kasliwal.

A Familiar Glow, Then a Surprise

The observations confirmed that the eruption of light had faded fast and glowed at red wavelengths—just as GW170817 had done eight years earlier. In the case of the GW170817 kilonova, the red colors came from heavy elements like gold; these atoms have more electron energy levels than lighter elements, so they block blue light but let red light pass through.

Then, days after the blast, AT2025ulz started to brighten again, turn blue, and show hydrogen in its spectra—all signs of a supernova not a kilonova (specifically a “stripped-envelope core-collapse” supernova). Supernovae from distant galaxies are generally not expected to generate enough gravitational waves to be detectable by LIGO and Virgo, whereas kilonovae are. This led some astronomers to conclude that AT2025ulz was triggered by a typical ho-hum supernova and not, in fact, related to the gravitational-wave signal.

What Might Be Going On?

Kasliwal says that several clues tipped her off that something unusual had taken place. Though AT2025ulz did not resemble the classic kilonova GW170817, it also did not look like an average supernova. Additionally, the LIGO–Virgo gravitational-wave data had revealed that at least one of the neutron stars in the merger was less massive than our Sun, a hint that one or two small neutron stars might have merged to produce a kilonova.

Neutron stars are the leftover remains of massive stars that explode as supernovae. They are thought to be around the size of San Francisco (about 25 kilometers across) with masses that range from 1.2 to about three times that of our Sun. Some theorists have proposed ways in which neutron stars might be even smaller, with masses less than the Sun’s, but none have been observed so far. The theorists invoke two scenarios to explain how a neutron could be that small. In one, a rapidly spinning massive star goes supernova, then splits into two tiny, sub-solar neutron stars in a process called fission.

In the second scenario, called fragmentation, the rapidly spinning star again goes supernova, but, this time, a disk of material forms around the collapsing star. The lumpy disk material coalesces into a tiny neutron in a manner similar to how planets form.

A Supernova Giving Birth to a Kilonova

With LIGO and Virgo having detected at least one sub-solar neutron star, it is possible, according to theories proposed by co-author Brian Metzger of Columbia University, that two newly formed neutron stars could have spiraled together and crashed, erupting as a kilonova that sent gravitational waves rippling through the cosmos. As the kilonova churned out heavy metals, it would have initially glowed in red light as ZTF and other telescopes observed. The expanding debris from the initial supernova blast would have obscured the astronomers’ view of the kilonova.

In other words, a supernova may have birthed twin baby neutron stars that then merged to make a kilonova.

“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.”

But while this theory is tantalizing and interesting to consider, the research team stresses that there is not enough evidence to make firm claims.

The only way to test the superkilonovae theory is to find more. “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.”

Reference: “ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger” by Mansi M. Kasliwal, Tomás Ahumada, Robert Stein, Viraj Karambelkar, Xander J. Hall, Avinash Singh, Christoffer Fremling, Brian D. Metzger, Mattia Bulla, Vishwajeet Swain, Sarah Antier, Marion Pillas, Malte Busmann, James Freeburn, Sergey Karpov, Aleksandra Bochenek, Brendan O’Connor, Daniel A. Perley, Dalya Akl, Shreya Anand, Andrew Toivonen, Sam Rose, Theophile Jegou du Laz, Chang Liu, Kaustav Das, Sushant Sharma Chaudhary, Tyler Barna, Aditya Pawan Saikia, Igor Andreoni, Eric C. Bellm, Varun Bhalerao, S. Bradley Cenko, Michael W. Coughlin, Daniel Gruen, Daniel Kasen, Adam A. Miller, Samaya Nissanke, Antonella Palmese, Jesper Sollerman, Niharika Sravan, G.C. Anupama, Smaranika Banerjee, Sudhanshu Barway, Joshua S. Bloom, Tomás Cabrera, Tracy Chen, Chris Copperwheat, Alessandra Corsi, Richard Dekany, Nicholas Earley, Matthew Graham, Patrice Hello, George Helou, Lei Hu, Yves Kini, Ashish Mahabal, Frank Masci, Tanishk Mohan, Natalya Pletskova, Josiah Purdum, Yu-Jing Qin, Nabeel Rehemtulla, Anirudh Salgundi and Yuankun Wang, 15 December 2025, The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ae2000

The paper was funded by 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.

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