Astronomers May Have Seen the First 'Superkilonova'

Astronomers say they may have caught the universe pulling off a rare double feature: a stellar blast that appears to be both a supernova (the explosion of a star) and a kilonova (usually the collision of two neutron stars). The event, described in a December 15 paper in Astrophysical Journal Letters, has been dubbed a "superkilonova" and, if confirmed, would be the first of its kind ever observed, reports Smithsonian.

The discovery began with gravitational waves—tiny ripples in space-time—picked up on Aug. 18 by detectors in the US and Italy, including the twin LIGO facilities in Washington and Louisiana. The waves suggested two compact objects had merged. Within hours, an instrument at Caltech's Palomar Observatory spotted a fading red source, later named AT2025ulz, about 1.3 billion light-years away in the same region of sky. Its fast-fading, red glow resembled a known neutron star merger observed in 2017, which produced heavy elements like gold and uranium that absorb bluer light.

But then the object did something unexpected. After a few days, AT2025ulz brightened again, shifted toward bluer wavelengths, and showed hydrogen in its spectrum—signals more typical of a supernova. "At first, for about three days, the eruption looked just like the first kilonova in 2017," study co-author Mansi Kasliwal says in a Caltech statement. "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."

The team's interpretation: a massive star exploded as a supernova, leaving behind not one but two neutron stars, which quickly spiraled together and merged in a kilonova. Gravitational-wave data hint that at least one of the merging objects had less mass than the sun, an unusually low value that has led researchers to consider scenarios involving a rapidly spinning star splitting into two neutron stars or ejecta re-forming into a second one. Other explanations were examined and ruled out, Kasliwal tells Scientific American. Still, astronomers caution that more examples are needed to confirm superkilonovas as a distinct cosmic phenomenon. As Kasliwal put it, "Nature is very creative. When we attempt to unlock its mysteries, we should do so with eyes wide open."