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| One possible origin for GRB 211211A, shown in this illustration, is a pair of compact stars merging (bright dots in the center) and emitting jets of radiation (green and purple beams). Heavy elements forming in the clouds of matter surrounding the stars emit light that is known as a kilonova. SAMUELE RONCHINI/GSSI 2022 |
Astronomers have spotted a bright gamma-ray burst that upends previous
theories of how these energetic cosmic eruptions occur.
For decades, astronomers thought that GRBs came in two flavors, long and
short — that is, lasting longer than two seconds or winking out more
quickly. Each type has been linked to different cosmic events. But about a
year ago, two NASA space telescopes caught a short GRB in long GRB’s
clothing: It lasted a long time but originated from a short GRB source.
“We had this black-and-white vision of the universe,” says astrophysicist
Eleonora Troja of the Tor Vergata University of Rome. “This is the red flag
that tells us, nope, it’s not. Surprise!”
This burst, called GRB 211211A, is the first that unambiguously breaks the
binary, Troja and others report December 7 in five papers in Nature and
Nature Astronomy.
Prior to the discovery of this burst, astronomers mostly thought that there
were just two ways to produce a GRB. The collapse of a massive star just
before it explodes in a supernova could make a long gamma-ray burst, lasting
more than two seconds. Or a pair of dense stellar corpses called neutron
stars could collide, merge and form a new black hole, releasing a short
gamma-ray burst of two seconds or less.
But there had been some outliers. A surprisingly short GRB in 2020 seemed to
come from a massive star’s implosion. And some long-duration GRBs dating
back to 2006 lacked a supernova after the fact, raising questions about
their origins.
“We always knew there was an overlap,” says astrophysicist Chryssa
Kouveliotou of George Washington University in Washington, D.C., who wrote
the 1993 paper that introduced the two GRB categories, but was not involved
in the new work. “There were some outliers which we did not know how to
interpret.”
There’s 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 being forged after a neutron star smashup.
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| This shows the glow of a kilonova that followed the oddball gamma-ray burst called GRB 211211A, in images from the Gemini North telescope and the Hubble Space Telescope.M. Zamani/International Gemini Observatory/NOIRLab/NSF/AURA, NASA, ESA |
“Although we suspected it was possible that extended emission GRBs were
mergers … this is the first confirmation,” says astrophysicist Benjamin
Gompertz of the University of Birmingham in England, who describes
observations of the burst in Nature Astronomy. “It 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 run-of-the-mill long gamma-ray burst,” says astrophysicist Wen-fai Fong of
Northwestern University in Evanston, Ill.
It was relatively close by, as GRBs go. So that allowed Fong’s and Troja’s
research groups to independently continue closely observing the burst in
great detail using telescopes on the ground, the teams report in Nature.
As the weeks wore on and no supernova appeared, the researchers grew
confused. Their observations revealed that whatever had made the GRB had
also emitted much more optical and infrared light than is typical for the
source of a long GRB.
After ruling out other explanations, Troja and colleagues compared the
burst’s aftereffects with the first kilonova ever observed in concert with
ripples in spacetime called gravitational waves. The match was nearly
perfect. “That’s when many people got convinced we were talking about a
kilonova,” she says.
In retrospect, it feels obvious that it was a kilonova, Troja says. But in
the moment, it felt as impossible as seeing a lion in the Arctic. “It looks
like a lion, it roars like a lion, but it shouldn’t be here, so it cannot
be,” she says. “That’s exactly what we felt.”
Now the question is, what happened? Typically, merging neutron stars
collapse into a black hole almost immediately. The gamma rays come from
material that is superheated as it falls into the black hole, but the
material is scant, and the black hole gobbles it up within two seconds. So
how did GRB 211211A keep its light going for almost a minute?
It’s possible that the neutron stars first merged into a single, larger
neutron star, which briefly resisted the pressure to collapse into a black
hole. That has implications for the fundamental physics that describes how
difficult 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. And
the process of the black hole eating the neutron star took longer.
Or it could have been 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, something 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 could still produce a kilonova if the white dwarf is very
heavy.
The resulting object could be a highly magnetized neutron star called a
magnetar. The magnetar could have continued pumping energy into 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 is
important because we wanted to understand, what on Earth are these events?”
Kouveliotou says.
Figuring out what caused it could illuminate how heavy elements in the
universe form. And some previously seen long GRBs that scientists thought
were from supernovas might actually be actually from mergers.
To learn more, scientists need to find more of these binary-busting GRBs,
plus observations of gravitational waves at the same time. Trejo thinks
they’ll be able to get that when the Laser Interferometer Gravitational-Wave
Observatory, or LIGO, comes back online in 2023.
“I hope that LIGO will produce some evidence,” Kouveliotou says. “Nature
might be graceful and give us a couple of these events with gravitational
wave counterparts, and maybe [help us] understand what’s going on.”
References:
B. Gompertz et al. The case for a minute-long merger-driven gamma-ray burst
from fast-cooling synchrotron emission. Nature Astronomy. December 7, 2022.
Doi: 10.1038/s41550-022-01819-4.
A. Mei et al Gigaelectronvolt emission from a compact binary merger. Nature.
December 7, 2022.
Doi: 10.1038/s41586-022-05405-7.
J. Rastinejad et al. A kilonova following a long-duration gamma-ray burst at
350 Mpc. Nature. December 7, 2022.
Doi: 10.1038/s41586-022-05390-w.
E. Troja et al. A nearby long gamma-ray burst from a merger of compact
objects. Nature. December 7, 2022.
Doi: 10.1038/s41586-022-05327-3.
J. Yang et al. A long-duration gamma-ray burst with a peculiar origin.
Nature. December 7, 2022.
Doi: 10.1038/s41586-022-05403-8.
C. Kouveliotou et al. Identification of two classes of gamma-ray bursts. The
Astrophysical Journal Letters. Vol. 413, August 1993, p. L101.
Doi: 10.1086/186969.
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Space & Astrophysics