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

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

“This detection shatters our standard idea of ​​gamma-ray bursts,” said co-author Eve Chase, a postdoc at Los Alamos National Laboratory. “We can no longer assume that all short-duration bursts come from neutron star mergers, while long-duration bursts come from supernovae. We now realize that gamma-ray bursts are much more difficult to classify. This detection pushes our understanding of gamma-ray bursts to the limits.”

As we did previously reported, gamma-ray bursts are extremely high-energy explosions in distant galaxies that last from just milliseconds to several hours. The first gamma ray bursts were observed at the end of the 60s, thanks to the launch of the sailing US satellites. They were intended to detect telltale gamma-ray signatures of 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 couple of months ago, several space detectors spotted one powerful burst of gamma rays passing through our solar system, sending astronomers around the world scrambling to train their telescopes on this part of the sky to gather vital data about the event and its glow. Called GRB 221009A, it was the most powerful gamma-ray burst yet recorded and could possibly be the “birth cry” of a new black hole.

There are two types of gamma ray bursts: short and long. Classical short-lived GRBs last less than two seconds, and were previously thought to occur only from the merger of two ultradense 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 at the Fermi and Swift telescopes simultaneously detected this latest gamma-ray burst last December and pinpointed the location in the constellation. Boots. This quick identification allowed other telescopes around the world to turn their attention to this sector, allowing them to capture the kilonova in its early stages. And it was remarkably close to 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 farthest GRB yet recorded traveled for 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 associated with a supernova, the death of a massive star, because it was too close. It was a completely different kind of optical signal, one we associate with a kilonova, the explosion triggered by the collision of neutron stars”.

As two binary neutron stars begin to spin in their death spiral, they emit powerful gravitational waves and separate neutron-rich matter. The stars then collide and merge, producing a hot cloud of debris that glows with light of multiple wavelengths. It is the neutron-rich debris that astronomers believe creates the visible and infrared light of a kilonova: the glow is brighter in the infrared than in the visible spectrum, a distinctive signature of this event that results from elements heavy in the ejecta that block visible light but let infrared through.

This signature is what further analysis of GRB211211A revealed. And because the subsequent decay of a merging neutron star produces heavy elements like gold and platinum, astronomers now have a new means of studying how these heavy elements form in our universe.

Several years ago, the late astrophysicist Neil Gehrels suggested that longer gamma-ray bursts could be produced by neutron star mergers. It seems 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 figured out,” said co-author Jillian Rastinejad, a Ph.D. student at Northwestern University. “Astronomers often take it for granted that the origins of GRBs can be identified by how long GRBs are, but this discovery shows us that there is still much more to understand about these amazing events.”

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