Possible ‘superkilonova’ exploded not once but twice
Astronomers at W. M. Keck Observatory in Hawaiʻi have observed AT2025ulz, a possible first-of-its-kind superkilonova that appears to have exploded twice — first as a supernova, then as a kilonova.
Artist interpretation depicts a hypothesized event known as a superkilonova. Initially, a massive star explodes in a supernova, which generates elements like carbon and iron (left). In the aftermath, two neutron stars are born, at least one of which is believed to be less massive than our Sun (middle). The neutron stars spiral together, sending gravitational waves rippling through the cosmos, before merging in a dramatic kilonova (right). Kilonovae seed the universe with the heaviest elements, such as gold at platinum, which glow in red light as depicted in the animation. Credit: Caltech/K. Miller and R. Hurt (IPAC)
The event was first detected on August 18, 2025, when gravitational wave detectors in the United States and Italy recorded a faint signal called S250818k. The Laser Interferometer Gravitational Wave Observatory (LIGO) and its European counterpart Virgo issued an alert indicating a compact-object merger.
Almost simultaneously, the Zwicky Transient Facility at Palomar Observatory captured a rapidly changing light source in the same part of the sky, approximately 399 megaparsecs away, or about 1.3 billion light-years. The optical brightness rose quickly and faded within days, displaying the red colors typical of kilonovae.
Kilonovae are explosions created when two neutron stars collide, ejecting heavy, neutron-rich material that produces elements such as gold and uranium. Their light glows red because the newly forged heavy atoms absorb blue wavelengths.
For about three days, the light from AT2025ulz looked strikingly similar to the 2017 kilonova known as GW170817. That earlier event confirmed that colliding neutron stars could emit both gravitational waves and visible light, linking two forms of cosmic messengers for the first time. But AT2025ulz behaved differently after its initial flash, becoming unexpectedly bright again and shifting to blue tones.
The unexpected transformation
Follow-up observations came from the W. M. Keck Observatory in Hawaiʻi using its Low Resolution Imaging Spectrograph through a rapid Target of Opportunity program. The Keck team, coordinated by Caltech astronomer Mansi Kasliwal, obtained spectra confirming that the source faded rapidly, then brightened again several days later.
The later data revealed hydrogen and helium absorption features consistent with a stripped-envelope, core-collapse supernova. Expansion speeds reached about 17 000 km/s (10 600 miles/s), and the light curve developed a second peak typical of a supernova powered by radioactive nickel decay.
This created a puzzle. Supernovae at such distances cannot normally generate gravitational waves detectable by LIGO or Virgo. Yet the optical source appeared in the same region and timeframe as the gravitational-wave alert. The simplest interpretation is that both signals were related, suggesting a hybrid event.
Kasliwal described the phenomenon as initially resembling a kilonova, then transforming into a supernova. The unique dual signature prompted the team to propose a new term for the process: superkilonova.
How a star can die twice
A superkilonova could occur when a massive, rapidly spinning star collapses and splits its core during the explosion. Instead of forming a single neutron star, the core divides into two smaller neutron stars, less massive than the Sun. The pair orbits briefly before merging again.
Another possible mechanism involves disk fragmentation. During the collapse, a rotating accretion disk forms around the core. Instabilities cause parts of the disk to condense into one or more tiny neutron stars. These fragments spiral inward, collide, and generate gravitational waves and a secondary flash of light within hours.
Both processes could explain the gravitational signal followed by a luminous, evolving optical display. Computer simulations indicate that such mergers could produce rapid neutron capture, or r-process nucleosynthesis, generating rare elements while enclosed in supernova debris.
In this case, the first explosion may have produced two sub-solar-mass neutron stars, each smaller than 1 solar mass. They then collided almost immediately, creating the secondary kilonova-like burst seen in the data. Such low-mass neutron stars have never been directly observed, making this event especially valuable for testing stellar evolution models.
Data modeling and supporting evidence
The research team modeled the light using both kilonova and supernova templates. The early phase matched a two-component kilonova with about 0.02 solar masses of fast ejecta and 0.09 solar masses of slower disk-wind material. The later phase matched a Type IIb supernova with approximately 3 solar masses of ejecta and a kinetic energy near 1.6 × 1051 erg.
Spectroscopy showed a redshift of 0.0848, corresponding to a distance of 399 megaparsecs, which equals roughly 1.30 billion light-years. That measurement aligns with the gravitational-wave data. The brightness exceeded GW170817 by about one astronomical magnitude, implying a more energetic process.
Statistical modeling suggested that the chance of a random supernova aligning with a gravitational-wave event at the same time and position was less than five percent. Although not conclusive, this strengthens the hypothesis of a physical link between the two.
The object’s double-peaked light curve and changing color over time fit neither a pure kilonova nor an ordinary supernova. Instead, the data point toward a composite explosion involving both mechanisms.
Theoretical meaning of a superkilonova
In astrophysics, the term superkilonova describes a rare phenomenon where a supernova and kilonova occur almost simultaneously. The concept was theorized several years ago to explain collapsars capable of producing both heavy-element synthesis and gravitational waves.
The 2025 event expands that definition. It suggests that rapidly rotating, massive stars could spawn twin neutron stars that merge quickly after the initial explosion. The result would be a luminous optical display with accompanying gravitational radiation.
If verified, superkilonovae could help explain how certain galaxies accumulate large quantities of heavy elements faster than standard models predict. They might also clarify why some gravitational-wave events lack strong gamma-ray signals yet still produce optical afterglows.
Such findings would extend the reach of multimessenger astronomy, the discipline that unites light, gravitational waves, and particles to study the same phenomena. Each new detection type brings a richer understanding of how the universe creates its heaviest elements.
Challenges and remaining uncertainty
Despite compelling evidence, the classification of AT2025ulz as a true superkilonova remains tentative. The gravitational-wave signal had a high false-alarm probability, meaning it was below the usual detection confidence threshold.
No strong X-ray or radio afterglow has yet been observed, and these emissions are typical in energetic mergers. The optical spectrum also displayed unusual shapes in hydrogen lines, suggesting asymmetry in the ejecta or contamination from the surrounding medium.
Future studies will require infrared spectroscopy from instruments like the James Webb Space Telescope to confirm the presence of heavy r-process elements. Continued radio monitoring could reveal faint jets or outflows hidden beneath the supernova shell.
The Caltech team emphasizes that even without full confirmation, AT2025ulz demonstrates the importance of quick, coordinated observation across the electromagnetic spectrum and gravitational-wave network.
Expanding the frontiers of cosmic observation
The next generation of observatories promises to clarify whether superkilonovae are rare or common. LIGO’s upcoming O5 observing run, combined with the Vera C. Rubin Observatory’s deep surveys, will increase the rate of gravitational-wave and optical transient detections dramatically.
Automated systems will help identify faint, short-lived signals like AT2025ulz within minutes, allowing telescopes to pivot quickly for detailed spectroscopy. With each coordinated response, astronomers can refine models of stellar collapse and heavy-element formation.
If another event of this type is found with a higher-confidence gravitational-wave counterpart, it could confirm that superkilonovae truly exist. Such a discovery would connect stellar death, nuclear physics, and cosmic chemistry in a single chain of evidence.
As Mansi Kasliwal explained, future kilonovae may not resemble those already known, and some may be hidden within supernovae. Astronomers now know to look more closely because the universe occasionally allows a star to die twice.
References:
1 “Superkilonova” A Star So Nice, It Explodes Twice – W.K. Keck Observatory – December 16, 2025
I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.
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