Astronomers Scrutinize AT2025ulz as a Candidate for the First 'Super-Kilonova'

The astronomical community is intensely focused on the event designated AT2025ulz, which is being considered as potentially the first observable instance of a 'super-kilonova.' This theoretical phenomenon represents a catastrophic merger of two distinct cosmic events: a supernova explosion followed by a kilonova event. The initial detection occurred on August 18, 2025, sparking immediate interest due to the S250818k signal registered by the LIGO and Virgo gravitational-wave observatories. This signal strongly suggested the merger of compact objects, crucially indicating that the mass of at least one component was unexpectedly low, leading researchers to hypothesize the collision of two sub-solar mass neutron stars.

Mere hours after the gravitational-wave registration, the Zwicky Transient Facility (ZTF), situated at the Palomar Observatory, spotted a rapidly fading red glow approximately 1.3 billion light-years away from Earth. Initially, this optical counterpart resembled the kilonova GW170817, which was confirmed in 2017 as the cosmic forge for heavy elements like gold. However, the subsequent evolution of AT2025ulz proved anomalous. The object began to brighten significantly and shifted its emission toward the blue spectrum, characteristics aligning perfectly with a stripped-envelope core-collapse supernova.

Mansi Kasliwal of the California Institute of Technology (Caltech), who spearheaded the investigation published in The Astrophysical Journal Letters, pointed out that the observed sequence—the initial kilonova signature followed by the supernova signature—provides the rationale for coining the term 'super-kilonova.' A theoretical contingent, which includes Brian Metzger from Columbia University, posits a scenario where the event commenced with a supernova explosion that birthed two newly formed, unusually small neutron stars. These two sub-solar mass neutron stars might have merged almost instantaneously, thus triggering the initial red kilonova emission that was partially obscured by the expanding debris from the preceding supernova blast.

David Reitze, Director of the LIGO Laboratory, stressed the paramount importance of the mass data, confirming that at least one of the colliding bodies possesses a mass below the typical threshold for a standard neutron star. Theorists speculate that such sub-solar neutron stars could originate either from the fission of an extremely rapidly spinning star or through the fragmentation of surrounding material during a stellar collapse. While AT2025ulz stands as a compelling contender, the researchers are quick to caution that the super-kilonova theory remains unconfirmed at this juncture.

Future observations are essential to definitively verify the status of this hybrid event and to gauge its true frequency across the cosmos. The upcoming fifth observing run (O5 run) of LIGO/Virgo will play a pivotal role, complementing the data gathered by the Vera C. Rubin Observatory. This ground-based facility, hosting an 8.36-meter telescope atop Cerro Pachón in Chile, is designed to conduct wide-field surveys of the sky every three nights over a decade. A successful confirmation, or even a refutation, of the super-kilonova model using powerful instruments like the Rubin Observatory promises to unlock a new era in understanding the evolution of the universe’s most massive stars and the processes of nucleosynthesis within them.