New kilonova causes astronomers to rethink what we know about gamma-ray bursts

A year ago, astronomers discovered a powerful gamma-ray burst (GRB) lasting nearly two minutes, dubbed GRB 211211A. Now, this unusual event upends the long-held assumption that longer GRBs are the distinctive signature of a massive stellar 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. Because 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 detection breaks our standard idea of ​​gamma-ray bursts,” said co-author Eve 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 detection pushes our understanding of gamma-ray bursts to its limits. »

As we reported earlier, gamma-ray bursts are very high-energy explosions in distant galaxies that last anywhere from milliseconds to hours. The first gamma-ray bursts were observed in the late 1960s, thanks to the launch of the Vela satellites by the United States. They were meant to detect telltale gamma-ray signatures of nuclear weapons testing following the 1963 nuclear test ban treaty with the Soviet Union. The United States feared the Soviets would conduct secret nuclear tests, violating the treaty. In July 1967, two such satellites picked up a flash of gamma radiation that was clearly not the signature of a nuclear weapons test.

Just a few months ago, several space detectors detected a powerful burst of gamma rays streaking through our solar system, sending astronomers around the world scrambling to train their telescopes on this part of the sky to collect vital data on the event and its aftermath. . Dubbed GRB 221009A, it was the strongest gamma-ray burst ever recorded and could possibly be the “birth cry” of a new black hole.

There are two types of gamma-ray bursts: short and long. Classic short-term GRBs last less than two seconds, and were previously thought to occur only from the merger of two ultra-dense objects, like binary neutron stars, producing an accompanying kilonova. Long GRBs can last anywhere from minutes to 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 Boötes. This quick identification allowed other telescopes around the world to turn their attention to this sector, allowing them to catch the kilonova in its infancy. 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. (Light from the farthest GRB on record traveled about 13 billion years.)

“It was something we had never seen before,” said co-author Simone Dichiara, a Penn State University astronomer and 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 type of optical signal, the 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 send out powerful gravitational waves and pull neutron-rich matter apart. Then the stars collide and merge, producing a hot cloud of debris that glows with light of multiple wavelengths. It is the neutron-rich debris that astronomers say 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 such an event that results from heavy elements in ejecta that block visible light but allow infrared to pass.

This signature is what subsequent analysis of GRB211211A revealed. And because the subsequent decay of a neutron star merger produces heavy elements like gold and platinum, astronomers now have a new way to study the formation of these heavy elements 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 only fitting that NASA’s Swift Observatory, 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 clear reminder that the Universe is never fully understood,” said co-author Jillian Rastinejad, who holds a Ph.D. student at Northwestern University. “Astronomers often take it for granted that the origins of GRBs can be identified by their length, but this discovery shows us that there is still much to understand about these amazing events.”

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