Did Astronomers Just Uncover a 'Superkilonova' Double Explosion? Perhaps.
Astronomers might have witnessed the first-ever 'superkilonova,' a fusion of a supernova and a kilonova. This groundbreaking discovery could revolutionize scientists' understanding of stellar birth and death.
When a massive star dies, astronomers term the ensuing violent thermonuclear explosion a supernova. These dramatic deaths leave behind remnants, where the core of the old star forms a densely packed neutron star or even a black hole if there's sufficient mass. Supernovas are commonplace in the universe, with astronomers observing approximately 20,000 of them annually.
In contrast, kilonova events are exceptionally rare, with only one confirmed occurrence in 2017. Dimmer than supernovas but more discernible in gravitational wave data, these explosions arise from the collision of two neutron stars. They are believed to be the primary source of heavy elements in the universe, including platinum and uranium.
A team of researchers now believes they've observed a supernova followed by a kilonova within hours from the same source, raising questions about the relationship between these events.
The potential 'superkilonova' occurred on August 18, 2025, and was initially detected by the Laser Interferometry Gravitational-wave Observatory (LIGO). Named AT2025ulz, the event closely mirrored the gravitational wave signals produced by the 2017 kilonova.
Upon notification from LIGO, other telescopes that observe using electromagnetic signals (visible light, x-rays, infrared, and radio) swiftly turned their attention to the aftermath.
"Initially, for about three days, the eruption resembled the first kilonova in 2017," says Mansi Kasliwal, director of Caltech's Palomar Observatory, in a press release. "Everyone was intensely trying to observe and analyze it, but then it started to look more like a supernova, and some astronomers lost interest. Not us."
What Kasliwal's team observed was an initially strong signal in red wavelengths, indicating the presence of heavy elements typically found after a kilonova. However, over time, the signal brightened and turned blue, suggesting the presence of hydrogen gas, which is more characteristic of a supernova.
This puzzle remains unsolved, but Kasliwal's team has proposed a plausible theory to explain the phenomenon.
Perhaps, when the star that triggered AT2025ulz went supernova, it left behind not one core but two. These tiny twin neutron stars subsequently collided, resulting in the double superkilonova explosion.
There are several ways this could have occurred, but both scenarios necessitate that the initial parent star be spinning rapidly. In one scenario, after the supernova, the core is split into two neutron stars through a process called fission. In the second, a single neutron star forms with a disk of material around it, which later clumps into a small neutron star, akin to planet formation in a solar system.
Both theories remain unproven. Neutron stars of such small sizes have never been observed before (though their existence is not ruled out), and it's even possible that the gravitational wave event and the subsequent electromagnetically observed supernova originated from different, but nearby, sources.
"We do not know with certainty that we found a superkilonova, but the event is undoubtedly eye-opening," Kasliwal states.
The only definitive way to confirm a superkilonova is to continue monitoring for similar events in the future. Following AT2025ulz, the astronomical community is sure to keep a vigilant eye for more such occurrences.
This research was published on December 15 in The Astrophysical Journal Letters.
