The mystery of fast radio bursts.
On the night of August 24, 2001, a powerful blast of radio waves washed over Earth. By mere chance, the 64-meter radio dish at Parkes Observatory in New South Wales, Australia, happened to be pointed in the right direction and scooped up the signal. Although the burst appeared more than 100 times brighter than ambient noise--at levels that saturated the observing system--it vanished after just 5 milliseconds. Astronomers didn't even notice the all-too-brief event.
That is, until years later. In 2006 David Narkevic (then an undergraduate at West Virginia University) was wading through 480 hours' worth of archived Parkes data when he stumbled upon the brilliant pulse. And it wasn't just bright. It was also smeared over a wide range of radio frequencies, with lower-frequency waves noticeably delayed, a signature that implied the radio waves had traveled some 3 billion light-years to Earth.
If that was really the case, then only something extreme--radiating as much energy in a few milliseconds as the Sun emits in thousands of years--could have sparked such a powerful signal. "We made the bold claim based on one object that it was the prototype of a new population," says Duncan Lorimer, Narkevic's advisor.
Now, 15 years after the first radio flash, astronomers have discovered more than a dozen so-called fast radio bursts (FRBs), and they suspect a new one arrives at Earth every 15 seconds. But for many years, the idea of brilliant radio flashes from mysterious and faraway sources fueled controversy.
A Brilliant Flash, Then Nothing
After that first, bright burst, astronomers expected to see fainter examples, but years went by with no other finds. Matthew Bailes (Swinburne University of Technology, Australia), who was also on the discovery team, had a nagging concern that the burst might not have been astrophysical at all.
"I remember I actually had trouble sleeping," he says. "It was a little bit worrying, because I couldn't imagine how you could have something so far away and yet not see other examples of it that were a lot fainter but still above our detection threshold.
"The Lorimer burst, as it came to be known, almost seemed too good to be true," Bailes adds.
Then in 2011, a series of wacky bursts threw astronomers into further confusion. Sarah Burke-Spolaor (then at Swinburne University of Technology and CSIRO's Australia Telescope National Facility) and team searched through archival data and found 16 radio bursts they dubbed perytons. Like the Lorimer burst, these brief pulses of radio waves smeared across frequencies, initially indicating faraway sources. But unlike the Lorimer burst, the perytons shone brightly in 13 adjacent fields --a telltale sign that the signals were more likely of terrestrial (and probably manmade) origin.
"The original burst was marred with suspicion," Lorimer recalls. Although the peryton events didn't look exactly like the Lorimer burst, they were similar enough to cast doubt. Even members of the discovery team, including Bailes, turned a skeptical eye toward the first event.
Lorimer's faith that the event was truly astrophysical never faltered, but his proposals to search for more bursts were rejected year after year. "I was trying to get tenure back then and ... I also needed a contingency plan," Lorimer says. While he admits that the daredevil in him would have loved to pursue FRBs full-time, he went back to his previous research to guarantee productivity. The buzz surrounding the Lorimer burst grew quiet.
But slowly, the tides began to turn. In 2013, seven years after the Lorimer burst was discovered, graduate student Dan Thornton (then at University of Manchester) was tasked with poring once again through Parkes' archives--and to everyone's surprise, four faint FRBs popped up. Then, Laura Spitler (Max Planck Institute for Radio Astronomy, Germany) and colleagues found one in real-time with the Arecibo Observatory in Puerto Rico. Kiyoshi Masui (University of British Columbia, Canada) and colleagues caught another one with the Robert C. Byrd Green Bank Telescope in West Virginia.
The new discoveries eliminated most doubts. But astronomers really started to relax when they traced perytons, which curiously tended to cluster around lunchtime, to the microwave oven in the Parkes Observatory's kitchen. Despite the unfortunately similar dispersion, that terrestrial source wasn't to blame for the Lorimer burst.
By early 2015 it became clear that FRBs were the latest astrophysical mystery waiting to unfold.
Flaring Stars Nearby, or Faraway Exotics?
Before astronomers could determine just what might spark such a short and violent burst, they had to determine where these bursts originated. If they called the Milky Way home, they might be run-of-the-mill sources. But if their light crossed vast cosmological distances, then they would require unknown phenomena, perhaps pushing the boundaries of physics.
The answer lay in the Lorimer burst's smeared appearance in frequency and time: its higher-frequency radio waves arrived roughly 300 milliseconds before their lower-frequency cousins. This so-called dispersion occurs when radio waves travel through clouds of plasma, such as those found between stars and between galaxies.
The speed of light may be a constant 300,000 kilometers per second (671 million mph) in a vacuum, but it's slightly slower in plasma. How slow depends on the light's frequency: lower-frequency photons interact more with free electrons, which slows their passage. By comparing the arrival of the same radio signal at low and high frequencies, astronomers can measure how many electrons lie between Earth and the source. Assuming those electrons are spread out randomly along our line of sight, a higher measure of dispersion means more electrons and therefore a more distant source.
Because the Lorimer burst and the following FRBs had dispersion measures that couldn't be explained by the electrons in our galaxy alone, most astronomers thought they had to originate at cosmological distances.
But Abraham Loeb (Harvard University and Harvard-Smithsonian Center for Astrophysics) wholeheartedly disagrees. Loeb argues that, while dispersion tells astronomers the number of electrons along the line of sight, it doesn't tell them where those electrons are located. So what if most of the electrons are in thick clouds around the source itself?
At first, Loeb thought that FRBs could be flares emitted from stars within our own galaxy. These stars' thick coronas could pack electrons tightly enough to have the same effect on passing radio waves as billions of light-years of the mostly empty intergalactic medium.
But few agree with Loeb's initial speculation--and even Loeb admits he enjoys going against the grain. Burke-Spolaor says, "It's important to have the contrarian in the mix to make everyone double-think their statistics." Nevertheless, she and many others are convinced that FRBs are extragalactic.
Here's why: although measuring dispersion can't tell astronomers exactly where the electrons are located, it can provide a pretty important clue. If those electrons are part of cold and sparse plasma, such as the stuff between stars, the delay decreases with frequency in a particular way (proportional to frequency squared). If, however, those electrons are part of the hotter and denser plasma found in a star's atmosphere, then the delay wouldn't follow that rule. But the smear of all FRB signals to date suggests that the pulses have mostly traveled through the sparser intergalactic medium.
The FRB detected at the Green Bank Telescope closed the distance debate. Not only did Masui and colleagues see that their FRB followed the dispersion measure expected for cold plasma, they also saw a subtle stretching out of the pulse's shape. This asymmetry implies the radio waves scattered off a thick envelope of plasma right after they were emitted. Whatever this envelope is, it's too big to support Loeb's idea of flare-prone stars--and it must lie in a distant galaxy.
Although Loeb agrees that his initial idea of flaring stars is out, he's still not sure these stars need to be cosmological. "The one thing to keep in mind ... is that we should be agnostic," he maintains. "The mistake that many people make is they jump into conclusions when the data are very scarce."
Placing FRBs on the Cosmic Map
To truly get a fix on an FRB's distance, astronomers need to pin it down to a host galaxy. It's a surprisingly hard task given that the Parkes receiver only sees details down to 14.4 arcminutes across--about half the angular size of the full Moon. Thousands of galaxies could fit in such a vast region. But if groups of radio telescopes working together could spot the burst, or its afterglow, they could narrow down where it came from. For example, the Australia Telescope Compact Array (ATCA), a collection of six 22-meter dishes, resolves regions as small as 1 arc-second across. Astronomers could easily pinpoint a host galaxy within that smaller area on the sky.
But first, astronomers need to catch an FRB in real time rather than in archived data--not an easy feat. In 2013 Parkes began sending all of its observations to "Green II," a supercomputer at Swinburne University of Technology. Within 30 seconds of a flash, the supercomputer pings 30 astronomers at 10 different institutions and alerts them to the burst's location so they can obtain follow-up observations.
The first several searches came up empty. But on April 18, 2015, the ATCA caught a slowly fading radio signal in an elliptical galaxy 5 billion light-years away. Initially, astronomers thought this signal was surely associated with an FRB that had occurred two hours earlier. If true, it would have proved that this FRB originated at a vast distance. It would even have hinted at a progenitor.
The study, however, was quickly disputed. Follow-up observations show the radio source is still there and has in fact brightened since the initial observations. It may be related to a supermassive black hole gobbling gas at the galaxy's center, but astronomers remain in limbo.
Nevertheless, with the field now poised to catch FRBs in action, and with the help of radio arrays that can image sources with much higher precision, it won't be long before astronomers pinpoint a source's location.
When Theorists Come Out to Play
Even as astronomers continue to hunt for more definitive observations, they can still speculate about what creates FRBs. There are three solid clues: the bursts are brief, bright, and probably far away.
To astronomers, a brief signal points to a small source. In the case of a pulse only a millisecond long, the source must be small enough that a beam of light could cross it within that time--several hundred kilometers across, or roughly the length of Maine's coastline. And if these sources are placed at cosmological distances, they would have to be extremely powerful. What source that small could pack so much power?
Theorists' favorite answer is neutron stars. These crushed stellar corpses pack the mass of half a million Earths into a sphere only tens of kilometers across. And they offer plenty of opportunities that could produce the right energetics, Lorimer says: "comets crashing into neutron stars, neutron stars inspiraling, neutron stars collapsing into black holes." The list goes on.
The FRB Masui detected with the Green Bank Telescope might narrow the range of possibilities. That FRB's radio waves rotated in a corkscrew motion as they traveled through space. It's a signature caused by a powerful magnetic field, quite possibly originating from the source itself. So magnetars--neutron stars with powerful magnetic fields--might be the culprit, producing flares when starquakes break the magnetar's brittle crust. Alternatively, collapsing stars may generate jets of electrons that race along magnetic field lines.
A more exotic idea that Lorimer finds particularly tantalizing is cosmic strings: these large defects in the structure of spacetime could distort, releasing bursts of energy. He admits the idea is highly speculative. But "we should still keep an open mind and not just closet ourselves to thinking that it's a neutron star or the merger model of a collapsed black hole," he says.
Yet James Cordes (Cornell University) thinks just the opposite. "I wish they were something truly exotic," he says. "Everybody would like them to be really exotic: evaporating black holes, cosmic strings, something like that. But that's not what I would bet on at this stage."
The reason it's still a betting game is that, at least until very recently, every FRB was a one-hit wonder. If we didn't catch it the first time around, it was lost forever.
That all changed this year, when astronomers detected 10 additional outbursts from the FRB first detected with Arecibo. The repeated pulse ruled out cataclysmic scenarios, at least for this particular FRB. An evaporating black hole, for example, would never burst again.
Instead, Spitler, who led the team, is sticking to the neutron star theory. She thinks these pulses came from a pulsar--a neutron star that's spinning rapidly--like the one sitting in the Crab Nebula deep within the Milky Way. Although pulsars typically send out regular pulses of radiation, some (including the Crab pulsar) are known to occasionally give off much stronger bursts.
The trouble is, to be spotted from faraway galaxies the pulsar would have to flare so powerfully it would be unlike anything ever seen before. The team is on the lookout for additional pulses that would help narrow down the burst's location and distance--and its source.
In the meantime, at least Bailes can sleep at night again: he admits that the repeating source is the one that finally convinced him FRBs really are astrophysical.
Cosmological Probes Unlike Any Other
Regardless of their source, as long as FRBs are extremely distant, astronomers suspect that they have the potential to become unique cosmological probes--opening a new window on the universe.
That's because if observers can find an FRB's host galaxy, and correspondingly the source's distance, then its smeared-out radio waves can divine the density of the intergalactic medium. Cataloged in sufficient numbers, FRBs could be used to scan electron density across billions of light-years, a sort of MRI of the universe. FRBs in relatively nearby galaxies would allow astronomers to account for all of the universe's visible matter.
FRBs in more distant galaxies would enable astronomers to scan dark matter and dark energy. Astronomers typically compute a host galaxy's distance by its redshift, the redward shift light makes as it travels across the universe. But translating this straightforward measurement into distance can get a little messy: the calculation involves complex mathematical models that invoke dark matter and dark energy. By calculating distance in a second way, using the FRB's dispersion measure, astronomers can get a handle on these cosmological parameters.
Not that it'll be easy. Astronomers will need thousands of FRB detections before they can start grappling with cosmology. "When I first saw that and realized we needed 1,000, I thought 'you're dreaming,"' Bailes says. But, he adds, if history tells us anything, then it might only take a decade or two to catalog that many.
But it might not take that long. "The race is on in various parts of the world to try to get more of these things," says Simon Johnston (Commonwealth Scientific and Industrial Research Organization, Australia). He expects that a precursor to the Square Kilometer Array in South Africa will see several FRBs a day by mid-2016. And the Canadian Hydrogen Intensity Mapping Experiment (CHIME) might see a few dozen a day a year later.
What began as a single controversial discovery has now become a full-fledged field. Students' PhD projects are devoted to the topic, astronomers are organizing their first FRB-dedicated meeting, and new telescopes are coming online to aid the search.
As for Lorimer, he can start studying these bursts full-time--a luxury he didn't have 10 years ago.
As a freelance science journalist, Shannon Hall spends her days pondering the wonders of the universe from a local coffee shop.
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|Title Annotation:||Faraway Flashes|
|Publication:||Sky & Telescope|
|Date:||Jul 1, 2016|
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