Caltech
A double explosion may have produced gravitational waves and light
So far, only one kilonova has been unequivocally confirmed. In this case, the kilonova candidate, named AT2025ulz, was the result of a supernova explosion that occurred hours earlier.
When the most massive stars reach the end of their lives, they explode in spectacular supernovaewhich seed the Universe with heavy elements like carbon and iron.
There is another type of explosion — the kilonova — which occurs when a pair of very dense dead stars, called neutron stars, collide, forging even heavier elements, such as gold and uranium. These heavy elements are among the fundamental building blocks of stars and planets.
Until now, only one kilonova has been confirmed unequivocally: the historic event known as , which occurred in 2017. In this case, two neutron stars collided, sending ripples in spacetimeknown as gravitational wavesas well as light waves through the cosmos.
Now, astronomers report evidence of a possible second kilonova event — although the case is not yet closed. In fact, the situation is much more complex, because it is thought that the kilonova candidate, named AT2025ulzwill result from a supernova explosion that occurred hours beforewhich ended up obscuring the observation by astronomers.
“At first, for about three days, the eruption looked exactly like the first kilonova of 2017”, says the astronomer Mansi Kasliwalprofessor of astronomy and director of Caltech’s Palomar Observatory in .
“Everyone was trying to observe and analyze it intensively, but then it started to look more like a supernova and some astronomers lost interest. We didn’t”, adds the researcher.
Kasliwal is the lead author of one describing the results, recently published in The Astrophysical Journal Letters. In the article, the astronomer and her colleagues present evidence that this unusual event could be a unprecedented “superkilonova” — that is, a kilonova triggered by a supernova.
A phenomenon of this type had already been proposed in theory, but never observed.
The first signs of the possible rarity appeared on August 18, 2025, when LIGO’s two detectors in Louisiana and Washington, as well as Virgo in Italy, recorded a new signal.
Within minutes, the team operating the gravitational wave detectors, an international collaboration that also includes the organization responsible for the KAGRA detector in Japan, sent an alert to the astronomical communityreporting that gravitational waves had been recorded from what appeared to be the merger of two objects — at least one of them unusually small.
The alert included a approximate map of source location.
“Although it doesn’t have the same level of confidence as some of our alerts, this quickly caught our attention as a potentially very intriguing candidate,” he says. David Reitzeexecutive director of LIGO and research professor at Caltech. “We continue to analyze the data, and it is clear that at least one of the colliding objects has less mass than a neutron star typical.”
A few hours later, the Zwicky Transient Facility (ZTF), a tracking camera installed at the Palomar Observatory, was the first to spot a rapidly fading red object 1.3 billion light-years away that is thought to have had origin in the same region of the sky than the source of gravitational waves.
The event, initially designated ZTF 25abjmnpswas later renamed AT2025ulz by the International Astronomical Union’s Transient Name Server.
About a dozen other telescopes aimed at the target to collect more data, including the WM Keck Observatory in Hawaii, the Fraunhofer telescope at the Wendelstein Observatory in Germany, and a cluster of telescopes around the world that were previously part of the GROWTH (Global Relay of Observatories Watching Transients Happen) program, led by Kasliwal.
Observations confirmed that the luminous eruption quickly extinguished and glowed in red wavelengths — just as GW170817 had done eight years earlier.
In the case of kilonova GW170817, the red colors came from elements heavy as gold: these atoms have more electronic energy levels than lighter elements, so block blue lightbut they let the red light pass.
Then, days after the explosion, AT2025ulz started to increase in brightnessturning blue and showing hydrogen in its spectra — typical signs of a supernova and not a kilonova (in particular, a core-collapse supernova with “shell removed”).
In general, supernovae in distant galaxies are not expected to generate enough gravitational waves to be detected by LIGO and Virgo, whereas kilonovae can do it.
This led some astronomers to conclude that AT2025ulz was triggered by a banal supernova and that, in reality, it was not related to the gravitational wave signal.
Kasliwal says that several pieces of evidence suggested to him that something unusual had happened. Although AT2025ulz did not resemble the “classic” kilonova GW170817, it also It didn’t look like an ordinary supernova.
Furthermore, LIGO–Virgo gravitational wave data revealed that at least one of the neutron stars in the merger was less massive than the Sun—a hint that one or two small neutron stars could have merged to produce a kilonova.
Neutron stars are the remains left by massive stars that explode like supernovae. They are thought to be approximately the size of San Francisco (about 25 kilometers in diameter) and masses ranging from 1.2 to about three times the mass of the Sun.
Some theorists have proposed mechanisms by which neutron stars could be even smaller, with masses lower than the Sun — but, to date, none were observed.
The only way to test the superkilonova theory is find more examples. “Future kilonova events may not look like GW170817 and could be confused with supernovae,” says Kasliwal.
“We don’t know for sure whether we found a superkilonova, but still, This event opens our eyes”, he concludes.
