Astronomers trace mysterious fast radio burst to extreme, rare star

Sifting through a trove of radio telescope data in 2007, Duncan Lorimer, an astrophysicist at West Virginia University, spotted something unusual. Data obtained six years earlier showed a brief, energetic burst, lasting no more than 5 milliseconds. Others had seen the blip and looked past it, but Lorimer and his team calculated that it was an entirely new phenomenon: a signal emanating from somewhere far outside the Milky Way.

The team had no idea what had 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 of the Milky Way -- some repeating and others ephemeral, single chirps. Astrophysicists have been able to pinpoint their home galaxies, but they've struggled to identify the cosmic culprit, putting forth all sorts of theories, from exotic physics to alien civilizations. 

On Wednesday, a trio of studies in the journal Nature describes the source of the first FRB discovered within the Milky Way, revealing the mechanism behind at least some of the highly energetic radio blasts.

The newly described burst, dubbed FRB 200428, was discovered and located after it pinged radio antennas in the US and Canada on April 28, 2020. A hurried hunt followed, with teams of researchers around the globe focused on studying the FRB across the electromagnetic spectrum. It was quickly determined that FRB 200428 is the most energetic radio pulse ever detected in our home galaxy. 

In the suite of new papers, astrophysicists outline their detective work and breakthrough observations from a handful of ground- and space-based telescopes. Linking together concordant observations, researchers pin FRB 200428 on one of the most unusual wonders of the cosmos: a magnetar, the hypermagnetic remains of a dead supergiant star. 

It's the first time astrophysicists have been able to finger a culprit in the intergalactic whodunit -- but this is just the beginning. "There really is a lot more to be learned going forward," 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 have to understand where a star ends.

Stars many times larger than the sun are known to experience a messy death. After they've exhausted all their fuel, physics conspires against them; their immense size places unfathomable pressure on their core. Gravity forces the star to fold in on itself, causing an implosion that releases huge amounts of energy in an event known as a supernova. 

The star's crumpled core, born under extreme pressure, is left behind. Except now it's very small, only about the size of a city, and around 1 million times more dense than the 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're a mysterious and intriguing class unto themselves. Astronomers call them "magnetars," and they're 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 nickname it Mag-1. It was first discovered in 2014 and is located around 30,000 light-years from Earth. On April 27, 2020, NASA's Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope picked up a spike in 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 Survey for Transient Astronomical Radio Emission 2 (STARE2) -- picked up an extremely energetic radio burst coming from the same region of space: FRB 200428. The FRB and Mag-1 were in the exact same galactic neighborhood. Or rather, they seemed to be in the same galactic house. 

"These observations point to magnetars as a smoking gun of an FRB," says Lorimer, lead author on the 2007 discovery of the first radio burst. Magnetars had been theorized as potential FRB sources previously, but the data provides direct evidence linking the two cosmic phenomena together.

However, just co-locating the burst with the magnetar doesn't explain everything.

"Magnetars occasionally produce bursts of bright X-ray emission," says Adam Deller, an astrophysicist at Swinburne University in Melbourne, Australia, "but most magnetars have never been seen to emit any radio emission."

Don't stop me now

Associating Mag-1 with FRB 200428 is just the beginning of a long-term investigation. 

In the cosmic whodunit, astronomers have found a culprit, but they're not exactly sure of the murder weapon. 

Studying the FRB, researchers were able to determine it was highly energetic but paled in comparison to some deep space FRBs previously discovered. "It was almost as luminous as the weakest FRBs we've detected," says Marcus Lower, an astronomy Ph.D. at Swinburne University studying neutron stars. This suggests magnetars may be responsible for some FRBs but not all of them -- some seem far too energetic to be produced in the same way FRB 200428 was.

Another paper in Nature on Wednesday sees researchers using China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) to study Mag-1 during one of its X-ray outbursts. The telescope did not pick up any radio emission from the magnetar during its outbursts. That means it's unlikely such an outburst, alone, is responsible for spewing highly energetic FRBs. "It's definite that not every magnetar X-ray burst fires off an accompanying radio burst," says Deller. 

Deller also notes that FRB 200428 shows characteristics similar to those seen in repeating FRBs from outside the Milky Way.

This is important because, at present, astronomers have observed two types of FRBs in other galaxies. There are those that  flash to life and disappear, and others that appear to be repeating with regular rhythm. FRB 200428 looks like a repeater, but much weaker. Further observations by the CHIME telescope in October detected more radio bursts from the magnetar, though this work hasn't yet been published.

All in all, there's still some uncertainty. "We cannot say for certain if magnetars are the sources of all of the FRBs observed to date," Weltman notes. 

Another question: How did Mag-1 generate the FRB? Two different mechanisms have been proposed. 

One suggestion is magnetars produce radio waves just as they do X-rays and gamma-rays in their magnetosphere, the huge region of extreme magnetic fields surrounding the star. The other is a little more complex. "The magnetar could live in a cloud of material hanging around from previous outflows," says Adelle Goodwin, an astrophysicist at Curtin University who was not affiliated with the study. This cloud of material, Goodwin notes, could then be slammed into by an X-ray or gamma-ray outburst, transferring energy into radio waves. Those waves then travel through the cosmos and ping Earth's detectors as an FRB. 

It's not clear which mechanism resulted in FRB 200428 -- or if something more exotic might be happening. Other researchers have suggested FRBs may even be caused by asteroids slamming into a magnetar, for instance. But one thing now seems certain: it's not alien civilizations trying to contact us. Sorry.

Radio ga-ga

There's still a great deal of work to be done in unraveling the mystery of fast radio bursts. 

For Deller, the hunt continues. Part of his work is focused on where FRBs originate. He says his team still needs to collect more data, but there's a chance that repeating FRBs may inhabit different types of galaxies from those FRBs which don't repeat. Weltman notes the search for other signals will also intensify, with astronomers looking for electromagnetic radiation and neutrinos that are generated from any magnetar-produced FRB. 

The investigation will, ultimately, change the way we see the universe. Duncan Lorimer notes that if FRBs can be definitively linked to neutron stars, it would provide a way to take a census of those extreme cosmic entities. Current methods can't identify neutron star types with great 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 FRBs to solve a decades-old problem about the universe's "missing matter." 

Lorimer says many of the predictions his team made after discovering the first FRB in 2007 "have been realized in some way" and he always hoped FRBs could become part of the mainstream. As the mysteries deepen, they've surpassed his expectations. They've become one of astrophysics' most perplexing but intriguing phenomena. 

"It continues to be a fascinating adventure," he says.

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