It’s a kilonova! It’s a supernova! It’s a superkilonova?

Carnegie Mellon Researchers Investigate a Cosmic Oddity

Astronomers thought they knew what to expect from a kilonova — a rare cosmic explosion sparked by the collision of two dead stars. But when a faint ripple in spacetime reached Earth on Aug. 18, 2025, it set off a chain of observations that revealed something far stranger. The event, AT2025ulz, did not resemble the classic kilonova nor did it look like an average supernova, even though it had attributes of both. 

Researchers from Carnegie Mellon University, Caltech, Ludwig Maximilian University of Munich and other institutions are working to understand what caused this unusual event. One tantalizing possibility is a never-before-seen phenomenon known as a superkilonova, or a kilonova spurred by a supernova. 

A number of cosmic events can generate ripples in spacetime called gravitational waves. They are emitted when two compact objects crash into each other, and they are the remnants left over from a supernova explosion. In August, after gravitational-wave detectors picked up a signal of a possible cosmic collision, Caltech’s Zwicky Transient Facility (ZTF) pinpointed a rapidly fading red object that appears to have originated in the same location as the source of the gravitational waves. It looked like it could be a kilonova. 

Carnegie Mellon graduate student Xander Hall was meeting with their Caltech collaborators when the ZTF discovery was made. Hall quickly arranged for the Fraunhofer telescope at Wendelstein Observatory in Germany to take a look. They also coordinated with their advisor, CMU Physics Assistant Professor Antonella Palmese, who launched additional follow-up observations with several telescopes, including the Gemini North telescope in Hawaii. 

“We vigorously followed up the initial discovery and accumulated a large amount of data for this event,” said Palmese, a member of Carnegie Mellon’s McWilliams Center for Cosmology and Astrophysics. 

The result was an extensive multiwavelength campaign that spanned X-ray to radio observations. The Carnegie Mellon team’s findings are described in papers published in and submitted to The Astrophysical Journal Letters. 

Their observations confirmed that the eruption of light had faded fast and glowed at increasingly redder wavelengths, similarly to another event, GW170817 — the only confirmed multimessenger kilonova — did in 2017. Then, days after the Aug. 18 blast, AT2025ulz started to brighten again, turn bluer and show hydrogen in its spectra, all signs of a supernova not a kilonova. 

“This event emphasizes the difficulty in identifying electromagnetic counterparts to the gravitational waves produced by binary neutron star mergers,” explained Hall, a second-year graduate student in the Department of Physics. “It takes significant observing time to distinguish kilonovae from impostors.” 

Using data gathered over several days at the Wendelstein Observatory, the CMU group and their collaborators Malte Busmann and Daniel Gruen, a graduate student and professor, respectively, at Ludwig Maximilian University of Munich, determined that AT2025ulz initially appeared to be a kilonova because it had the telltale signature of a binary neutron star merger, but days later developed spectral features characteristic of a Type IIb supernova. 

Carnegie Mellon’s Palmese and Hall also collected and analyzed data using the optical/infrared Gemini North telescope. Their analysis was consistent with the initial Wendelstein findings. They also conducted a deep X-ray and radio wave search, which indicated that the transient was a stripped-envelope supernova. 

In addition, the CMU team, including McWilliams Fellow Ignacio Magaña Hernandez, analyzed data obtained by the Dark Energy Spectroscopic Instrument (DESI) in Arizona to look more closely at AT2025ulz’s host galaxy. While the host galaxy properties are consistent with both those from similar supernovae and possible neutron star mergers, they found that the 3D location of the AT2025ulz host galaxy is consistent with that of the gravitational wave alert. These statistical calculations were necessary but not sufficient conditions to claim an association with the gravitational wave alert. They needed a physical model to explain the transient. 

Could a supernova spark a kilonova? 

In a companion study published in The Astrophysical Journal Letters, Caltech astronomer Mansi Kasliwal and colleagues, including Carnegie Mellon’s Hall and Palmese, suggest that AT2025ulz may represent the first example of a superkilonova — a kilonova triggered by a supernova. This type of event has been theorized but never observed. 

According to models developed by co-author Brian Metzger of Columbia University, the process could unfold like this: a massive star explodes in a supernova. In the aftermath, two neutron stars are born, at least one of which is believed to be less massive than our Sun. The neutron stars spiral together and collide, unleashing a kilonova. The initial supernova would have masked the kilonova’s signature, creating a hybrid event unlike anything astronomers have seen before. 

The theory is tantalizing and interesting to consider, but the research team emphasized that there is not enough evidence to confirm their theory. 

The only way to test the superkilonova theory is to find more. 

A clue from this event is that one of the two objects that collided had a mass that’s less than the Sun’s. According to Palmese, no one really knows how these sub-solar objects are formed so astronomers don’t exactly know how to look for them in the vast cosmos. 

“When we're looking for an electromagnetic transient involving sub-solar mass compact objects, we probably should not be looking for something that is the same as a standard neutron star merger, because the emission could be different,” Palmese said. 

Another sub-solar event detected just two months later underscores the need for a framework to interpret these unusual mergers. “If superkilonovae are real, we’ll eventually see more of them,” Palmese said. “And if we keep finding associations like this, then maybe this was the first.”