A research team led by Caltech has potentially identified the first-ever superkilonova, a rare cosmic event involving a star that explodes in two distinct phases. This discovery stems from a series of observations linked to a gravitational wave detected on August 18, 2025. The findings suggest this phenomenon may be the first documented instance of a supernova followed by a kilonova, shedding light on the complex processes governing stellar explosions.
Supernovas occur when massive stars, significantly larger than the Sun, collapse and explode, typically leaving behind a neutron star. Kilonovas, in contrast, arise from the energetic collisions of two neutron stars, often originating from binary systems. These dramatic events generate gravitational waves that ripple through spacetime, creating detectable signals across the universe.
When the gravitational waves were observed by the LIGO-Virgo-KAGRA collaboration, astronomers quickly initiated a search for the explosion’s source. Within hours, they identified an intriguing and rapidly fading object, designated AT2025ulz, located approximately 1.3 billion light-years away. This event shares characteristics with GW170817, the first confirmed kilonova discovered in 2017, which marked a significant milestone in gravitational wave astronomy.
In the case of AT2025ulz, the initial observations indicated the presence of heavy elements, such as gold, which are typical of kilonovas. However, after a brief period, the object’s spectrum revealed hydrogen emissions, characteristic of supernovae. This duality has led researchers to propose that AT2025ulz may represent both a supernova and a kilonova.
Research has previously suggested that supernovae could occasionally produce two neutron stars from their rapidly spinning debris. If these stars were to merge immediately, they could generate a gravitational-wave signal akin to that of a kilonova. Brian Metzger, an astronomer at Columbia University and a co-author of the study, explained that this merger may have taken place “within the exploding star,” which would obscure the kilonova signal under the mass expelled during the explosion.
Significantly, the colliding objects involved in this potential kilonova included a surprisingly small body. David Reitze, a laser physicist at LIGO and another co-author, noted that “at least one of the colliding objects is less massive than a typical neutron star.” This finding is notable, as the formation of such sub-stellar neutron stars presents ongoing challenges in understanding stellar evolution.
Theoretical models suggest that sub-stellar neutron stars can form through two processes: fission, where a rapidly spinning massive star undergoes a supernova and splits into two neutron stars, or fragmentation, where a large spinning gas disk forms and subsequently breaks into smaller clumps that collapse into low-mass neutron stars.
The implications of this discovery extend beyond just the identification of a new type of astronomical event. It highlights the necessity for continued research into the myriad phenomena the universe presents, as well as the complexities that can arise from stellar explosions. Mansi Kasliwal, the study’s lead author and an astronomer at Caltech, emphasized that “future kilonova events may not look like GW170817 and may be mistaken for supernovae.”
As researchers delve further into the characteristics of AT2025ulz and similar occurrences, the universe continues to reveal its mysteries, reminding scientists and enthusiasts alike of the vast complexities of cosmic events. The findings from this study have been published in The Astrophysical Journal Letters, marking a significant contribution to the field of astrophysics and cosmic exploration.
