New kilonova astronomers are revising what we know about gamma-ray bursts

A year ago, astronomers discovered a powerful gamma-ray burst (GRB) lasting about two minutes, called GRB 211211A. Now, this unusual event overturns the long-held hypothesis that longer GRBs are a distinctive signature of a giant star going supernova. Instead, two independent groups of scholars called the source “kilonova,” according to began with the merger of two neutron stars new paper Published in the journal Nature. Since neutron star mergers are thought to produce only short GRBs, the discovery of a hybrid event involving a long GBR and a kilonova is quite surprising.

“This detection disrupts our standard idea of ​​gamma-ray bursts” said co-author Eve Chasand a postdoc at Los Alamos National Laboratory. “We can no longer assume that all short bursts are due to neutron star mergers and long bursts are from supernovae. We now understand that gamma-ray bursts are more difficult to classify. This detection pushes our understanding of gamma-ray bursts to the limit.”

As we have previously reported, gamma-ray bursts are extremely high-energy bursts in distant galaxies that last from mere milliseconds to several hours. First gamma-ray bursts was observed in the late 1960s thanks to its launch sailboat Satellites by the US. They were designed to detect the gamma-ray signatures of nuclear weapons tests after the 1963 Nuclear Test Ban Treaty with the Soviet Union. The US feared that the Soviets would conduct secret nuclear tests in violation of the treaty. In July 1967, two of those satellites caught a gamma-ray glow that was clearly not the signature of a nuclear weapons test.

Just a few months ago, many space-based detectors a powerful gamma-ray burst As it passes through our solar system, it sends astronomers worldwide to train their telescopes on that part of the sky to gather vital information about the event and its afterglow. Dubbed GRB 221009A, it was the most powerful gamma-ray burst ever recorded and is likely to be the “birth cry” of a new black hole.

There are two types of gamma-ray bursts: short and long. Classical short-duration 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. 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 last gamma-ray burst last December and pinpointed the constellation. boots. This rapid identification allowed other telescopes around the world to focus on that sector, allowing them to catch the kilonova in its earliest stages. And it was pretty 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. (The light from the most distant GRB yet recorded traveled about 13 billion years.)

“It was something we had 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 a supernova, the death of a giant star, because it was so close. It was an entirely different kind of optical signal, the explosion we associate with a kilonova, the explosion of colliding neutron stars.

When two binary neutron stars begin orbiting in a death spiral, they send out powerful gravitational waves and rip apart neutron-rich matter. The stars then collide and merge, forming a cloud of hot debris that glows with light of many wavelengths. Astronomers believe neutron-rich debris produces the kilonova’s visible and infrared light—a glow brighter than the visible infrared spectrum, a hallmark of such an event caused by heavy elements blocking visible light in the ejecta. allows infrared transmission.

This signature emerged from further analysis of GRB211211A. Because the subsequent decay of neutron star mergers produces heavy elements like gold and platinum, astronomers now have new tools to study how these heavy elements form in our universe.

A few years ago, the late astrophysicist Neil Gehrels suggested that longer gamma-ray bursts could be produced by neutron star mergers. NASA’s Swift Observatory, named after him, played a key role in the discovery of GRB 211211A and appears to be the first direct evidence of this connection.

“This discovery is a stark reminder that the Universe has never been fully defined” said co-author Jillian Rastinejad, PhD student at Northwestern University. “Astronomers often take for granted that the origin of GRBs can be determined by how long the GRBs last, but this discovery shows us that there is still much to understand about these amazing phenomena.”

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