AT2025ulz is a puzzling transient about 1.3 billion light-years away that combined gravitational waves with evolving light signatures. LIGO and Virgo detected a merger-like signal on 18 August 2025, and the Zwicky Transient Facility found a rapidly fading red flash hours later — consistent with a kilonova. Days after, the source brightened, turned blue, and showed hydrogen lines typical of a supernova, prompting a hybrid hypothesis: a supernova fragmented a star into two neutron stars that later merged inside the debris. Researchers caution the evidence is suggestive but not yet definitive.
Astronomers May Have Spotted a Star Shatter and Rejoin — A Possible 'Superkilonova' Discovery
Credit: Caltech/K. Miller and R. Hurt (IPAC)
Astronomers report a puzzling and potentially revolutionary transient event, AT2025ulz, that may represent a new class of cosmic explosion: a "superkilonova." The observations combine gravitational-wave detections with unusual, rapidly evolving light signatures that at first appeared to conflict — and then suggested a remarkable two-stage sequence of events.
Mixed Signals From Gravitational Waves and Light
The source, located about 1.3 billion light-years away, triggered strong gravitational-wave alerts on 18 August 2025 from the LIGO detectors in the United States and Virgo in Europe. The waveforms were consistent with a merger of compact objects such as neutron stars. Hours later, the Zwicky Transient Facility (ZTF) in California identified a rapidly fading red optical transient at the same location — a color and fading pattern expected from a kilonova, where merging neutron stars synthesize heavy elements and emit redder light.
An Unexpected Turn
Several days after the initial detection the object brightened, its emission shifted toward the blue, and optical spectra began to show hydrogen lines — features normally associated with supernova explosions. This combination of a short-lived red phase followed by a blue, hydrogen-rich phase challenged simple interpretations as either a standard kilonova or a normal supernova.
How a 'Superkilonova' Could Explain the Data
To reconcile the conflicting signals, the research team proposes a hybrid scenario. In this model, an initial supernova partially disrupted the progenitor star and produced two smaller neutron-star remnants (rather than a single compact core). Those two neutron stars, embedded in the expanding supernova debris, later spiraled together and merged, creating a second explosion — a kilonova — whose early optical signature was obscured by surrounding material. Analysis of the gravitational-wave data reportedly indicates at least one merging object had a mass below one solar mass, a result that would be unusual and consistent with an unconventional formation pathway.
"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," said lead author Mansi Kasliwa of Caltech.
While the term "superkilonova" is not yet an established category, the proposed scenario is intriguing because it could reveal previously unseen pathways for stellar death and element production. The authors stress the findings are suggestive but not conclusive; further observations and modeling are needed to test whether AT2025ulz truly represents a split-and-recombine event.
Why This Matters
If confirmed, a superkilonova would expand our understanding of how heavy elements form, how compact-object binaries can arise, and how complex interactions between explosions and mergers shape transient signals. The event highlights the growing power of multi-messenger astronomy — combining gravitational waves and electromagnetic observations — to reveal surprising astrophysical phenomena.
Next steps: continuing monitoring of the source, reanalysis of the gravitational-wave data, and targeted theoretical work will be crucial to confirm or refute the superkilonova interpretation.
