Strange afterglow hints at a superkilonova with two explosions

When big stars die, they explode as supernovae, scattering essential elements like carbon and iron across space.

But there’s an even rarer blast called a kilonova. This happens when two super-dense dead stars, called neutron stars, collide.

That collision makes super-heavy materials such as gold and uranium, which later help form planets, stars, and even life itself.

So far, astronomers have confirmed only one kilonova beyond doubt: GW170817, observed in 2017. That event was a cosmic symphony, gravitational waves rippling through space-time, light flashing across the cosmos, and telescopes worldwide racing to capture it.

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Now, scientists may have stumbled upon a second, but this one is far stranger.

On August 18, 2025, detectors at LIGO in the U.S. and Virgo in Italy picked up a new gravitational-wave signal. Within minutes, astronomers were alerted: two objects had merged, and at least one was unusually tiny.

“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, executive director of LIGO. “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.”

Soon after, the Zwicky Transient Facility (ZTF) at Caltech’s Palomar Observatory spotted a fading red glow 1.3 billion light-years away. The object was christened AT2025ulz. For three days, it looked eerily similar to the 2017 kilonova.

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“At first, for about three days, the eruption looked just like the first kilonova in 2017,” recalls Mansi Kasliwal, Caltech astronomer and lead author of the new study. “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.”

Telescopes worldwide joined the chase. Initially, AT2025ulz glowed red, just like GW170817, whose crimson light came from heavy elements such as gold. But then, unexpectedly, the object brightened again, turned blue, and revealed hydrogen in its spectra. These were unmistakable signs of a supernova, not a kilonova.

This left astronomers puzzled. Supernovae don’t usually produce detectable gravitational waves. Could this be something new?

Kasliwal and her team suspect AT2025ulz may represent the first-ever superkilonova, a kilonova triggered by a supernova.

Neutron stars are usually 1.2 to 3 times the mass of our Sun. Still, the gravitational-wave data hinted at something smaller: sub-solar neutron stars, which theorists have long imagined but never seen.

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“The only way theorists have come up with to birth sub-solar neutron stars is during the collapse of a very rapidly spinning star,” explains Brian Metzger of Columbia University. “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.”

In other words, a supernova may have birthed two tiny neutron stars that quickly spiraled together, colliding in a kilonova hidden beneath the supernova’s glare.

The evidence is tantalizing but not conclusive. Kasliwal emphasizes caution:

“Future kilonovae events may not look like GW170817 and may be mistaken for supernovae. We can explore new possibilities in data from ZTF and the Vera Rubin Observatory, as well as in upcoming projects such as NASA’s Nancy Roman Space Telescope, NASA’s UVEX, Caltech’s Deep Synoptic Array-2000, and Caltech’s Cryoscope in Antarctica. We do not know with certainty that we found a superkilonova, but the event

Nevertheless, it is eye-opening.”

AT2025ulz may be rewriting the rulebook of stellar death. If confirmed, it would mark the first superkilonova ever seen, a hybrid explosion where a supernova gives birth to neutron stars that immediately collide.

Whether this is truly a new class of cosmic fireworks or just a trick of timing, astronomers agree: the universe has once again surprised us.

Journal Reference:

1. Mansi M. Kasliwal, Tomás Ahumada, Robert Stein, Viraj Karambelkar, et al. ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ae2000