A collective of researchers spearheaded by the California Institute of Technology might have identified the inaugural superkilonova, an astronomical occurrence characterized by a star’s dual detonation through vastly dissimilar mechanisms.
Their meticulous examination of a sequence of astronomical observations, initiated by the detection of gravitational waves earlier this year, could potentially furnish evidence of the very first supernova to be subsequently followed by a kilonova event.
Supernovae transpire when stars exhibiting extreme rotational velocity and significantly exceeding the Sun’s mass undergo gravitational collapse and subsequent explosive expulsion, typically culminating in the formation of a neutron star.
Conversely, kilonovas emerge from the extraordinarily energetic amalgamation of two neutron stars, which frequently originate within a binary stellar configuration. These formidable cosmic occurrences propagate gravitational waves that propagate through the fabric of spacetime, causing ripples akin to the resonance of a celestial bell.
Consequently, upon the detection of gravitational waves by the LIGO-Virgo-KAGRA consortium on August 18, 2025, astronomers commenced an intensive search for indications of a catastrophic cosmic merger.
Within mere hours, the global astronomical community systematically surveyed the heavens for the precise origin of these waves, identifying an intriguing, rapidly diminishing celestial object situated approximately 1.3 billion light-years distant.
In several aspects, this particular event, now designated AT2025ulz, bore a resemblance to the sole other “unambiguously confirmed” kilonova that was documented in 2017. Referred to as GW170817, that discovery represented a landmark advancement, marking the first instance where scientists successfully pinpointed the locus and source of gravitational waves.
Similar to GW170817, the residual luminescence observed at AT2025ulz’s location exhibited a reddish hue, signifying the synthesis of heavy elements such as gold and pointing towards a high-energy collision. However, subsequent to the fading of its red glow after several days, AT2025ulz experienced a resurgence in brightness, this time displaying spectral signatures indicative of hydrogen, a characteristic commonly associated with supernovae rather than kilonovas.
Therefore, the prevailing question arises: was this a supernova or a kilonova? The researchers posit that it was, remarkably, both.
Prior theoretical investigations have put forth the hypothesis that supernovae, on exceedingly rare occasions, might eject two neutron stars from their rapidly rotating accretion disks, rather than a single stellar remnant. Should these two newly formed neutron stars subsequently engage in an immediate merger, they could potentially generate the gravitational wave signature characteristic of a kilonova.
Ordinarily, such mergers transpire in the vacuum of space, affording an unimpeded vantage point for the observation of their emitted radiation.
Brian Metzger, an astronomer affiliated with Columbia University and a co-author of the study, elucidated to ScienceAlert via electronic correspondence that in this specific instance, the merger took place “within the exploding star itself, meaning any kilonova signal would have been obscured by the significantly larger mass expelled from the detonating star.”
Crucially, one of the two colliding entities responsible for the kilonova was surprisingly diminutive. “At least one of the colliding objects possesses a mass lower than that of a typical neutron star,” states David Reitze, a laser physicist at LIGO and a contributing author to the research.
This finding, in itself, is highly unusual, as the formation mechanisms for such theoretical sub-stellar neutron stars present a “major challenge to stellar evolution.”
Current astrophysical models predict that neutron stars possess a mass range typically falling between 2.2 and approximately three solar masses, although theoretically, they could exist with masses as low as 0.1 solar masses.
Within the realm of theoretical astrophysics, there are only two proposed pathways for the creation of sub-stellar neutron stars from a supernova event. The first involves fission, wherein a hyper-rotating massive star undergoes supernova and bifurcates into two neutron stars instead of one. The second mechanism is known as fragmentation.
In the latter scenario, a hyper-rotating massive star (possessing at least 20 solar masses) undergoes gravitational collapse, leading to the formation of a substantial spinning gaseous disk weighing several solar masses.
Mere seconds after its formation, this disk undergoes fragmentation due to its own gravitational forces, breaking into “a multitude of smaller fragments that subsequently collapse into low-mass neutron stars, again within a matter of seconds,” as explained by Metzger.
This process bears a conceptual similarity to the manner in which planets are formed within the circumstellar disks surrounding nascent stars, Metzger conveyed to ScienceAlert.
Regardless of the precise mechanism, this yet-to-be-definitively resolved outcome serves as a potent reminder that the cosmos continually presents us with unforeseen phenomena and profound enigmas. It also underscores the possibility that such captivating astrophysical events may harbor multiple interpretations concealed within the observational data.
Further scientific inquiry is imperative to conclusively validate the superkilonova and analogous celestial occurrences.
“Future kilonova events may depart from the characteristics of GW170817 and could potentially be misidentified as supernovae,” posits Mansi Kasliwal, an astronomer at Caltech and the lead author of the research paper.
This groundbreaking research has been disseminated in The Astrophysical Journal Letters.
