When massive stars die, they go out in fiery explosions called supernovae. On rare occasions, two dying stars collide to produce dim but equally intense kilonovas. On rare occasions, supernovas and kilonovas overlap in a superkilonova-at least, that’s the best explanation yet.
In a recent paper in The Astrophysical Journal Letters, astronomers led by the California Institute of Technology describe unusual explosions that may be supernova and kilonova. In reality, a supernova gave birth to two neutron stars—dense, nearly dead ones—that then merged to form a kilonova.
If confirmed, the signal, called AT2025ulz, would be the second kilonova to be detected and the first of its kind to appear in such a complex manner.
A burst of ripples
When stars explode at the end of their lives, the explosion helps seed the universe with heavy elements like carbon and iron. Kilonovas, on the other hand, release even heavier elements, such as gold and uranium, which become the building blocks of many stars and rockier planets.
Such cataclysmic events create ripples in space—gravitational waves—that are detected by detectors like LIGO on Earth. Mankind had recorded kilonovas only once, back in 2017, and with LIGO. So astronomers were very happy when, in August of this year, the same place sent a warning to the public, advising them of a sign that seemed to be similar to that historic discovery.
Almost immediately, another surveillance camera confirmed rapidly fading red lights—a sign of heavy element production from kilonovas—from the same area. A few days later, the source flared up again, but this time blue, like a supernova.
Stellar detectives
“At first, for about three days, the explosion looked like the first kilonova of 2017,” Mansi Kasliwal, lead author of the study and an astronomer at Caltech, said in a statement. “Everyone was trying very hard to see it and analyze it, but then it started to look like a supernova, and other astronomers lost interest. Not us.”
For Kasliwal, there were too many unanswered questions about AT2025ulz to conclude that it was a supernova. For one thing, it didn’t resemble a typical supernova—or, for that matter, the kilonova seen in 2017. Furthermore, the gravitational wave data indicated the merger of two objects, at least one of which was unusually bright.
“No neutron star has ever been observed before with a mass less than that of the Sun, and it is believed to be impossible,” said Brian Metzger, study author and Columbia University philosopher, in a statement. But that’s what LIGO found: a subsolar neutron star engaged in an explosive merger.
The investigation has not been completed
Theoretically, the best explanation for light neutron stars would be the product of a rapidly rotating giant star that splits in two during a supernova, Metzger said. But the chaos common to the whole process will also force the baby neutron stars into a deadly storm that ends in a kilonova, he added.
All that said, this explanation, while “interesting,” needs to be tested again, the researchers agree. After all, the AT2025ulz – if it’s a kilonova – just stands for it the second time kilonova to be discovered.
“Future kilonovae events may not be as visible as GW170817 and may be considered supernovae,” Kasliwal said. “We don’t know for sure that we found a superkilonova, but the event was an eye-opener.”
