The new kilonova is causing astronomers to rethink what we know about gamma-ray bursts

A year ago, astronomers detected a powerful gamma-ray burst (GRB) that lasted nearly two minutes, called GRB 211211A. Now, this unusual event overturns the long-held assumption that longer GRBs are the hallmark of a massive star going supernova. Instead, two independent teams of scientists identified the source as the so-calledkilonova”, caused by the merger of two neutron stars, according to a new paper published in the journal Nature. Since neutron star mergers were thought to produce only short GRBs, the discovery of a hybrid event involving a kilonova with a long GBR is quite surprising.

“This discovery disrupts our standard understanding of gamma-ray bursts,” said co-author Yves Chase, a postdoctoral fellow at Los Alamos National Laboratory. “We can no longer assume that all short-lived bursts come from neutron star mergers, while long-lived bursts come from supernovae. We now realize that gamma ray bursts are much more difficult to classify. This discovery pushes our understanding of gamma-ray bursts to the limits.”

As do we reported earlier, gamma-ray bursts are extremely high-energy explosions in distant galaxies lasting between milliseconds and several hours. The first one gamma rays were observed in the late 1960s, thanks to the marketing of Vela satellites from the USA. They were designed to detect telltale gamma signals from nuclear weapons tests following the 1963 Nuclear Test Ban Treaty with the Soviet Union. The US feared that the Soviets were conducting secret nuclear tests in violation of the treaty. In July 1967, two of these satellites picked up a flash of gamma radiation that was clearly not the signature of a nuclear weapons test.

Just a few months ago, multiple space detectors picked up a powerful gamma-ray burst passing through our solar system, sending astronomers around the world scrambling to point their telescopes at this part of the sky to gather vital data about the event and its afterglow. Called GRB 221009A, it was the most powerful gamma-ray burst ever recorded and could possibly be the “birth cry” of a new black hole.

There are two types of gamma rays: short and long. Classical short-duration GRBs last less than two seconds and were previously thought to arise only from the merger of two superdense objects, such as binary neutron stars, producing an accompanying kilonova. Long GRBs can last from a few minutes to several hours and are thought to occur when a massive star goes supernova.

Astronomers from the Fermi and Swift telescopes simultaneously detected this latest gamma-ray burst last December and pinpointed its location in the constellation Boots. This quick identification allowed other telescopes around the world to turn their attention to this sector, allowing them to catch the kilonova in its earliest stages. And it was remarkably close for a gamma-ray burst: about 1 billion light-years from Earth, compared to about 6 billion years for the average gamma-ray burst detected so far. (The light from the most distant GRB ever recorded traveled about 13 billion years.)

“It was something we had never seen before,” said co-author Simone DiChiara, an astronomer at Penn State University and a member of the Swift team. “We knew it wasn’t related to a supernova, the death of a massive star, because it was too close. It was a completely different kind of optical signal that we associate with a kilonova, the explosion caused by a collision of neutron stars.

When two binary neutron stars begin their death spiral, they send out powerful gravitational waves and strip each other of neutron-rich matter. The stars then collide and merge, producing a hot cloud of debris that glows with multi-wavelength light. It’s the neutron-rich debris that astronomers believe is creating the visible and infrared light of the kilonova—the glow is brighter in the infrared than in the visible spectrum, a hallmark of such an event that results from heavy elements in the ejecta that block visible light but let infrared rays through.

This signature is what subsequent analysis of GRB211211A revealed. And because the subsequent decay of merging neutron stars produces heavy elements like gold and platinum, astronomers now have new tools to study how these heavy elements form in our universe.

A few years ago, the late astrophysicist Neil Gehrels suggest that longer gamma-ray bursts may be produced by merging neutron stars. It seems only fitting that NASA’s Swift Observatory, which is named in his honor, played a key role in the discovery of GRB 211211A and the first direct evidence of this connection.

“This discovery is a stark reminder that the universe is never fully understood,” said co-author Gillian Rastinejad, a doctoral student at Northwestern University. “Astronomers often take for granted that the origin of GRBs can be identified by how long the GRBs are, but this discovery shows us that there is much more to understand about these amazing events.”

DOI: Nature, 2022. 10.1038/s41550-022-01819-4 (About DOI).