Astronomers have spotted a bright gamma-ray burst that upends previous theories of how energetic cosmic eruptions occur.
For decades, astronomers thought that GRBs came in two flavors, long and short, that is, one that lasts longer than the other, or decays more quickly. Each species is connected to different cosmic events. But about a year ago, two NASA space telescopes caught a short GRB in the cloak of a long GRB: it lasted a long time, but it originated from a short GRB source.
“We had this black and white vision of the universe,” says astrophysicist Eleonora Troja Tor Vergata of the University of Rome. “It’s a red flag that says, no, it’s not. Surprise!”
This burst, called GRB 211211A, is the first of a binary, Troja and others report on December 7 in five papers. nature and Nature Astronomy.
Before the discovery of this burst, astronomers generally thought that GRBs could be produced in only two ways. The collapse of a massive star shortly before it explodes in a supernova emits a long gamma-ray burst and continues for more than two fractions of a second (SN: 10/28/22). Or two dense stellar bodies, called neutron stars, collide, fuse and form a new black hole, releasing a short gamma-ray burst in a second or less.
But there was a certain manor. A surprising short GRB in 2020 appeared to come from the implosion of a massive star (SN: 8/2/21). And some long-term GRBs dating back to 2006 were missing supernova after the fact, raising questions about their origins.
“We’ve always known about LINO,” said Chryssa Kouveliotou of George Washington University in Washington DC, who wrote a 1993 paper that introduced two types of GRB, but was not involved in the new work. “There were some that remained that we did not know how to interpret.”
There is no such mystery about GRB 211211A: the burst lasted for more than 50 seconds and was clearly accompanied by a kilonova, a characteristic of the new elements created after a neutron star smashup.
“Although we suspected it was possible that the extended emission of GRBs was mergers … this is the first confirmation,” says astrophysicist Benjamin Gompertz of the University of Birmingham in England, who described the burst observations. Nature Astronomy. “He’s got a kilonova, that’s the smoking gun.”
NASA’s Swift and Fermi space telescopes detected an explosion on December 11, 2021, in a galaxy about 1.1 billion light-years away. “We thought it was a run-of-the-mill gamma-ray burst for a long time,” says astrophysicist Wen-fai Fong at Northwestern University in Evanston, Ill.
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It was a close second, as GRBs go. So that allowed Fong’s and Troy’s research group to continue to independently observe the eruption using ground-based telescopes, the teams reported. nature.
When no supernova appeared for weeks on end, researchers were confused. Their observations revealed that whatever caused the GRB also emitted much more optical and infrared light than is typical of the GRB’s long-lived origin.
After other developments, Troja and his colleagues compared the effects of bursts with the first kilonova, always observed in one of the circles of space, which are called gravitational waves.SN: 10/16/17). The match was almost perfect. “That’s what convinced many people, we were talking about kilonova,” he said.
Remember, it feels obvious to be a kilonova, Troja says. But at the moment, the arctic lion seemed impossible, he felt. “He looks like a lion, he roars like a lion, but he is not here, so that he is not,” he said. “This is exactly what we felt.”
Now the question is, what? Typically, collapsing neutron stars fall into a black hole almost immediately. Gamma rays flow from the superheated matter into the black hole, but the material is small, and the black hole reenters within two seconds. So how did GRB 211211A keep its light for almost a minute?
It is possible that the neutron stars first collapsed into a single, larger neutron star that briefly resisted the pressure to fall into the black hole. That has implications for the fundamental physics that describes how hard it is to crush 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. But the process of the black hole eating the neutron star took longer.
Or it could be something else entirely: a neutron star merging with a white dwarf, astrophysicist Bing Zhang of the University of Nevada, Las Vegas and colleagues suggest in nature. “We suggest a third type of progenitor, quite different from the previous two types,” he said.
White dwarfs are the remnants of smaller stars like the Sun, not as dense or dense as neutron stars. A collision between a white dwarf and a neutron star could still produce a kilonova if the white dwarf is supermassive.
The resulting object is a highly magnetic neutron star called a magnetar.SN: 12/1/20). The magnetar could continue to pump energy into gamma rays and other wavelengths of light, extending the life of the burst, Zhang says.
Whatever its origin, GRB 211211A has a lot of physics. “It is important because we wanted to understand, what on earth are these events?” Kouveliotou says.
Figuring out what happened could shed light on how heavy elements form in the universe. And some GRBs have been seen long before what scientists thought were actually supernovae from mergers.
To learn more, scientists need to find more from these binary busting GRBs, plus observations of gravitational waves at the same time. Trejo thinks he can achieve that when the Laser Interferometer Gravitational-Wave Observatory, or LIGO, comes back online in 2023.
“I hope there will be some LIGO document,” says Kouveliotou. “Nature would be nice and give us two opposites of gravity wave events, and maybe [help us] to understand what is going on. “
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