Fast radio bursts (FRBs) are intense, short-lived bursts of radio waves originating from outside the Milky Way that can emit the same amount of energy in just thousandths of a second as it takes the Sun three days to emit.
However, despite their power and the fact that up to 10,000 FRBs can erupt in the skies above Earth every day, these blasts of radio waves remain mysterious. One of the biggest puzzles surrounding FRBs is why most flashes once and then disappears, while a small minority (less than 3 percent) repeat the flash. This has prompted scientists to discover the mechanisms that launch FRBs. Some even believe that different celestial bodies can produce both repeating and non-repeating FRBs.
University of Toronto scientists used the Canadian Hydrogen Intensity Mapping Experiment (CHIME) to focus on properties of polarized light associated with 128 non-repeating FRBs. This showed that the one-off FRBs appear to have originated in distant galaxies that look a lot like our own Milky Way, unlike the extreme environments that launch their repeating cousins. The results could finally bring scientists closer to solving the lingering celestial puzzle of FRBs.
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"Until now, when we've thought about FRBs, we've just looked at them in the same way we would look at a star in the sky, thinking about how bright it is, and perhaps figuring out how far away it is , things like that," lead researcher Ayush Pandhi, a Ph.D. student at the Dunlap Institute for Astronomy & Astrophysics and the David A. Dunlap Department of Astronomy & Astrophysics at the University of Toronto, told Space.com "FRBs are, though special because they also emit polarized light, meaning the light coming from these sources is all focused in one direction."
The main difference with this research is that it really focuses on research into polarized light.
Polarized light consists of waves that are oriented in the same way: vertically, horizontally, or at an angle between these two directions. Changes in polarization could explain the mechanism that launched the FRB and thus reveal what its source was. Polarization can also reveal details about which environments the FRB had to pass through before reaching our detectors on Earth. This study represented the first large-scale look at the non-repeating 97% of FRBs in polarized light.
A gap has emerged in research on non-repeating FRBs because it is much easier to observe repeating FRBs because astronomers already know where they will occur. This means that it is possible to point any radio telescope at that patch of sky and wait. With non-repeating FRBs, astronomers must have a telescope that can look at a large portion of the sky at once, because they don't really know where the signal will come from.
"They can show up anywhere in the sky. CHIME is unique in that way because it looks at such a large part of the sky at once," Pandhi said. "Plus, people haven't really looked at that polarization yet, because it's much harder to detect just on a technical level.
"Other studies have looked at the polarization of perhaps ten non-repeating FRBs, but this is the first time we've looked at more than 100. It allows us to reconsider what we think FRBs are and see how repetitive and non-repeating FRBs may be different."
To repeat or not to repeat?
In 2007, astronomers Duncan Lorimer and David Narkevic, Lorimer's student at the time, discovered the first FRB. It was a non-repeating burst of energy now commonly referred to as the 'Lorimer Burst'. Five years later, in 2012, astronomers discovered the first repeating FRB: FRB 121102. After that, more repeating bursts gradually emerged.
Astronomers naturally wonder whether there is another phenomenon behind these two types of FRBs. And Pandhi's team did indeed find that non-repeating FRBs are a bit different from repeating FRBs, as most of the former appear to come from galaxies like our own Milky Way.
Although the origin of FRBs is shrouded in mystery, these bursts of radio waves may act as messengers from the environments they pass through as they race toward Earth. That information is encoded in their polarization.
"As the polarized light passes through electrons and magnetic fields, the angle at which it is polarized rotates, and we can measure that rotation," Pandhi said. "So as an FRB goes through more material, it will rotate more. As it passes through." less, it will rotate less."
The fact that the polarization of non-repeating FRBs is smaller than that of repeating FRBs indicates that the former appears to pass through less material or weaker magnetic fields than the latter. Pandhi added that while repeating radiation blasts appear to come from more extreme environments (such as the remains of stars that have died during supernova explosions), their non-repeating brethren appear to appear in slightly less violent environments.
"Non-repeating FRBs usually come from environments with weaker magnetic fields or less stuff around them than repeating FRBs," Pandhi continued. "So repeating FRBs seems a little more extreme in that sense."
(Image credit: Jingchuan Yu, Beijing Planetarium)
Are neutron stars off the hook?
One of the big surprises this research yielded for Pandhi was that the polarization of non-repeating FRBs appears to elucidate one of the main suspects behind their launch: highly magnetized, rapidly spinning neutron stars, or "pulsars."
"We know how pulsars work and we know the types of polarized light we expect to see from a pulsar system. Surprisingly, we don't see that much similarity between FRBs and pulsar light," Pandhi said. "If these things come from If they are the same type of object, you would expect them to have some similarities, but it looks like they are actually quite different."
When it comes to figuring out which objects launch FRBs, Pandhi thinks expanding our understanding of the polarization of these bursts of radio waves could help refine theoretical predictions.
"If we're confused between multiple different theories, we can now look at the polarized light and say, 'Okay, does this rule out theories that we haven't ruled out yet?'" he said. "It provides another parameter, or even a few extra parameters, to help us rule out theories about what they might be until we get one that sticks."
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Pandhi went on to explain that this study laid the foundation for future FRB studies; he himself is working on a way to disentangle the polarization of FRBs that occurred in the Milky Way from the polarization that occurred in their other galaxies and closer to the source of their emission.
This should help us better understand the mechanisms behind the launch of FRBs, but for Pandhi, it's the mysterious nature of these cosmic energy blasts that will keep him investigating them for some time to come.
"I mean, what's more mysterious than explosions happening thousands of times a day all over the sky, and you have no idea what's causing them?" Pandhi said. "If you're anything like a detective who likes to solve mysteries, FRBs are just a mystery begging to be solved."
The team's research was published Tuesday (June 11) in the Astrophysical Journal.
