Caltech and LIGO/Virgo Unveil the Cosmic Drama of AT2025ulz

Scientists from Caltech and the LIGO/Virgo collaboration have announced the observation of an astronomical event, AT2025ulz, which could indicate a previously unrecorded type of cosmic explosion. According to their data, this event combines characteristics of both a supernova and a kilonova, potentially representing the first observed instance of a so-called ‘superkilnova’. Supernovae occur during the explosion of massive stars and are considered the primary source of medium-mass elements like carbon and iron. Kilonovae are associated with the merging of neutron stars, believed to be responsible for the creation of the heaviest elements, including gold and uranium. Until 2025, only one kilonova had been reliably confirmed-GW170817, which was registered in 2017 and confirmed through gravitational waves and electromagnetic radiation.

The AT2025ulz event was registered on August 18 and immediately drew attention for its ambiguous nature. The LIGO and Virgo detectors identified a weak gravitational wave signal indicating a merger involving at least one object with an unusually small mass, atypical of known neutron stars.

In the first three days, the flash appeared almost like the kilonova of 2017. However, subsequent observations showed more complex behavior: the emission first dimmed and shifted towards the red part of the spectrum, like a kilonova, and then intensified, became more ‘blue’, and displayed signs of hydrogen-characteristic markers of a supernova, noted by Mansi Kasliwal from Caltech.

The authors of the study suggest that AT2025ulz could be a superkilnova-a kilonova triggered by a supernova. Under this hypothesis, the explosion of a massive star leads to the formation of two neutron stars that quickly merge. The light from the kilonova is partially shielded by the expanding shells of the supernova. Gravitational wave data also point to the possible existence of a neutron star with a mass less than that of the sun-an object whose existence was previously only discussed theoretically. Two mechanisms were considered for their formation: the division of a rapidly rotating star and the fragmentation of matter around a collapsing core.

The authors emphasize that the presented data are not definitive proof. Nevertheless, the unusual properties of AT2025ulz are already influencing the strategies for searching and interpreting cosmic collisions. Future sky surveys, especially using new-generation observatories, may uncover other similar events that previously went unnoticed.