Double Cosmic Explosion Gives Birth to Unprecedented ‘Superkilonova’

• Explosions to ripples
• Stellar detectives
• An unfinished investigation

When massive stars die, they go out in fiery explosions called supernovas. On rarer occasions, two nearly dead stars collide to create dimmer yet similarly intense kilonovas. On even rarer occasions, the supernovas and kilonovas overlap in a superkilonova—at least, that’s the best explanation at the moment.

In a recent paper for The Astrophysical Journal Letters, astronomers led by the California Institute of Technology describe an odd stellar explosion highly likely to be somehow both a supernova and a kilonova. To be exact, a supernova birthed two neutron stars—dense, practically dead stellar cores—which then merged to create a kilonova.

If confirmed, the signal, dubbed AT2025ulz, would be the second kilonova ever to be detected and the first of its kind to have emerged in such a complex manner.

Explosions to ripples

When stars explode at the end of their lifespans, the explosions help seed the universe with heavier elements like carbon and iron. Kilonovas, on the other hand, release even heavier elements, such as gold and uranium, which then become the basic building blocks of more stars and rockier planets.

Such cataclysmic events create ripples in spacetime—gravitational waves—that detectors like LIGO pick up on Earth. Humanity had documented kilonovas only once, back in 2017, also with LIGO. So astronomers were ecstatic when, in August this year, the same facility sent an alert to the community, advising them of a signal that appeared to resemble that historical detection.

Almost immediately, another survey camera confirmed rapidly fading red lights—a sign of heavy element production from kilonovas—originating from the same location. A few days later, the source flared again, but this time in blue, more like a supernova.

Stellar detectives

“At first, for about three days, the eruption looked just like the first kilonova in 2017,” Mansi Kasliwal, study lead author and an astrophysicist at Caltech, said in a release. “Everybody 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.”

For Kasliwal, there were too many unanswered questions about AT2025ulz to conclude it was a supernova. For one, it didn’t resemble an average supernova—or, for that matter, the kilonova observed in 2017. What’s more, the gravitational wave data pointed to the merger of two objects, at least one of which was unusually light.

“No neutron star had ever been observed before with a mass less than that of the Sun, and it was believed to be theoretically impossible,” said Brian Metzger, study co-author and a theoretical physicist at Columbia University, in a statement. But that was what LIGO found: a sub-solar neutron star engaged in an explosive merger.

An unfinished investigation

Theoretically, the best explanation for the lightweight neutron stars would be the product of a rapidly spinning massive star splitting into two during a supernova, Metzger said. But the general chaos throughout the process would additionally force the baby neutron stars into a deadly spiral that ends with a kilonova, he added.

All that said, this explanation, while “tantalizing,” must be tested further, the researchers admitted. After all, AT2025ulz—if it is a kilonova—represents just the second kilonova to be detected.

“Future kilonovae events may not look like GW170817 and may be mistaken for supernovae,” Kasliwal said. “We do not know with certainty that we found a superkilonova, but the event nevertheless is eye-opening.”