What shines brighter than the Sun, appears for only a split second and lights up Earth’s skies thousands of times each day?
If you’re stumped, don’t worry—experts are too. For nearly a decade, astrophysicists have been struggling to explain perplexing millisecond chirps of radio waves pinging through the heavens. Now, several new studies are bringing researchers closer to solving the mystery by narrowing the search for the radio flashes’ origins to youthful stellar outbursts in distant galaxies.
Dubbed “fast radio bursts,” or FRBs, the first of these bright, brief events was announced in 2007 by the West Virginia University astrophysicist Duncan Lorimer and colleagues, based on data from the Parkes radio telescope in Australia. The radio signal that streamed into the Parkes dish was curiously smeared out, with its high-frequency waves arriving a fraction of a second earlier than its low-frequency counterparts—an effect attributed to scattering by diffuse plasmas that fill interstellar and intergalactic space. The more smeared a radio signal is, the more plasma it has passed through, and the farther it has presumably traveled through space. Analyzing the smear, Lorimer and his collaborators made a rough estimate that the burst could’ve come from up to a few billion light-years away. If they were in fact coming from so far away, and if more could be found, FRBs offered a way for astronomers to better measure vast cosmological distances and to probe deeper into the dark spaces between stars and galaxies. The search was on.
Based on small surveys of tiny patches of sky, since 2007 astronomers have published more than a dozen additional FRB detections, and are discussing another dozen or so that have yet to appear in the literature. Extrapolated across the entire sky, these meager results suggest FRBs are flashing overhead as often as ten thousand times per day. And if, as most researchers believe, FRBs originate far beyond our Milky Way, that means each one is releasing anywhere between an hour’s to a year’s worth of our Sun’s total energy output in the span of a few milliseconds.
Still, the sources for FRBs remain unknown. What could cause such intense, frequent events? Researchers have proposed so many answers over the years that there are now more theories for FRB origins than there are observed FRBs.
Too Many Theories
For a time, some skeptics favored earthbound explanations, guessing that the FRBs seen at Parkes were related to human-made interference from cell phones and other electronic devices that has long plagued the observatory; those concerns were dismissed earlier this year when astronomers at Parkes discovered that, unlike the randomly-timed arrivals of genuine FRBs, rogue terrestrial radio signals showed up in their data around lunchtime, produced by an errant microwave oven in the observatory break room. More far-out speculations have postulated that FRBs are caused by evaporating black holes, or by oscillating primordial cosmic strings, or by, yes, talkative aliens. None of these ideas, however, hold a great deal of weight with theorists: FRBs simply seem to occur too often to be explained by such suggestions.
Broadly speaking, the most compelling theories can be lumped into two broad categories: Either FRBs are caused by relatively rare explosive collisions between pairs of aging, degenerate stellar remnants like old neutron stars or white dwarfs, or they are caused by an assortment of far more common powerful, potentially periodic outbursts from younger, rapidly-spinning neutron stars called pulsars. Both groups of theories can account for the prevalence of FRBs, though they have different implications: collision-based models suggest FRBs are extremely energetic one-off events often taking place billions of light-years away, while outburst-based models suggest FRBs are less energetic, potentially repeatable, and would tend to be “only” hundreds of millions of light-years distant.
Finding the Source
Now, an FRB observed using the National Science Foundation’s Green Bank Telescope in West Virginia is strengthening the case that the bizarre phenomenon indeed originates in other galaxies, arising from youthful stellar outbursts rather than old stellar collisions. The findings are published in Nature (Scientific American is part of Nature Publishing Group).
The University of British Columbia astrophysicist Kiyoshi Masui and colleagues measured the FRB’s smeared-out radio waves, estimating its source to be as far as six billion light-years away. But they didn’t stop there—they also measured its polarization, its orientation as it propagated through space—as well as asymmetries in its smearing caused by traversing particularly dense regions of plasma. The FRB’s polarization was twisted like a corkscrew, indicating that the radio waves had passed through a powerful magnetic field, while its asymmetric smearing suggested it had also traveled through a “dispersing region,” a thick screen of plasma shortly after its emission—likely a clue to its emergence in another galaxy.
“Our main result is that the source of the burst is surrounded by a region with a magnetic field and dense gas, like that of a star-forming nebula, a supernova remnant, or galaxy’s central region,” Masui says. Such environments tend to harbor several varieties of young, highly active pulsars rather than old neutron stars and white dwarfs, leaving the former as the most probable progenitor for this particular FRB.
A second study, submitted to the Monthly Notices of the Royal Astronomical Society by the Max Planck Institute for Radioastronomy astrophysicist David Champion and colleagues, deals another blow to FRB models relying on one-shot collisions between old stars. The study reports the discovery of an FRB at Parkes that burst once, then burst again.
“We have one burst, then a gap of several milliseconds, then another equally large and short burst,” Champion says. A double burst is difficult to reconcile with neutron-star or white-dwarf mergers, which are thought to result in the near-instantaneous obliteration of both colliding objects—after the short, sharp burst generated by the collision, there wouldn’t be much left to generate a second follow-up burst.
Instead of through collisions, the double burst could be produced by an ultra-massive star collapsing to form a black hole, or a giant radio pulse from a standard pulsar, or a humongous flare produced by a “starquake” on a magnetar—a rare pulsar with an extremely powerful magnetic field.
A third study, uploaded by the California Institute of Technology astrophysicist Shrinivas Kulkarni and colleagues to the arXiv pre-print repository on November 30th, examines an FRB announced last year by a team using the Arecibo radio telescope in Puerto Rico. Like Masui and coworkers, Kulkarni’s team found telltale evidence in the FRB’s properties suggesting that it was emitted in the vicinity of a dense, magnetized region of plasma, probably a star-forming nebula in a distant galaxy. Kulkarni’s preferred culprit? A garguantuan outburst from a magnetar.
Keep It Simple?
Even so, certainty is elusive. “We’re considering rare events that we haven’t directly seen in our own galaxy,” Champion admits. “We’re looking at phenomena that we do see, and asking if they could rarely occur at much higher energies.”
For that reason, James Cordes, an astrophysicist at Cornell University, likes to keep his FRB theories simple. He prefers to explain FRBs as giant pulses from relatively run-of-the-mill pulsars rather than products of exotic magnetar starquakes. A pulsar in the nearby Crab Nebula regularly emits very large pulses, Cordes says, though no one has seen it produce one big enough to create an FRB—yet. “What’s nice about pulsars is that there are just a lot of them to play with,” Cordes says—more than a hundred quadrillion in the observable universe. With those numbers, only one giant pulse per neutron star every ten billion years could explain the estimated prevalence of FRBs. More realistically, however, such pulses can’t be seen clear across the universe, Cordes says. “That means you just need many more giant pulses per object, and what that implies is that these things have to repeat.” But, so far, no repeating FRBs have been found, though several teams are ardently seeking them along with their host galaxies.
The allure of simple, common sources for FRBs has led some researchers to propose they aren’t cosmological phenomena at all, and instead come from rather ordinary flaring stars within the Milky Way and other very nearby galaxies. But according to the Nature study co-author and West Virginia University astrophysicist Maura McLaughlin, such models are incompatible with her team’s measurement of an FRB’s asymmetric smearing by a thick region of plasma. The cloud of plasma around their FRB was at least ten times larger than the Earth-Sun distance, McLaughlin notes—an order of magnitude larger than the plasma sheaths that surround typical stars. “This rules out a flare-star origin,” McLaughlin says. “I do believe we are very close to solving this mystery.”
Other researchers aren’t so sure. Avi Loeb, a Harvard University astrophysicist and proponent of the flare-star model, says that the Nature study is by no means the end of the story. If the plasma that scatters an FRB’s radio waves is in fact near the FRB source, Loeb says, “then the original argument in favor of a cosmological origin—namely that the scattering is in intergalactic space—is lost.” No one has ever seen very large plasma clouds around a flare star, he acknowledges—but no one has ever seen the extremely large magnetar starquakes or giant pulses required by competing models, either.
“Nature is often more imaginative than theorists,” Loeb says. “I truly wish that FRBs come from exotic cosmological sources, but for now we must collect more data and test more conservative alternatives.” In November, Loeb and his student Ben Margalit submitted a paper to the Monthly Notices of the Royal Astronomical Society suggesting a new method for pinning down the distances to FRBs to help solve the mystery once and for all.
Lorimer, the researcher who helped discover FRBs in 2007, says more surprises are in store. The double-burst observed by Champion and colleagues was almost certainly not produced by colliding neutron stars, but theorists have already shown there seem to be many ways to make FRBs. “It could be that there are multiple populations out there,” Lorimer says, and researchers may soon find them, thanks to ramped-up searches using radio telescope arrays and opportunistic multi-wavelength observations with space telescopes. “What’s clear,” he says, “is that the whole field of FRBs is now exploding with possibilities.”