Astronomers detect rare kilonova explosion

Astronomers studying short-lived gamma-ray bursts (GRBs) have detected a rare kilonova explosion in which two neutron stars appear to have merged to form a larger neutron star called a magnetar.

“I’ve been studying these short gamma-ray bursts for a decade now,” says Wen-fai Fong, an astrophysicist at Northwestern University, US. “Just when you think you understood them, they throw a new twist at you. The Universe produces such a diversity of explosions.”

Short GRBs are brief flashes of gamma-rays, heralding exciting events in distant galaxies. “We think they come from the merger of two neutron stars,” Fong says. As the name implies, they happen quickly: there and gone in the course of a couple of seconds. But they can be followed by an afterglow of everything from X-rays to radio and infrared emissions.

That means that when a short GRB is detected, it’s all hands on deck to turn as many types of astronomical instruments on it as possible, before the afterglow fades. “It is a fast-fading signal,” Fong says. “The burst is fading from the time you eat lunch to the time you eat dinner,” (though there’s usually still time to study it for a few days).

This afterglow, she says, suggests that some short GRBs are from kilonova explosions, in which the merger of the neutron stars ejects part of those stars’ mass into space. (The name means that they are about 1000 times more powerful than stellar nova explosions, though much less powerful than the supernova explosions that mark the deaths of giant stars.)

Fong compares it to what would happen if you tried to make a smoothie and forgot to put the lid on the blender – though in this case, it’s chunks of neutron star that get blasted out all over the place.

These neutrons rapidly coalesce into unstable isotopes of heavy elements that then quickly decay into more stable ones, releasing heat, light, X-rays, and radio waves in the process. But for a short GRB detected on 22 May (called GRB 200522A), something didn’t fit the model.

When the Hubble Space Telescope was free to pause other observations and turn toward GRB 200522A’s source, three-and-a-half days after the GRB, astronomers found that it was emitting 10 times more infrared light than a normal kilonova. “Given what we know about the radio and X-rays from this blast, it just didn’t match up,” Fong says.

Gradually, her team realised they’d seen something truly unusual.

Normally, neutron star mergers produce black holes. But the only explanation her team could come up with for how such a merger could produce an afterglow ten times too bright in the infrared was that they had witnessed the birth of a magnetar.

Magnetars are neutron stars with extremely strong magnetic fields. As it spun rapidly after the collision that created it, this magnetar’s field would transfer energy to the debris created by the kilonova explosion, heating it up and causing it to glow in exactly the manner observed by the Hubble.

That in itself is exciting enough. But even though GRB 200522A is in a galaxy far, far away, the finding is also relevant to our lives here on Earth.

Scientists once thought that nuclear reactions in supernova explosions were what created elements heavier than iron, many of which later found their way into planets.

But that theory is passé. Now, scientists think, if you have a gold ring, odds are that its atoms were forged in the brief fires of something akin to a long-ago kilonova. “We think a lot of our heavy elements come from these neutron-star mergers,” Fong says.

The next step will be the launch of NASA’s James Webb Space Telescope in October 2021. That instrument, she says, will be sensitive enough that if there’s another burst like GRB 200522A, it will not only be able to observe its afterglow, but obtain a spectrum of it, thereby picking out the specific elements being created in the kilonova.

Meanwhile, Fong’s team’s research is scheduled to be published in The Astrophysical Journal and available now on the pre-print arXiv.