An unusual signal initially picked up by the LIGO and Virgo gravitational wave detectors and subsequently by optical telescopes around the world may have been a “superkilonova” – that is, a kilonova that took place inside a supernova. According to a team led by astronomers at the California Institute of Technology (Caltech) in the US, the event in question happened in a galaxy 1.3 billion light years away and may have been the result of a merger between two or more neutron stars, including at least one that was less massive than our Sun. If confirmed, it would be the first superkilonova ever recorded – something that study leader Mansi Kasliwal says would advance our understanding of what happens to massive stars at the end of their lives.
A supernova is the explosion of a massive star that occurs either because the star no longer produces enough energy to prevent gravitational collapse, or because it suddenly acquires large amounts of matter from a disintegrating nearby object. Such explosions are an important source of heavy elements such as carbon and iron in the universe.
Kilonovae are slightly different. They occur when two neutron stars collide, producing heavier elements such as gold and uranium. Both types of events can be detected from the ripples they produce in spacetime – gravitational waves – and from the light they give off that propagates across the cosmos to Earth-based observers.
An unusual new kilonova candidate
The first – and so far only – confirmed observation of a kilonova came in 2017 as a result of two neutron stars colliding. This event, known as GW170817, produced gravitational waves that were detected by the Laser Interferometer Gravitational-wave Observatory (LIGO), which is operated by the US National Science Foundation, and by its European partner Virgo. Several ground-based and space telescopes also observed light signals associated with GW170817, which enabled astronomers to build a clear picture of what happened.
Kasliwal and colleagues now believe they have evidence for a second kilonova with a very different cause. They say that this event, initially called ZTF 25abjmnps and then renamed AT2025ulz, could be a kilonova driven by a supernova – something that has never been observed before, although theorists predicted it was possible.
A chain of detections
The gravitational waves from AT2025ulz reached Earth on 18 August 2025 and were picked up by the LIGO detectors in Louisiana and Washington and by Virgo in Italy. Scientists there quickly alerted colleagues to the signal, which appeared to be coming from a merger between two objects, one of which was unusually small. A few hours later, the Zwicky Transient Facility (ZTF) at California’s Palomar Observatory identified a rapidly fading red object 1.3 billion light-years away that appeared to be in the same location as the gravitational wave source.
Several other telescopes that were previously part of the Kasliwal-led GROWTH (Global Relay of Observatories Watching Transients Happen) programme, including the W M Keck Observatory in Hawaiʻi and the Fraunhofer telescope at the Wendelstein Observatory in Germany, picked up the event’s trail. Their observations confirmed that the light eruption had faded fast and glowed at red light wavelengths – exactly what was observed with GW170817.
A few days later, though, something changed. AT2025ulz grew brighter, and telescopes began to pick up hydrogen lines in its light spectra – a finding that suggested it was a Type IIb (stripped-envelope core-collapse) supernova, not a kilonova.
Two possible explanations
Kasliwal, however, remained puzzled. While she agrees that AT2025ulz does not resemble GW170817, she argues that it also doesn’t look like a run-of-the-mill supernova.
In her view, that leaves two possibilities. The first involves a process called fragmentation in which a rapidly spinning star explodes in a supernova and collects a disc of material around it as it collapses. This disc material subsequently aggregates into a tiny neutron star in much the same way as planets form. The second possibility is that a rapidly spinning massive star exploded as a supernova and then split into two tiny neutron stars, both much less massive than our Sun, which later merged. In other words, a supernova may have produced twin neutron stars that then merged to make a kilonova inside it – that is, a superkilonova.
“We have never seen any hints of anything like this before,” Kasliwal says. “It is amazing to me that nature may make tiny neutron stars smaller than a solar mass and more than one neutron star may be born inside a stripped-envelope supernova.”
While Kasliwal describes the data as “tantalizing”, she acknowledges that firm evidence for a superkilonova would require nebular infrared spectroscopy from either the W M Keck Observatory or the James Webb Space Telescope (JWST). On this occasion, that’s not going to be possible, as the event occurred outside JWST’s visibility window and was too far away for Keck to gather infrared spectra – though Kasliwal says the team did get “beautiful” optical spectra from Keck.
The only real way to test the superkilonova theory would be to find more events like AT2025ulz, and Kasliwal is hoping to do just that. “We will be keeping a close eye on any future events in which there are hints that the neutron star is sub-solar and look hard for a young stripped envelope supernova that could have exploded at the same time,” she tells Physics World. “Future superkilonova discoveries will open up this entirely new avenue into our understanding of what happens to massive stars.”
The study is detailed in The Astrophysical Journal Letters.
The post Unusual astronomical event might be a ‘superkilonova’ appeared first on Physics World.
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