A double cosmic explosion could be the first known ‘superkilonova’

Astronomers have spotted a distant star that seems to have exploded twice. The blast may be the first-ever known “superkilonova” — a chimera of a supernova and a neutron star merger, researchers report in the Dec. 20 Astrophysical Journal Letters.

“The reason it would be amazing if true is that this would be producing objects we’ve never seen before in the universe,” says astronomer Cole Miller of the University of Maryland at College Park, who was not involved in the study.

In August, the U.S.-based Laser Interferometer Gravitational-wave Observatory, or LIGO, and the Virgo detector in Italy detected ripples in spacetime coming from a pair of neutron stars merging about 1.8 billion light-years away.

One thing about this signal stood out: At least one of the neutron stars seemed to have less mass than the sun.

“It was really puzzling,” says Caltech astronomer Mansi Kasliwal. Stellar physics predicts that neutron stars — ultradense clumps of stellar ashes left after massive stars explode as supernovas — should have a mass greater than about 1.4 times that of the sun. And every other neutron star astronomers have found is more massive than the sun.

Kasliwal and her colleagues followed up on the event at the Palomar Observatory in California. Within a few hours, they found a smear of red light that seemed to come from the same distance and direction as the merger. Eleven other observatories gathered data in a range of light wavelengths over the next several days.

The event initially looked like another neutron star merger caught in 2017. The wealth of data collected by dozens of observatories during that event showed that it produced a kilonova, characterized by the glow of heavy elements such as gold and platinum being forged as atomic nuclei gobble up neutrons.

The new event was reddish and faded quickly, features it shared with the 2017 event, Kasliwal says. But as the days passed, the object started to brighten again and show signs of containing hydrogen, a trait more characteristic of a supernova.

“That’s when we realized, if you put these two pieces together, what this could be was a kilonova inside a supernova,” or a superkilonova, Kasliwal says.

She and her colleagues propose that a star exploded in a supernova and left behind a rapidly spinning neutron star. That whirling neutron star may have then split into two smaller ones, or it could have formed a rotating disk that clumped into smaller neutron stars, like how planets form out of a dusty disk encircling a young sun. In either case, the smaller neutron stars could have collided with each other, producing the kilonova.

Miller is not yet convinced. The gravitational wave signal could have been from Earthly noise, such as a truck passing by the detectors. Further analysis from LIGO will rule that possibility in or out. It’s also not clear if the light source is really from the same event as the gravitational waves.

“Is the current evidence such that you’re going to sell your house to buy tickets for [the superkilonova theory]? No,” Miller says. “But it’s possible.”

Kasliwal agrees that the evidence could be stronger. “We try to be very careful to say this is a candidate, not slam dunk evidence,” she says. The best way to confirm the hypothesis would be to find other similar events, preferably closer to Earth.

But those discoveries might be a long time coming. The new find is only the second kilonova ever observed with both electromagnetic and gravitational waves.

“It means nature doesn’t do this all the time,” Kasliwal says. “I wish we’d have one a day. But it does what it does, and these are relatively rare.”