Possible "Superkilonova" Exploded Not Once But Twice

When the most massive stars reach the ends of their lives, they blow up in spectacular supernova explosions, which seed the universe with heavy elements such as carbon and iron. Another type of explosion—the kilonova—occurs when a pair of dense dead stars, called neutron stars, smash together, forging even heavier elements such as gold and uranium. Such heavy elements are among the basic building blocks of stars and planets.

So far, only one kilonova has been unambiguously confirmed to date, a historic event known as GW170817, which took place in 2017. In that case, two neutron stars smashed together, sending ripples in space-time, known as gravitational waves, as well as light waves across the cosmos. The cosmic blast was detected in gravitational waves by the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and its European partner, the Virgo gravitational-wave detector, and in light waves by dozens of ground-based and space telescopes around the world.

Now, astronomers are reporting evidence for a possible second kilonova event, but the case is not closed. In fact, this situation is much more complex because the candidate kilonova, named AT2025ulz, is thought to have stemmed from a supernova blast that went off hours before, ultimately obscuring astronomers' view.

"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 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.

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.

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.

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 star 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 star in a manner similar to how planets form.

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."

The paper, titled "ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger," 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. 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 funded by the NSF and an international collaboration of partners. Additional support comes from the Heising-Simons Foundation and from Caltech. ZTF data are processed and archived by IPAC, an astronomy center at Caltech.