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

One year ago, astronomers discovered a powerful gamma-ray burst (GRB) that lasted nearly two minutes, dubbed GRB 211211A. Now that unusual event is changing the long-held assumption that longer GRBs are the distinctive signature of a massive star going supernova. Instead, two independent teams of scientists identified the source as the so-called “kilonovatriggered by the merger of two neutron stars, according to a new paper published in the journal Nature. Because neutron star mergers were only supposed to produce 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 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 limit.”

as we have previously reportedGamma-ray bursts are extremely high-energy explosions in distant galaxies that last from mere milliseconds to several hours. First gamma ray bursts were observed in the late 1960s, thanks to the release of the Candle US satellites were meant to detect telltale gamma-ray signatures from nuclear weapons tests in the wake of the 1963 Nuclear Test Ban Treaty with the Soviet Union. The United States feared that the Soviets were conducting secret nuclear tests, in violation of the treaty. In July 1967, two of those 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, multiple space-based detectors detected a powerful gamma-ray burst passing through our solar system, sending astronomers from around the world in a hurry to point their telescopes at that part of the sky to collect vital data about the event and its afterglow. Dubbed GRB 221009A, it was the most powerful gamma-ray burst yet recorded and could likely be the “birth cry” of a new black hole.

There are two types of gamma-ray bursts: short and long. Classical short-term GRBs last less than two seconds, and were previously thought to only occur from the merger of two ultra-dense objects, such as binary neutron stars, producing a companion 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. That quick identification allowed other telescopes around the world to turn their attention to that 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 with about 6 billion years for the average gamma-ray burst detected to date. (Light from the most distant GRB recorded so far traveled over 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 Swift’s team. “We knew it was not associated with a supernova, the death of a massive star, because it was too close. It was a completely different type of optical signal, which we associate with a kilonova, the explosion caused by the collision of neutron stars.”

When two binary neutron stars begin to spin in their death spiral, they send out powerful gravitational waves and push neutron-rich matter apart. 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 such an event resulting from heavy elements in ejecta that block visible light but let infrared through.

That signature is what the subsequent analysis of GRB211211A revealed. And since the subsequent decay of a neutron star merger 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 neutron star mergers could produce longer gamma-ray bursts. It seems 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 that connection.

“This discovery is a stark reminder that the Universe is never fully figured out.” said co-author Jillian Rastinejad, a PhD student at Northwestern University. “Astronomers often take for granted that the origins of GRBs can be identified by the length of the GRBs, 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 DOIs).