A research team led by Caltech has potentially identified the first-ever superkilonova, a remarkable cosmic event characterized by a star exploding in two distinct ways. This groundbreaking discovery stems from observations triggered by gravitational waves detected on August 18, 2025, which sparked an intense search for the origins of this astronomical phenomenon.
Supernovas typically occur when rapidly spinning stars, significantly more massive than the Sun, collapse and explode, often resulting in the formation of a neutron star. In contrast, kilonovas are produced by the merger of two neutron stars, events that generate powerful gravitational waves rippling through spacetime. The gravitational waves detected by the LIGO-Virgo-KAGRA collaboration led astronomers to investigate a rapidly fading object located approximately 1.3 billion light-years away, now designated as AT2025ulz.
Initial observations of AT2025ulz revealed characteristics similar to the only other confirmed kilonova, known as GW170817, discovered in 2017. That event marked a significant milestone in astronomy, as it was the first to pinpoint the source of gravitational waves. The analysis of AT2025ulz showed red emissions indicative of heavy element formation, such as gold, suggesting an energetic collision had taken place. However, just days later, the object brightened again, this time exhibiting hydrogen in its spectra, which is more typical of a supernova.
This duality led researchers to propose that AT2025ulz was both a supernova and a kilonova. According to past studies, it is possible for supernovae to occasionally produce two neutron stars from their rapidly spinning debris. If such neutron stars were to immediately collide, they might generate the gravitational wave signature of a kilonova.
Brian Metzger, an astronomer at Columbia University and a co-author of the study, remarked that this specific merger occurred “within the exploding star.” Consequently, any kilonova signal would have been obscured by the substantial mass ejected during the supernova event.
Another intriguing aspect of this discovery is the presence of a surprisingly small object involved in the merger. David Reitze, a laser physicist at LIGO and co-author of the study, noted, “At least one of the colliding objects is less massive than a typical neutron star.” This finding is significant, as the formation of such sub-stellar neutron stars presents a considerable challenge in the field of stellar evolution.
Neutron stars typically fall within a mass limit of approximately 2.2 to 3 solar masses, although they could theoretically range as low as 0.1 solar masses. There are two known mechanisms for creating these lighter neutron stars from a supernova: the fission of a rapidly spinning massive star or a process called fragmentation. In the fragmentation scenario, a massive star collapses to form a large spinning gas disk, which then fragments under its own gravity into smaller clumps that can collapse into low-mass neutron stars.
This discovery underscores the complexity and unpredictability of the universe and highlights the potential for multiple interpretations of astronomical data. Further research is essential to confirm the existence of the superkilonova and to understand similar cosmic events.
Mansi Kasliwal, an astronomer at Caltech and the study’s lead author, emphasized the need for caution in interpreting future kilonovae events, stating, “They may not look like GW170817 and may be mistaken for supernovae.” The findings from this research have been published in The Astrophysical Journal Letters, contributing valuable insights to the ongoing exploration of cosmic phenomena.
