New kilonova makes astronomers rethink what we know about gamma-ray bursts
A year ago, astronomers discovered a powerful gamma-ray burst (GRB) lasting nearly two minutes called GRB 211211A. Now that unusual event turns the long-standing assumption that longer GRBs are the signature signature of a massive star going supernova on its head. Instead, two independent teams of scientists identified the source as a so-called “kilonova,” caused by the merger of two neutron stars, according to one new paper published in the journal Nature. Since neutron star mergers were believed to produce only short GRBs, the discovery of a long GBR kilonova hybrid event is quite surprising.
“This detection breaks with 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 bursts come from neutron star mergers, while long 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.”
Like we did previously reported, gamma-ray bursts are extremely high-energy explosions in distant galaxies that last from just a few milliseconds to several hours. The first gamma ray bursts were observed in the late 1960s, thanks to the launch of the Vela satellites across the US. They were intended to detect telltale gamma rays from nuclear weapons tests in the aftermath of 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 those satellites caught a flash of gamma rays that were clearly not the hallmark of a nuclear weapons test.
Just a few months ago, multiple space-based detectors have a powerful gamma ray burst which passes through our solar system and sends astronomers around the world 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 on record and could likely be the “birth cry” of a new black hole.
There are two types of gamma-ray bursts: short and long. Classic short-duration GRBs last less than two seconds and were previously thought to be formed only by the merger of two ultra-dense objects, such as binary neutron stars, producing an accompanying kilonova. Long GRBs can last from a few minutes to a few hours and are believed to occur when a massive star goes supernova.
Astronomers from the Fermi and Swift Telescopes simultaneously discovered this latest gamma-ray burst last December and pinpointed its location in the constellation Boots. That rapid identification allowed other telescopes around the world to turn their attention to that sector, helping them capture 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. (Light from the most distant GRB on record to date traveled some 13 billion years.)
“It was something we’d never seen before,” said co-author Simone Dichiara, an astronomer at Penn State University 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 very different kind of optical signal, one that we associate with a kilonova, the explosion caused by colliding neutron stars.”
As two binary neutron stars begin to orbit in their death spiral, they emit powerful gravitational waves and strip neutron-rich matter from each other. Then the stars collide and merge, creating a hot cloud of debris that glows with light of multiple wavelengths. It’s the neutron-rich debris that astronomers say creates a kilonova’s visible and infrared light — its glow is brighter in the infrared than in the visible spectrum, a distinguishing feature of such an event that results from heavy elements in the ejecta that block visible light but allow infrared to pass through.
That signature is what later analysis of GRB211211A revealed. And since the subsequent decay of a neutron star merger produces heavy elements such as gold and platinum, astronomers now have a new way to study how these heavy elements form in our universe.
Several years ago, the late astrophysicist Neil Gehrels suggested that longer gamma-ray bursts may be produced by neutron star mergers. It seems only fitting that NASA’s Swift Observatory, named after him, played a key role in the discovery of GRB 211211A and the first direct evidence for that connection.
“This discovery is a clear reminder that the universe will never be fully understood,” said co-author Jillian Rastinejad, a Ph.D. student at Northwestern University. “Astronomers often assume 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.”
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