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What Causes Mysterious Radio Bursts From Deep Space? (And Why Are They Always Blamed on Aliens?)

On April 27, the Swift Burst Alert Telescope—an instrument aboard the Neil Gehrels Swift Observatory satellite, launched into low Earth orbit in 2004—detected a sequence of gamma radiation bursts from the remnants of an ancient star within our galaxy, 30,000 light years from Earth.

The energy was emanating from a celestial body known as a soft gamma repeater, which emits a recurring pattern of gamma and x-ray radiation. Known as SGR 1935+2154 (the name indicating coordinates in the sky), it was the eighth soft gamma repeater discovered since March 5, 1979, when a wave of gamma radiation 2000 times the baseline tripped multiple sensors in our solar system, including two Soviet Venus probes, a joint German-U.S. sun probe and the Vela satellites put up by the Department of Defense to detect nuclear detonations.

But subsequent observations of SGR 1935+2154 revealed something new: phenomena previously recorded only from distant galaxies, providing data that may confirm the source of intensely "bright" radio signals, called fast radio bursts—or FRBs—that flare for mere thousandths of a second.

A few hours after detection, the Swift team reported a "forest of bursts" to The Astronomer's Telegram, where astronomers from around the world share newly observed phenomena. The next day, a Canadian radio telescope called CHIME—four half-pipe shaped antennas built over more than an acre of alpine field in British Columbia—registered a radio signal described as "a bright millisecond-timescale radio burst."

SGR 1935+2154 wasn't just emitting gamma rays, but also radio waves, from the opposite end of the electromagnetic spectrum.

"As the Earth rotates, the sky moves above the telescope, so as sources go in and out of CHIME's field of view, we detect them," astronomer Paul Scholz, part of the CHIME team at the Dominion Radio Astrophysical Observatory in B.C. told Newsweek. "Radio telescopes aren't perfectly sensitive just to the area where you point them, they also have low-level sensitivity all the way out to the horizon. So when this thing went off it was so bright that we detected it 23 degrees away from the point where CHIME was most sensitive. It lit up our telescope."

Fast radio bursts were first described by West Virginia University Professor of Physics and Astronomy Duncan Lorimer, who revealed a fast radio burst uncovered in archival pulsar survey data in a 2007 article for the academic journal Science. The FRB that came to be known as the "Lorimer Burst" prompted more questions than answers.

Lorimer and his co-authors speculated that this "entirely new class of radio source" may be produced "by exotic events at cosmological distances," proposing merging neutron stars or evaporating black holes in far-flung galaxies as potential sources for the brief, but incredibly powerful radio bursts. But a new theory soon emerged, connecting FRBs to a highly magnetized form of neutron star, known as a magnetar.

Since FRBs have a duration measured in milliseconds, they remain difficult to detect and are mostly picked up from cosmological sources halfway across the universe—typically recorded as incidental bursts among other radio sources. After discovering a fast radio burst in 2014, astronomer Laura Spitler, from the Max Planck Institute for Radio Astronomy in Bonn, Germany, estimated an average of seven bursts per minute, somewhere in the observable universe. But instruments aren't often pointed at the right place at the right time, so only a few hundred have ever been detected.

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The Parkes telescope in Australia, part of the Commonwealth Scientific and Industrial Research Organization, has been the source for many FRB discoveries. NASA/CSIRO/Shaun Amy

Launched in 2017, The STARE2 program (Survey for Transient Astronomical Radio Emission) planned to search for FRBs by surveying a broader swath of the sky, but sticking to our own galaxy. It worked as intended, when it too picked up the April FRB from SGR 1935+2154.

"Lo and behold, we found a mega-jansky burst," Shrinivas Kulkarni, a professor of astronomy and planetary science at the California Institute of Technology, told Newsweek.

While a graduate student Kulkarni discovered the first "millisecond pulsar." A pulsar is a spinning neutron star. A neutron star is what's left after a massive star, more than ten times the size of ours, explodes in a supernova, then collapses back into a dense core of neutrons Kulkarni compares to the size of Manhattan island. But it packs into that space more than our sun's mass, so to break free of its gravity in a spaceship you'd need to accelerate to half the speed of light. A millisecond pulsar is one of those, except:

"This pulsar spun around 642 times a second," Kulkarni said in a phone interview. "Faster than your kitchen blender."

According to a 2015 NPR profile, he also raises rabbits and "dreams of being a bartender."

Rather than an immense and sensitive radio telescope array searching narrow spans of distant galaxies, Kulkarni and astrophysics grad student Christopher Bochenek opted for a wider field of view and a lower gain, at the cost of subsequent radio frequency interference emitted by everything from airplane radar to radio bursts from the sun. To filter out terrestrial sources they constructed three antennae, each about the size of a trash can, then installed them hundreds of miles apart—one at Caltech's Owens Valley Radio Observatory, a second near Barstow, California and the third in the small city of Delta, Utah. Get a simultaneous hit from a bright object in the sky at all three locations and you can be sure it's not coming from Earth.

Whereas CHIME picked up the fast radio burst as if out of the corner of its eye, STARE2 captured FRB 200428 (Fast Radio Burst YYMMDD) closer to head on, detecting more than 1.5 million janskys—a radio astronomy unit measuring the amount of power received by a surface (what we'd think of as "brightness" in the context of the visual light portion of the electromagnetic spectrum). When measured from Earth, most cosmological radio sources aren't very bright, but for a millisecond or more fast radio bursts can outshine whole galaxies.

"They are tremendously, tremendously bright," Kulkarni said. "Just unimaginable brilliance."

Further readings from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Guizhou province, China corroborated SGR 1935+2154 as the source of the fast radio burst. Altogether, the wave of international observations of SGR 1935+2154 and fast radio burst 200428 not only confirmed the first FRB ever detected within the Milky Way, but drew a line back to one likely source of the phenomena: the magnetar soft gamma repeaters. While STARE2 is still preparing its findings for publication, their April 29 post, seconding the CHIME observation on The Astronomer's Telegram, noted where the shared data pointed: "We conclude that active magnetars are a source of FRBs at extragalactic distances. We encourage follow-up observations."

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Radio telescopes around the world registered the FRB from SGR 1935+2154, including the Five-hundred-metre Aperture Spherical Radio Telescope (FAST) in Pingtang, in southwestern China's Guizhou province. STR/AFP via Getty Images

Magnetars and the fast radio bursts they emit are unique in the history of radio astronomy. In 1932, physicist and Bell Telephone Laboratories engineer Karl Jansky discovered the first deep space radio signals with a 100-foot-long rotating antenna, while trying to find the source of static interfering with trans-Atlantic phone calls. Jansky picked up a repeating signal—"a steady hiss type static," Jansky described in one of his papers—from the dense heart of our own galaxy. We now believe Jansky's signal marks the location of a supermassive black hole at the Milky Way's center, hidden from our sight wavelengths by clouds of cosmic dust.

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This composite image of the Crab Nebula combines radio waves, x-rays, infrared and visible light to reveal the energy still emanating from supernova remnants. At its center is the Crab Pulsar. NASA, ESA, NRAO/AUI/NSF and G. Dubner University of Buenos Aires

Radio astronomy turned up a universe full of radio sources, like the post-supernova remains of Cassiopeia A, splashed apart across 10 light years, or emanating from Jupiter's magnetic field and the crab nebula. There's even the pervasive relic microwave radio, left behind after the Big Bang and filling the space between stars. But the millisecond fast radio bursts are so different from every other galactic and cosmological radio source that only something as strange as magnetars might explain them.

"The store of energy for a pulsar is its rotation. But for a magnetar, the store of energy is its magnetic field," Kulkarni said. "Magnetars are the most magnetized objects in the universe."

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An infrared image of a mysterious ring around magnetar SGR 1900+14, taken by NASA's Spitzer Space Telescope. NASA/JPL-Caltech

Unlike the dipole configuration we're used to from toy magnets and our north and south poles, the magnetic field of a magnetar may not be so evenly balanced, creating the conditions for a fast radio burst. While the exact mechanisms for creating FRBs will be a focus for future research, Kulkarni described some of the powerful processes at play.

"There's a tremendous amount of strain inside this magnetar. The magnetic field is so powerful it's literally trying to reshape the star. However, gravity is more powerful. So the gravity keeps the star a nice, round sphere. So the big tension inside the magnetar releases like an earthquake," Kulkarni described. "So underneath, slowly, tension builds up, as the magnetic field is trying hard to push, but gravity is saying 'No.' Eventually, there's a rupture, just like on Earth. Instead of an earthquake, there's a magnetic quake. We believe it's in these magnetic quakes that fast radio bursts are emitted."

It's an elegant explanation, supplanting the more popular alien in the room.

Because we most often think of radio as a means of communication, deep space radio signals are intertwined with the possibility of extraterrestrial civilizations in the public imagination. When Karl Jansky's "mysterious static" from the center of the Milky Way hit the front page in May 1933, The New York Times made sure to confirm "no evidence of interstellar signaling."

Fast radio bursts have received similar treatment. Stories speculate whether they may be extraterrestrial emissions, or emphasize their mysterious qualities. "Gotta be aliens!" "Did Ancient Aliens Send Us Messages?" "Could be aliens," headlines read after new fast radio burst discoveries are announced.

"For fast radio bursts, and this would apply to any unexplained phenomena in radio astronomy, the problem with the mystery of FRBs isn't that we can't think of what could cause it. It's that we can think of many different natural phenomena that could power a fast radio burst, we just haven't been able to determine which ones are causing it," Scholz told Newsweek. "Because we have all those natural phenomena, there's really no reason to bring in extraterrestrial intelligence. That's a much more difficult theory to fit to the data."

But the connection between alien life and radio astronomy isn't completely unfounded either. Because human civilization emits radio waves out into space, it's reasonably surmised that extraterrestrial civilizations might do the same, making radio astronomy the backbone of programs like the Search for ExtraTerrestrial Intelligence (SETI). Even the Wow! Signal—the only compelling extraterrestrial candidate in the history of SETI—was logged on the same Big Ear Radio Observatory previously used in the groundbreaking Ohio Sky Survey, which identified more than 11,000 previously uncataloged extragalactic radio sources.

On August 15, 1977, astronomer Jerry Ehman found a signal which emanated from the same direction as a cluster of stars in the Sagittarius constellation—most approximately a giant orange star (larger but cooler than our own) 122 light-years away called Tau Sagittarii. Marked 6EQUJ5 on the printout (measurements denoting a high intensity narrowband signal) and marked by Ehman with the word "Wow!" in red ink, the signal appeared in a portion of the electromagnetic spectrum long theorized as the most likely range for extraterrestrial contact, due to minimal radio interference and its symbolic proximity to a frequency emitted from hydrogen, the most abundant element in the universe.

"The 'Wow!' signal is highly suggestive of extraterrestrial intelligent origin but little more can be said until it returns for further study," The Ohio State University Radio Observatory director John Kraus wrote in a summary sent to Carl Sagan in 1994. It hasn't been heard again.

But instead of strengthening the case for extraterrestrial explanations for deep space radio mysteries, the anomalous Wow! Signal instead provides a clear benchmark for why most radio signals are not the conscious transmissions of extraterrestrials. Unlike FRBs, the Wow! Signal matches with what astronomers had anticipated from an extraterrestrial civilization interested in communicating (even if it didn't transmit any detectable information). In most cases, extraterrestrials act as a placeholder hypothesis—possible but implausible—in the public imagination, until a better explanation comes along.

A good example of this came in 1967, when astrophysicist Susan Jocelyn Bell Burnell, a research student at the time, discovered Little Green Men on the 96 feet of chart paper spit up by a multi-acre radio telescope array constructed near the University of Cambridge. The repeating pattern, what she called "scruff," revealed a point source emitting a radio pulse every one and a third second. After eliminating interference from human sources, Burnell and radio astronomer Antony Hewish were left with a clockwork signal and no non-artificial explanation, hence the designation LGM-1, for little green men. Instead, they had discovered pulsars.

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British astrophysicist Jocelyne Bell Burnell, discoverer of pulsars, standing in front of a radio telescope in France in 2019. Photo by GUILLAUME SOUVANT/AFP via Getty Images

"We did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission," Burnell said, in an after-dinner speech delivered at the Eighth Texas Symposium on Relativistic Astrophysics in 1976. "We did not solve the problem that afternoon, and I went home that evening very cross. Here was I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us."

Perhaps one day we will hear a radio signal from an extraterrestrial civilization, but until then headlines touting radio astronomy discoveries as possible alien life are putting a proxy explanation before the vast array of natural phenomena still capable of surprising us. With pulsars, the explanation empowered whole new areas of study. It's looking like this will be true of magnetars as well.

"The space between galaxies and the space within galaxies is occupied respectively by the intergalactic medium (with a density of maybe a thousandth particle per cubic centimeter) and the interstellar medium (maybe one particle per cubic centimeter)," Kulkarni told Newsweek, describing how radio signals are slowed below the speed of light by the resulting plasma interference.

Unlike bright interstellar objects, the comparatively inert interstellar and intergalactic medium is incredibly difficult to study. Magnetars might help change that, opening up our understanding of the space between stars and galaxies. Higher frequencies in a fast burst radio signal experience less interference, creating what's called "chirp" in the gap between less and more obstructed frequencies. The chirp created by FRBs (and pulsars) can be used to measure the density of electrons along the beam's path to Earth, almost like a galactic core sampler.

"FRBs can now become the new digital meter of intergalactic plasma," Kulkarni said. "They're a way to study the intergalactic medium. Now astronomers can use fast radio bursts to understand a frontier field in astronomy."

What Causes Mysterious Radio Bursts From Deep Space? (And Why Are They Always Blamed on Aliens?) | Tech & Science