Astronomers have spotted a bright gamma-ray burst that overturns previous theories about how these energetic cosmic eruptions occur.
For decades, astronomers believed that there were two types of gamma-ray bursts: long and short, meaning they last longer than two seconds or disappear more quickly. Each type was associated with different cosmic events. But about a year ago, two NASA space telescopes recorded a short gamma-ray burst in the long form of a gamma-ray burst: it lasted a long time, but it came from a short gamma-ray burst source.
“We had this black-and-white vision of the universe,” says astrophysicist Eleanor of Troy at the University of Rome Tor Vergata. “That’s a red flag that tells us, no, it’s not. Surprise!”
This burst, named GRB 211211A, is the first to unequivocally disrupt the binary system, Troy and others report Dec. 7 in five papers in Nature and Nature Astronomy .
Before this burst was discovered, astronomers generally believed that there were only two ways to create a gamma-ray burst. The collapse of a massive star just before a supernova explosion can produce a long gamma-ray burst lasting more than two seconds. Or a pair of dense stellar corpses, called neutron stars, may collide, merge and form a new black hole, releasing a short gamma-ray burst lasting two seconds or less.
But there were some deviations. A surprisingly short GRB in 2020 appeared to be the result of the implosion of a massive star. And some long-duration gamma-ray bursts dating back to 2006 did not have a supernova after the fact, raising questions about their origins.
“We’ve always known there was an overlap,” says astrophysicist Chrissa Kuveliotou of George Washington University in Washington, DC, who wrote a 1993 paper outlining the two categories of GRBs but was not involved in the new work. “There were some abnormalities that we didn’t know how to interpret.”
There is no such mystery in GRB 211211A: the burst lasted more than 50 seconds and was clearly accompanied by a kilowatt, the characteristic glow of new elements that are formed after the breakup of a neutron star.
“Although we suspected that the gamma-ray bursts with extended emission were a merger … this is the first confirmation,” says astrophysicist Benjamin Gompertz of the University of Birmingham in England, who describes the observation of the flare in Nature Astronomy . “He has a kilonova which is a smoking gun.”
NASA’s Swift and Fermi space telescopes recorded an explosion on December 11, 2021 in a galaxy about 1.1 billion light-years away. “We thought it was a normal long gamma-ray burst,” says astrophysicist Wen-fai Fong of Northwestern University in Evanston, Illinois.
It was relatively close as GRBs. This allowed the research teams of Fong and Troy to independently continue to closely observe the explosion in great detail using ground-based telescopes, the teams report in Nature .
Weeks passed, and the supernova did not appear, researchers were puzzled. Their observations showed that whatever produced the gamma-ray burst also emitted much more optical and infrared light than is typical for a long gamma-ray burst source.
After ruling out other explanations, Troia and his colleagues compared the effects of the explosion to the first kilonova ever observed, along with ripples in space-time called gravitational waves. The match was almost perfect. “Then a lot of people were convinced that we were talking about kilonova,” she says.
In retrospect, it’s clear that it was a kilonova, says Troy. But at this point it was as impossible as seeing a lion in the Arctic. “It looks like a lion, it roars like a lion, but it’s not supposed to be here, so it can’t be,” she says. “That’s exactly how we felt.”
Now the question is what happened? As a rule, merging neutron stars collapse into a black hole almost instantaneously. The gamma rays come from material that is superheated as it falls into the black hole, but there is little material and the black hole absorbs it within two seconds. So how did GRB 211211A retain its light for nearly a minute?
It is possible that the neutron stars first merged into a single, larger neutron star that briefly resisted the pressure to collapse into a black hole. This has implications for the fundamental physics that describes how hard it is to squeeze neutrons into a black hole, Gomperz says.
Another possibility is that the 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 lasted longer.
Or it could have been something else entirely: a neutron star merging with a white dwarf, suggest astrophysicist Bing Zhang of the University of Nevada, Las Vegas, and his colleagues at Nature . “We propose a third type of progenitor that is very 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 can still produce a kilonova if the white dwarf is very massive.
The resulting object may be a strongly magnetized neutron star called a magnetar. The magnetar could continue to pump energy into gamma rays and other wavelengths of light, extending the life of the flare, Zhang says.
Whatever its origin, GRB 211211A is a big deal for physics. “It’s important because we wanted to understand what kind of events are these?” – says Kuveliotou.
Finding out why this is so could shed light on how the heavy elements in the universe form. And some previously seen long gamma-ray bursts, which scientists thought came from supernovae, could actually be the result of a merger.
To learn more, scientists need to find more of these binary gamma-ray bursts, as well as observe gravitational waves at the same time. Trejo thinks they’ll be able to get it when the Laser Interferometric Gravitational-Wave Observatory, or LIGO, goes live again in 2023.
“I hope that LIGO will provide some evidence,” says Kuveliotou. “Nature can be sophisticated and give us a couple of such events with analogues of gravitational waves, and maybe [допомогти нам] to understand what is happening.”
