A strange gamma-ray burst breaks the rules of these cosmic flares

Astronomers have spotted a bright gamma-ray burst that upends previous theories about how these energetic cosmic flares occur.

For decades, astronomers thought that GRBs came in two versions, long and short, i.e. they lasted longer than two seconds or died out faster. Each type has been linked to different cosmic events. But about a year ago, two NASA space telescopes captured a short GRB in long GRB clothes: it lasted a long time but came from a short GRB source.

“We had this black and white view of the universe,” says astrophysicist Eleonora Troja of Tor Vergata University in Rome. “It’s the red flag that tells us, no, it’s not. Surprise!”

This burst, called GRB 211211A, is the first to unambiguously break the binary, Troja and others report December 7 in five articles in Nature and natural astronomy.

Before the discovery of this burst, astronomers mainly thought that there were only two ways to produce a GRB. The collapse of a massive star just before it explodes as a supernova could produce a long gamma-ray burst, lasting more than two seconds (SN: 10/28/22). Or a pair of dense stellar corpses called neutron stars could collide, merge and form a new black hole, releasing a short burst of gamma rays lasting two seconds or less.

But there had been a few outliers. A surprisingly short GRB in 2020 seemed to come from the implosion of a massive star (SN: 02/08/21). And some long-lived GRBs dating back to 2006 had no supernova afterthought, raising questions about their origins.

“We always knew there was an overlap,” says astrophysicist Chryssa Kouveliotou of George Washington University in Washington, DC, who wrote the Article from 1993 which introduced the two GRB categories, but was not involved in the new work. “There were outliers that we didn’t know how to interpret.”

There is no such mystery about GRB 211211A: the burst lasted more than 50 seconds and was clearly accompanied by a kilonova, the characteristic glow of new elements forged after a neutron star crash.

This shows the glow from a kilonova that followed the bizarre gamma-ray burst called GRB 211211A, in images from the Gemini North Telescope and the Hubble Space Telescope.
This shows the glow from a kilonova that followed the bizarre gamma-ray burst called GRB 211211A, in images from the Gemini North Telescope and the Hubble Space Telescope.M. Zamani/Gemini International Observatory/NOIRLab/NSF/AURA, NASA, ESA

“Although we suspected that it was possible that long-emission GRBs were fusions…this is the first confirmation,” says astrophysicist Benjamin Gompertz of the University of Birmingham in England, who describes gust observations in natural astronomy. “He has the kilonova, which is the smoking gun.”

NASA’s Swift and Fermi space telescopes detected the explosion on December 11, 2021, in a galaxy about 1.1 billion light-years away. “We thought it was a mundane long gamma-ray burst,” says astrophysicist Wen-fai Fong of Northwestern University in Evanston, Illinois.

It was relatively close, as GRBs do. This therefore allowed Fong and Troja’s research groups to independently continue closely observe the highly detailed burst using ground-based telescopes, report teams in Nature.

As the weeks passed and no supernova appeared, the researchers became confused. Their observations revealed that whatever made the GRB also emitted far more optical and infrared light than is typical for the source of a long GRB.

After ruling out other explanations, Troja and his colleagues compared the aftermath of the burst with the first kilonova ever observed in concert with ripples in spacetime called gravitational waves (SN: 10/16/17). The match was almost perfect. “That’s when a lot of people were convinced that we were talking about a kilonova,” she says.

In retrospect, it seems obvious that it was a kilonova, says Troja. But at the time, it was as impossible as seeing a lion in the Arctic. “It looks like a lion, it roars like a lion, but it shouldn’t be there, so it can’t be there,” she said. “That’s exactly how we felt.”

Now the question is, what happened? As a rule, merging neutron stars almost immediately collapse into a black hole. The gamma rays come from material that is superheated when it falls into the black hole, but the material is rare and the black hole swallows it in two seconds. So how did GRB 211211A keep its light on for almost a minute?

It is possible that the neutron stars first merged into a single, larger neutron star, which briefly resisted the pressure to collapse into a black hole. This has implications for fundamental physics which describes how difficult it is to smash neutrons into a black hole, Gompertz says.

Another possibility is that a neutron star collided with a small black hole, about five times the mass of the sun, instead of another neutron star. And the process of the black hole eating the neutron star took longer.

Or it could have been something else: a neutron star merging with a white dwarfastrophysicist Bing Zhang of the University of Nevada, Las Vegas and his colleagues suggest in Nature. “We suggest a third type of progenitor, something quite different from the previous two types,” he says.

White dwarfs are the remnants of smaller stars like the Sun, and are not as dense or compact as neutron stars. A collision between a white dwarf and a neutron star could still produce a kilonova if the white dwarf is very heavy.

The resulting object could be a highly magnetized neutron star called magnetar (SN: 01/12/20). The magnetar could have continued to pump energy in gamma rays and other wavelengths of light, extending the life of the burst, Zhang says.

Whatever its origins, GRB 211211A is a big deal for physics. “It’s important because we wanted to understand, what are these events?” said Kouveliotou.

Understanding what caused it could shed light on how the heavy elements in the universe form. And some long GRBs already seen that scientists thought came from supernovae might actually come from mergers.

To find out more, scientists need to find more of these anti-binary GRBs, as well as gravitational wave observations at the same time. Trejo thinks they can get it when the Laser Interferometer Gravitational Wave Observatory, or LIGO, comes back online in 2023.

“I hope LIGO will produce evidence,” says Kouveliotou. “Nature might be gracious and give us some of these events with gravitational wave counterparts, and maybe [help us] understand what is happening. »

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