Researchers Observe Possible ‘Superkilonova’ of Dead Stars That 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 heavier 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. These 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.

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,” said California Institute of Technology’s (Caltech) Mansi Kasliwal, professor of astronomy and director of Caltech’s Palomar. “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-a-kind superkilonova, or a kilonova spurred by a supernova. Such an event has been hypothesized but never seen.

Columbia Professor Brian Metzger was brought in as a “theory consultant” on Kasliwal’s group’s observational work to help them interpret what they had seen. In two recent works, Metzger and his co-authors, including Columbia Professor Lam Hui, had shown how it might be possible to create a “superkilonova.” They also predicted that its creation would be coincident with a supernova explosion of a massive star, which is what Kasliwal and her team observed.

Evidence for the possible rare “superkilonova” event 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.

“No neutron star had ever been observed before with a mass less than that of the Sun, and it was believed to be theoretically impossible,” Metzger said. “LIGO seems to have detected merger of two neutron stars, one of which had a mass less than a Sun.” 

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 high confident as some of our alerts, this quickly got our attention as a potentially very intriguing event candidate,” said David Reitze, the executive director of LIGO and a research professor at Caltech. “We are continuing to analyze the data.”

With LIGO and Virgo having detected at least one sub-solar neutron star, it is possible, according to theories proposed by Metzger, that two newly formed neutron stars could have crashed into each other, 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 telescopes observed it doing in August. 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 said.  “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 said. “We do not know with certainty that we found a superkilonova, but the event nevertheless is eye opening.”

This article was adapted from a press release by California Institute of Technology. More information is available on their website.

The artist's animation above shows a hypothesized event known as a superkilonova. Researchers may, for the first time, have observed such an event. Credit: Caltech/K. Miller and R. Hurt (IPAC)