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Examining a trove of radio telescope data in 2007, Duncan Lorimer, an astrophysicist at West Virginia University, discovered something unusual. Data obtained six years earlier showed a brief energetic burst, lasting no more than 5 milliseconds. Others had seen the signal and looked further afield, but Lorimer and his team calculated that it was a completely new phenomenon: a signal emanating from somewhere far outside the Milky Way.
The team had no idea what caused it, but they published their results in Science. The mysterious signal became known as a “fast radio burst” or FRB. In the 13 years since Lorimer’s discovery, dozens of FRBs have been discovered outside the Milky Way, some repeating and others short-lived, unique chirps. Astrophysicists have been able to identify their home galaxies, but have struggled to identify the cosmic culprit, coming up with all manner of theories, from exotic physics to alien civilizations.
On Wednesday, a trio of studies in the journal Nature describe the source of the first FRB discovered inside the Milky Way, revealing the mechanism behind at least some of the highly energetic radio bursts.
The recently described explosion, dubbed FRB 200428, was discovered and located after it pinged radio antennas in the US and Canada on April 28, 2020. A hasty search followed, with teams of investigators from around the world. world focused on studying FRB across the electromagnetic spectrum. FRB 200428 was quickly determined to be the most energetic radio pulse ever detected in our galaxy.
In the set of new articles, the astrophysicists describe their detective work and groundbreaking observations from a handful of ground and space telescopes. By linking concordant observations, the researchers place FRB 200428 in one of the most unusual wonders of the cosmos: a magnetar, the hypermagnetic remains of a dead supergiant star.
It is the first time that astrophysicists have been able to pinpoint the culprit of the intergalactic crime novel, but this is only the beginning. “There really is a lot more to learn in the future,” says Amanda Weltman, an astrophysicist at the University of Cape Town and author of a Nature news article accompanying the discovery.
“This is just the first exciting step.”
Under pressure
To understand where FRB 200428 begins, you must understand where a star ends.
Stars many times larger than the sun are known to experience disorderly death. Once they have used up all their fuel, physics conspires against them; its immense size puts unfathomable pressure on its core. Gravity forces the star to fold in on itself, causing an implosion that releases enormous amounts of energy in an event known as a supernova.
The wrinkled core of the star, born under extreme pressure, is left behind. Except it’s very small now, only the size of a city and about 1 million times denser than Earth. This stellar zombie is known as a neutron star.
Some neutron stars have extreme magnetic fields, about 1,000 times stronger than typical neutron stars. They are a mysterious and intriguing class unto themselves. Astronomers call them “magnetars” and they are just as curious as FRBs, with only about 30 discovered so far.
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One such magnetar in the Milky Way is officially known as SGR 1935 + 2154, which refers to its position in the sky. To make things easier, let’s call it Mag-1. It was first discovered in 2014 and is located about 30,000 light years from Earth. On April 27, 2020, NASA’s Swift Neil Gehrels Observatory and the Fermi Gamma-ray Space Telescope detected a spike in the X-rays and gamma rays emanating from Mag-1.
The next day, two huge North American telescopes, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Transient Astronomical Radio Emissions Study 2 (STARE2), captured an extremely energetic radio burst from the same region of space. : FRB 200428. The FRB and Mag-1 were in exactly the same galactic neighborhood. Or rather, they seemed to be in the same galactic house.
“These observations point to magnetars as irrefutable evidence for an FRB,” says Lorimer, lead author of the 2007 discovery of the first radio burst. Magnetars had previously been theorized as possible sources for FRBs, but the data provides direct evidence linking the two cosmic phenomena together.
However, just locating the burst with the magnetar doesn’t explain everything.
“Magnetars occasionally produce bright X-ray emission bursts,” says Adam Deller, an astrophysicist at Swinburne University in Melbourne, Australia, “but most magnetars have never emitted any radio emission.”
Do not stop me now
Mag-1’s association with FRB 200428 is just the beginning of a long-term investigation.
In the cosmic novel, astronomers have found the culprit, but they are not exactly sure of the murder weapon.
By studying the FRB, the researchers were able to determine that it was very energetic, but it paled in comparison to some previously discovered deep space FRBs. “It was almost as bright as the fainter FRBs we have detected,” says Marcus Lower, a Ph.D. in astronomy. at Swinburne University studying neutron stars. This suggests that magnetars may be responsible for Some FRB, but not all – some seem too energetic to be produced in the same way as FRB 200428.
Another article published in Nature on Wednesday shows researchers using China’s 500-meter aperture spherical radio telescope (FAST) to study Mag-1 during one of its X-ray bursts. The telescope did not pick up any radio emission from the magnetar during his outbursts. That means such an outburst, alone, is unlikely to be responsible for dropping highly energetic FRBs. “Clearly, not all magnetar X-ray blasts trigger a radio blast,” says Deller.
Deller also notes that FRB 200428 shows similar characteristics to those seen in repeating FRB from outside the Milky Way.
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This is important because, today, astronomers have observed two types of FRBs in other galaxies. There are those that come alive and disappear, and others that seem to repeat themselves with a regular rhythm. FRB 200428 looks like a repeater, but much weaker. Other observations made by the CHIME telescope in October detected more radio bursts from the magnetar, although this work has yet to be published.
However, there is still some uncertainty. “We cannot say with certainty whether magnetars are the sources of all FRBs observed to date,” Weltman notes.
Another question: How did Mag-1 generate the FRB? Two different mechanisms have been proposed.
One suggestion is that magnetars produce radio waves in the same way that X-rays and gamma rays do in their magnetosphere, the huge region of extreme magnetic fields that surround the star. The other is a bit more complex. “The magnetar could live in a cloud of material hanging from previous spills,” says Adelle Goodwin, an astrophysicist at Curtin University who was not affiliated with the study. This cloud of material, Goodwin points out, could be hit by a burst of X-rays or gamma rays, transferring energy to radio waves. Then those waves travel through the cosmos and ping Earth’s detectors like an FRB.
It is unclear what mechanism resulted in FRB 200428, or if something more exotic might be happening. Other researchers have suggested that FRBs can even be caused by asteroids colliding with a magnetar, for example. But one thing now seems certain: it is not the alien civilizations that are trying to contact us. I’m sorry.
Radio ga ga
There is still much work to be done to unravel the mystery of fast radio bursts.
For Deller, the hunt continues. Part of his work focuses on where FRBs originate. He says his team still needs to collect more data, but there is a possibility that repeated FRBs could inhabit different. types of galaxies of those FRBs that do not repeat themselves. Weltman notes that the search for other signals will also intensify, with astronomers looking for electromagnetic radiation and neutrinos that are generated from any FRBs produced by magnetar.
Research will ultimately change the way we view the universe. Duncan Lorimer notes that if FRBs can be definitively linked to neutron stars, it would provide a way to conduct a census of those extreme cosmic entities. Current methods cannot identify neutron star types with high specificity, but FRBs could change that. And FRBs are already changing the way we see things. A study published in Nature earlier this year used FRB to solve a decades-long problem about the “missing matter” of the universe.
Lorimer says that many of the predictions his team made after discovering the first FRB in 2007 “have come to pass in some way” and he always hoped that FRBs could become part of the mainstream. As the mysteries deepen, they have exceeded your expectations. They have become one of the most puzzling yet intriguing phenomena in astrophysics.
“It’s still a fascinating adventure,” he says.
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