For years, mysterious blasts of radio waves coming from billions of light-years away have stumped scientists on Earth. Lasting a mere several thousandths of a second, the blasts – called fast radio bursts – appear randomly in the sky and are often discovered hiding in data sets, months or years after they’ve arrived on Earth. Scientists haven’t been able to figure out what the blazingly bright bursts are, alternately suggesting the culprits could be evaporating black holes, colliding dense objects or flaring dead stars, among other possibilities.
For a while, some even thought the enigmatic bursts were an artifact produced by life on Earth, rather than signals coming from outside our galaxy. (“Aliens” appeared to be the preferred explanation among readers of stories describing the mystery.)
Now, after studying how the incoming radio waves in a newly detected burst are twisted and scattered, a team of scientists has uncovered some crucial clues about the blast’s origin: It originated far, far away, in an area with dense, highly magnetized plasma – and traveled through two gas clouds before colliding with the Green Bank Telescope in West Virginia.
“It could be from a star-forming region, a supernova remnant, or the dense inner regions of a galaxy. But these all point toward a younger stellar population, a region that’s forming stars or where stars are dying and exploding,” says Kiyoshi Masui, of the University of British Columbia, who described the burst today in Nature. “There are a lot of models for what these fast radio bursts are. I wouldn’t make any strong bets on any one of them, but my favorite one is flares from magnetars,” he said, referring to a type of extremely magnetic, tempestuous neutron star.
Mining the Data
Masui and his colleagues found the burst, called FRB 110523, in data they’d collected while studying the large-scale structure of the universe. After becoming intrigued by fast radio bursts, the team decided to search for the brief-but-bright signals and wrote a computer program to sift through 650 hours of observations. The program came back with 6,496 candidate bursts – and the unlucky task of sorting through those by eye fell to Hsiu-Hsien Lin of Carnegie Mellon University, who easily identified the one real burst among the thousands of imposters.
The burst exploded on May 23, 2011 in the constellation Aquarius and lasted for roughly three milliseconds. Because of how the team was looking at the cosmos, the scientists were able to extract some important information about the burst’s origin. Mapping matter in the universe means getting detailed information about polarization, or how incoming radiation – such as light and radio waves – is oriented.
“They have to take very high quality, very highly calibrated data that includes full polarization information,” says astronomer Scott Ransom of the National Radio Astronomy Observatory. “That’s kind of overkill for most of the pulsar observations, which is where most fast radio bursts have been seen in the past.”
Hiding in that polarization data were some crucial clues. The radio waves had been twisted as they traveled through the cosmos, something that can only happen if they’d passed through a magnetic field. By measuring how twisted the waves were, the team could determine how strong the magnetic field was – and nothing in the Milky Way is strong enough to warp a radio wave to that degree.
“There’s just not enough magnetization there,” Masui says. “And along the line of sight, most of the distance between us and the burst is just really empty space…so the only thing left is that the magnetization came from the source itself.”
But there’s more. The team determined that in addition to originating near an intense magnetic field, the burst traveled through at least two clouds of ionized gas. As it did, the clouds scattered the radio waves and changed the shape of the burst, producing discernible signatures that only appeared when the team looked at the data in millionths-of-a-second intervals. The first of those clouds, Masui says, is at the signal’s origin; the second is somewhere in the Milky Way.
Lastly, the team figured out the burst couldn’t have traveled more than 6 billion light-years before arriving at Earth.
“Well, it could be between 6 billion and 100 million light-years away,” Masui says.
Astronomers who study these bursts say the team’s work is solid, and that the case for the signals coming from outside the galaxy is getting stronger.
“It’s amazing what they got out of such a small amount of data,” Ransom says. “If these things are really coming from outside of our galaxy, they’re just mindboggling – we just don’t understand them.”
Masui and his colleagues suspect the burst originated in a young, star-forming region in a distant galaxy. (But which galaxy? “There’s something like 100 candidate galaxies that it could be in – we don’t have any idea,” Masui says.) Star-forming regions are known for being dusty, turbulent and sporadically violent. Here, young stars ignite when the crush of gravity turns dusty lumps into nuclear furnaces, and the biggest, brightest stars live fast and die explosive deaths.
When some of those large stars die, their corpses turn into magnetars – young, highly magnetic, spinning neutron stars. These are incredibly dense, incredibly exotic objects with magnetic fields millions of times stronger than the strongest magnet we’d find on Earth. Occasionally, starquakes ripple through a magnetar’s crust and disrupt the dead star, producing enormous flares that emit intense gamma rays.
Now, astronomers suspect these flaring magnetars might also emit radio waves – and could be the culprits behind fast radio bursts.
“They are amongst the most powerful — apart from the sun, which happens to be next door — sources of high-energy radiation that we receive on Earth,” says Caltech astrophysicist Shrinivas Kulkarni, who doubted for years that the bursts came from outside the Milky Way.
Now, he says, the preponderance of the evidence suggests an extragalactic origin for the phenomena – a conclusion he published this week in a paper submitted to the arXiv.
“Every test I feel I’ve undertaken to show the bursts are nearby has failed,” he says.
In his recent paper, Kulkarni and colleagues took a close look at a burst that had been detected by the Arecibo Observatory in Puerto Rico. They independently reached conclusions that are very similar to those of Masui and his colleagues: That the burst came from outside the galaxy, in a region with dense, highly magnetized plasma – and that it could be the work of a magnetar.
So, while teams have only pulled this kind of information from two of the 16 known fast radio bursts, the results are still good news for scientists searching for the signals’ origin – a search that should become easier as a new generation of telescopes comes online.