Double cosmic explosion could be first known ‘superkilonova’

Astronomers have spotted a distant star that appears to have exploded twice. The explosion may be the the very first known “superkilonova” — a supernova chimera and a neutron star merger, researchers report in December 20 Letters from astrophysical journals.

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

In August, the US-based Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detector in Italy detected ripples in space-time coming from a pair of merging neutron stars about 1.8 billion light-years away.

One thing stood out from this signal: at least one of the neutron stars appeared to have less mass than the sun.

“It was really confusing,” says Caltech astronomer Mansi Kasliwal. Stellar physics predicts that neutron stars – ultradense clumps of stellar ash left after massive stars explode as supernovas – should have a mass greater than about 1.4 times that of the sun. And all other neutron stars discovered by astronomers are more massive than the sun.

Kasliwal and his colleagues followed the event at the Palomar Observatory in California. Within hours, they discovered a trail of red light that appeared to come from the same distance and direction as the merger. Eleven other observatories collected data in a range of light wavelengths over the following days.

The event initially resembled another neutron star merger that occurred in 2017. The wealth of data collected by dozens of observatories during this event showed that he produced a kilonovacharacterized by the glow of heavy elements such as gold and platinum being wrought as atomic nuclei gobble up neutrons.

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

“That’s when we realized that if you put these two things together, it could be 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 rotating neutron star. This spinning neutron star may have then split into two smaller stars, or it could have formed a rotating disk grouped into smaller neutron stars, like how planets form from a dusty disk surrounding a young sun. In either case, the smaller neutron stars could have collided with each other, producing the kilonova.

Miller isn’t convinced yet. The gravitational wave signal could come from terrestrial noise, such as that of a truck passing the detectors. Further analysis of LIGO may or may not rule out this possibility. It is also unclear whether the light source actually comes from the same event as the gravitational waves.

“Does the current evidence indicate that you will sell your house to buy tickets to [the superkilonova theory]? No,” Miller said. “But it’s possible.”

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

But these discoveries may take time to come. This new discovery is only the second kilonova ever observed with electromagnetic and gravitational waves.

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