AT2025ulz is a candidate "superkilonova" that may combine a core-collapse supernova with a subsequent kilonova when a collapsing massive star fragments into two neutron stars that then merge. Gravitational-wave detectors LIGO and Virgo recorded a signal on Aug. 18, 2025, and telescopes including ZTF and Keck observed an initially red transient that later rebrightened and showed hydrogen and helium. The team finds a 99% probability that at least one merging component was below one solar mass—a surprising result that supports a rapid-rotation collapse scenario. Confirmation requires more detections from upcoming surveys such as Rubin and NASA's Roman Telescope.
Astronomers May Have Observed the First 'Superkilonova' — A Star That Split and Exploded Twice
An artist's concept illustrates a never-before-seen superkilonova event. . | Credit: Caltech/K. Miller and R. Hurt (IPAC)
Scientists report a candidate for a never-before-seen cosmic hybrid: a massive star that may have briefly fragmented into two neutron stars and then merged, producing two distinct explosions. The transient, labelled AT2025ulz, was first associated with a faint gravitational-wave signal and an unusually evolving optical counterpart that together suggest a combined supernova + kilonova sequence.
How It Was Detected
On Aug. 18, 2025, gravitational-wave instruments operated by the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European partner Virgo recorded a subtle signal consistent with the merger of compact objects. Hours later, the Zwicky Transient Facility (ZTF) at Palomar Observatory discovered a rapidly fading red point of light in the same region of sky.
Why This Event Is Unusual
The early red glow resembled the only confirmed kilonova to date (GW170817 in 2017), with colors consistent with freshly synthesized heavy elements such as gold and platinum. Unexpectedly, the source then rebrightened and its spectrum shifted toward bluer wavelengths. Follow-up observations from about a dozen facilities, including Hawaii's Keck Observatory, revealed clear spectral signatures of hydrogen and helium—features typically associated with a core-collapse supernova rather than a standalone kilonova.
Proposed Explanation: A 'Superkilonova'
To reconcile the sequence, the authors propose that a massive, rapidly rotating star underwent core collapse and exploded as a supernova, but its core fragmented into two smaller neutron stars. Those newborn neutron stars then spiraled together and merged within hours, producing a kilonova inside the expanding supernova ejecta. In this scenario, the first explosion initially masks the kilonova, explaining the unusual light curve and spectral evolution.
“We do not know with certainty that we found a superkilonova, but the event nevertheless is eye opening,” said study lead author Mansi Kasliwal, professor of astronomy at Caltech.
Gravitational-Wave Constraints And Surprising Masses
Analysis of the gravitational-wave data cannot tightly measure each object's mass, but it disfavors scenarios in which both merging objects were heavier than the Sun. The team reports a 99% probability that at least one component had a mass below one solar mass—a surprisingly low value given theoretical expectations that neutron stars usually exceed ~1.2 solar masses. The authors note that such lightweight neutron stars could form if the progenitor collapsed while rotating extremely rapidly, consistent with their proposed model.
Caveats And Next Steps
The researchers emphasize that the overlapping electromagnetic and gravitational-wave signals are complex; it remains possible the detections arose from unrelated events that happened to occur close together in time and position. Definitive confirmation of a new class of "superkilonovae" will require additional examples from next-generation surveys such as the Vera C. Rubin Observatory and NASA's Nancy Grace Roman Space Telescope.
“If superkilonovae are real, we'll eventually see more of them. And if we keep finding associations like this, then maybe this was the first,” said study co-author Antonella Palmese of Carnegie Mellon University.
Findings reported Dec. 15 in The Astrophysical Journal Letters.
