Fast Radio Bursts: How close are we to decoding these strange space signals?
Mysterious radio signals from space have been coming in from all directions, and their source is still unknown. Extremely bright and short-lived, these fast radio bursts (FRB) have scientists puzzled, but the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope may help bring some answers to light. New Atlas spoke to Keith Bannister, an astronomer on the project, about what might be behind these strange signals and how the mystery might be unraveled.
Located in an isolated part of Western Australia, far from the radio interference of the city, one of ASKAP's main goals is to study FRB, and it picked up its first one just four days after it began the search, in January this year. Designated FRB170107, the signal originated from the constellation of Leo, and traveled some six billion light-years to get here.
What are fast radio bursts?
"Scientists aren't very original at naming things: they're fast, and they're radio, and they're bursts," Bannister explains to us. "So they're radio, you detect them with a radio telescope that looks like a big satellite dish. And they're fast – really fast. Click your fingers and they're gone, less than a millisecond usually in duration. And they're bursts. They happen once and that's the last time we see them."
The first FRB was identified in 2007, when scientists trawling through archival data from 2001 came across a short, sharp burst that couldn't be explained. Over the next few years, more of these anomalies were discovered in existing data, but the phenomenon wasn't detected live until 2015. In total, less than 30 of the signals have been identified so far.
What could be causing them?
Generally, astronomers aren't sure what could be giving off these signals, but they seem to have the most in common with pulsars and magnetars. These two types of neutron stars both have extremely large magnetic fields and give off similar radio bursts, but the difference is that they repeat themselves. Some are as regular as clockwork while others have more unpredictable patterns, but either way they usually happen more than once. FRB, on the other hand, are largely one-time events, and that leaves scientists scratching their heads over their origins.
"There are more theories than there are bursts," says Bannister. "They range from everything from scaled-up versions of what we've seen in our own galaxy, like a pulsar or a magnetar, to things that are much more exotic: someone proposed it's the signal of what alien civilizations look like when they're starting to explore their own galaxy."
While these outlandish ideas often pop up to help explain space mysteries, the general consensus tends to lean toward something a little more mundane.
"The community at the moment hasn't really settled on anything," says Bannister. "But the more conservative theory people are saying it's probably got to be something to do with a neutron star. So you can think of a pulsar as being a neutron star with a huge magnetic field, and then a magnetar is a neutron star with an even bigger magnetic field. So maybe if you can make one that's got an even bigger magnetic field, then eventually it can make FRB. That I think is probably the leading option, but it's not settled by any stretch."
How could astronomers solve the mystery?
The first steps toward answering the question of what FRB are is simply to find more of them. The more data there is to work with, the clearer the picture gets, and being able to detect them faster means more telescopes can be trained on that patch of sky to see what possible points of origin are lurking there.
"One big leap forward will be when a fast radio burst happens, to really pinpoint its location in the sky," says Bannister. "Then you can go and look and see if there's a galaxy there, and that will tell you a lot. So if it turns out FRB come from certain types of galaxies, then we can use what we know about those galaxies to try and work out what FRB are. We'll go back and we'll look for repeats. We take optical images, we look for x-ray images or gamma ray bursts, or all sorts of stuff. So once you start finding them, you can go back and look in that particular part of the sky with lots of different types of telescopes, and see what you can see."
By the same token, the more FRB that are studied, the more likely it becomes that a particularly enlightening one will be spotted.
"When you look at enough bursts, you never know when one of them is so unusual it kind of unlocks the whole thing," says Bannister. "So in that sense, jut collecting more sometimes is helpful because every now and then you'll find one that's a real oddball, and that actually turns out to be the key to understanding the whole bunch."
In fact, the oddball in question may have already been discovered. FRB121102 was first found by the Arecibo radio telescope in Puerto Rico, and so far it's the only fast radio burst to buck the trend of being a one-hit wonder, instead pulsing at least 16 times since its discovery in 2012. Whether it confounds the mystery or helps solve it remains to be seen.
"That's the oddball of the family," says Bannister. "We call it the Repeater, because there's only one. That's even more puzzling than the phenomenon of fast radio bursts. We think they're probably connected, but we don't exactly know how."
What else can FRB teach us?
Besides the thrill of nailing down a potentially new type of celestial object, FRB can give us a new understanding of our little corner of the Universe. They travel an extremely long distance through space before they reach us, and when they get here they carry with them clues about their journey.
"The key property of radio bursts in general, from pulsars and fast radio bursts, is that we can measure what's called the dispersion," says Bannister. "So the radio waves as they leave the pulsar or fast radio burst, they go at different speeds, and that's because of the matter that the radio waves are going through. When they get to our telescopes, the short wavelengths arrive first and the long wavelengths about a second afterwards, and that time delay tells us how much matter those waves have gone through."
By that system, when a fast radio burst is found to have a large dispersion, that indicates that the radio waves have passed through a lot of electrons. Astronomers can then compare that data with what they know about that region of the sky, and if there aren't many galaxies in its path, it tells them that the signal must have come from a long way away. When a fast radio burst goes off, studying its dispersion can tell scientists just how much matter there is along that line of sight, and with enough data, a 3D map of the Universe can effectively be built up.
How can ASKAP help?
ASKAP began searching for FRB in the first week of January this year, and within four days it had spotted its first signal. Since then two more FRB have been detected, and that was done using just eight of its eventual 36 dishes. Those dishes, each 12 m (40 ft) in diameter, will all point in slightly different directions, giving the array a wide viewing area, like a segmented fly's eye.
"This thing will see 36 different patches of the sky at the same time," says Bannister. "That's a huge increase in the amount of sky you can see, and that makes a huge difference in how many FRB you can catch every day."
The rest of these dishes are due to come online over the next 12 to 18 months, and when that happens, the number of FRB the system spots should increase dramatically.
"I think the next 12 months will be a pretty exciting time, because there are a lot of telescopes, like ASKAP, that are just starting up, and once they're really running, there'll be a lot of new information coming in," says Bannister.
A paper describing the ASKAP team's discovery was published in The Astrophysical Journal Letters.