A year ago, astronomers detected a powerful gamma-ray burst (GRB) that lasted about two minutes, and named it GRB 211211A. Now this unusual event upends the long-held assumption that longer GRBs are the signature signature of a massive star going supernova. Instead, two independent teams of scientists have identified the source as a so-called “kilonova,” caused by the merger of two neutron stars, according to a new paper published in the journal Nature. Since neutron star mergers were supposed to produce only short GRBs, the discovery of a hybrid event involving a kilonova with a long GBR is surprising.
“This discovery breaks our standard idea of gamma-ray bursts,” said co-author Eve Chase, a postdoctoral researcher at Los Alamos National Laboratory. “We can no longer assume that all short-duration bursts come from neutron star mergers, while long-duration ones come from supernovae. We now realize that categorizing gamma-ray bursts is much more difficult. This discovery pushes our understanding of gamma-ray bursts to an extreme.”
As mentioned earlier, gamma ray bursts are extremely high-energy explosions in distant galaxies that last between milliseconds to several hours. The first gamma-ray bursts were observed in the late 1960s, thanks to the United States’ launch of the Vela-Vela satellites. Their purpose was to detect gamma-ray signatures of nuclear weapons testing in the aftermath 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 not clearly a signature of a nuclear weapons test.
Just two months ago, several space detectors picked up a powerful gamma-ray burst passing through our solar system, sending astronomers around the world scrambling to train their telescopes on that part of the sky to collect vital data on the event and its afterglow. . Dubbed GRB 221009A, it was the most powerful gamma-ray burst recorded to date, and likely the “birth cry” of a new black hole.
There are two types of gamma ray bursts: short and long. Classic short-period GRBs last less than 2 seconds, and were previously thought to only occur from the merger of two ultra-dense objects, such as binary neutron stars, producing an accompanying kilonova. Long GRBs can last anywhere from a few minutes to several hours and are thought to occur when a massive star goes supernova.
Astronomers with the Fermi and Swift telescopes simultaneously detected the last gamma-ray burst last December and pinpointed the location in the Boötes constellation. This quick identification allowed other telescopes around the world to turn their attention to this sector, enabling them to capture a 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 has traveled from the farthest GRB so far recorded for about 13 billion years.)
“It was something we hadn’t seen before,” said co-author Simon DiShiara, an astronomer at Penn State 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 so close. It was a completely different kind of light signal, one that 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 strip neutron-rich matter from each other. Then the stars collide and merge, producing a hot cloud of debris that glows with multi-wavelength light. It’s the neutron-rich debris that astronomers believe produces visible and infrared kilonova light—the glow is brighter in the infrared than in the visible spectrum, a characteristic signature of such an event that results from heavy elements in the ejecta blocking visible light but letting in the rays. infrared pass through.
This signature is what was revealed by post hoc analysis of GRB211211A. And since the subsequent decay of a neutron star merger produces heavy elements like 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 even longer gamma-ray bursts could be caused by neutron star mergers. It only 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 such a link.
“This discovery is a clear reminder that the universe has never been fully discovered,” said co-author Jillian Rastenjad, PhD. Student at Northwestern University. “Astronomers often take it for granted that the origins of GRBs can be determined by how long the GRBs are, 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).
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