Two neutron stars may have formed the first known ‘superkilonova’

Double Explosion dying stars may be the first observed case of the long-hypothesised, but never proven, “superkilon”. Although astronomers are still searching for concrete answers, a study published in the journal Letters from an Astrophysical Journal can detail a historic explosion approximately 1.3 billion light-years from Earth.

Most of the universe's massive stars end their lives in a blaze of glory. supernovaebut this is not always the case. Sometimes the end of the road is a more spectacular event known as kilonova. It is believed that these explosions usually occur after two dense neutron stars collide with each other and produce an exponentially larger explosion. While supernovae help spread heavy elements like carbon and iron throughout space, kilonovae create even denser remains, including uranium and gold.

The most striking example of a kilonova in astronomy. GW170817was opened only in 2017. At that time, observing facilities such as the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo gravitational-wave detector in Europe detected gravitational and light waves resulting from the collision of two neutron stars. Since then, both professional and amateur astronomers have noted many possible kilonovae, but our understanding of these phenomena remains relatively tenuous.

The team at Caltech's Palomar Observatory thinks they may have another kilonova candidate, but the situation is a little more complicated. On August 18, 2025, both LIGO and Virgo detected gravitational wave signals, launching an alert system to the global astronomical community. Researchers at Palomar Observatory's Zwicky Transition Center soon discovered a rapidly fading red body about 1.3 billion light-years from Earth. Later classified as AT2025ulz, the object had the same fading red waves as GW170817.

“At first, for about three days, the eruption looked similar to the first kilonova in 2017,” said Palomar Observatory director and study co-author Mansi Kasliwal. says the statement.

However, after a few days, AT2025ulz began to brighten again, this time turning blue, indicating the presence of hydrogen. For many, this proved that the explosion was not another kilonova, but an ordinary supernova.

Kasliwal suspected something else was going on. The aggregate data for AT2025ulz admittedly did not resemble the kilonova GW170817, but it also did not correspond to a classical supernova. Moreover, gravitational waves suggested that at least one of the two neutron stars was less massive than the Sun. Neutron stars are small in nature—only about 15 miles wide—and have a mass between 1.2 and three times that of our Sun.

How could a neutron star be even smaller? Kasliwal's team offered two possible explanations. In the first scenario, a rapidly rotating star goes supernova and then splits into two subsolar neutron stars. The second theory suggests the same beginning as a supernova, but instead of fission, a disk of debris begins to form around the collapsing star. This material eventually coalesces into a small neutron star, much like the formation process of early planets.

Given the possible subsolar neutron star implied by the gravitational wave data, the researchers hypothesize that two newborn supernova neutron stars orbited each other, forming a separate kilonova. This could explain the early red wavelengths, since the kilonovae generate heavy metals in the red spectrum. As the supernova expanded, its blue spectrum eventually eclipsed the kilonova.

“The only way theorists have come up with the birth of subsolar neutron stars is through the collapse of a very rapidly rotating star,” added Columbia University astronomer and study co-author Brian Metzger. “If these 'forbidden' stars paired up and merged, emitting gravitational waves, it is possible that such an event would be accompanied by a supernova rather than being seen as a naked kilonova.”

Metzger, Kasliwal and their colleagues emphasize that their theory remains just that: a theory. However, this possibility is intriguing enough to continue searching for additional candidates in hopes of unambiguously identifying a superkylonova. Until then, Kasliwal explained the importance of continuing to study possible suspects, even if they start to look like an ordinary supernova.

“Everyone was trying hard to observe and analyze it, but then it started looking more like a supernova and some astronomers lost interest,” she said. “Not us.”