A new kilonova has astronomers rethinking what we know about gamma-ray bursts

A year ago, astronomers discovered a powerful gamma-ray burst (GRB) lasting about two minutes in the name GRB 211211A. Now, that unusual result supports the long-standing assumption that longer GRBs are the signature of a supernova star of its own size. But two teams of independent scientists point to a source of so-called “kilonova” radiation from the merger of two neutron stars, according to a new paper published in the journal Nature. Because neutron star mergers are thought to only produce short GRBs, the discovery of a hybrid event involving a kilonova-long GBR is quite surprising.

“This detection shatters our standard idea of ​​gamma-ray bursts,” said co-author Eva Chase, a postdoc at Los Alamos National Laboratory. “We can no longer assume that all short-duration bursts originate from neutron sinks, while long-duration bursts originate from supernovae. I now realize that gamma-ray bursts are much more difficult to identify. This detection pushes our understanding of gamma-rays to the limits.

As we reported earlier, gamma-ray bursts are very high-energy explosions in distant galaxies lasting between mere milliseconds to several hours. The first gamma-ray bursts were observed coming out in the 1960s thanks to the Vela satellites launched by the US. They understood the telltale gamma-ray signatures of nuclear weapons tested in the event of the Nuclear Test Ban Treaty of 1963 with the Soviet Union. The US feared that the Soviets were conducting secret nuclear tests, violating the treaty. In July 1967, two of those satellites picked up a gamma ray burst that was clearly the signature of a nuclear weapons test.

Just two months ago, many time-based detectors picked up a powerful gamma-ray burst passing through our solar system, sending astronomers scrambling to set up their telescopes in that part of the sky to gather vital data from the event and its afterglow. . Called GRB 221009A, it was the most powerful gamma-ray burst yet detected and could very well be the “birth cry” of a new black hole.

There are two types of gamma rays: short and long. Classic short-period GRBs last less than two seconds, and were previously thought to occur only from two-bulb 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 at the Fermi and Fast telescopes simultaneously detected this latest gamma-ray burst last December and pinpointed its location in the constellation Bootes. That quick identification allowed other telescopes around the world to turn their attention to that sector, so they can catch them in their early kilonova stages. And it was surprisingly close for a gamma-ray burst: it was detected about 1 billion light-years from Earth, compared to about 6 billion years for the average gamma-ray burst. (The light from the last GRB still travels some 13 billion years).

“It was something we’ve never seen before,” said collaborator Simone Dichiara, an astronomer at Penn State University and a member of the Rapid team. “We know that the supernova was not a companion, the massive death of a star, because it was too close. It was a completely different kind of optical signal, one that we associate with kilonova, the explosion of neutron stars that uses collisions.”

When two binary stars begin their neutron death spiral, they emit gravitational waves and collide with each other’s neutron-rich material. Then the stars collide and merge, producing a hot cloud of debris that glows with light of multiple wavelengths. It is neutron-rich debris that astronomers believe creates visible and infrared kilonova light – glowing brighter in the infrared than in the visible spectrum, such a distinctive signature event that results from heavy elements ejected that block visible light. emits infrared.

It is that signature that later analysis of GRB211211A revealed. And since the subsequent neutron decay of a merging star produces the heavy elements gold and platinum, astronomers now have a new way to learn how these heavy elements form 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 Fast Observatory, which is named in his honor, played a key role in discovering GRB 211211A, and is the first to have direct evidence of the link.

“This discovery is a clear reminder that the Universe has never been fully figured out,” said co-author Jillian Rastinejad, a Ph.D. student at Northwestern University. “Astronomers often take for granted that the origins of GRBs can be traced back to how long GRBs are, but this discovery shows that we understand much more about these amazing events.”

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