Fast radio bursts (FRBs) are intense, short-lived bursts of radio waves from beyond the Milky Way and can emit in a thousandth of a second the same energy that it takes the sun three days to emit.
However, despite their power and the fact that approximately 10,000 FRBs may explode in the sky on Earth each day, these radio wave bursts remain mysterious. One of the biggest puzzles surrounding FRBs is why most of them flash once and then disappear, and a small minority (less than 3 percent) repeat the flash. This led scientists to seek to discover the mechanisms that initiate FRBs. Some even believe that different celestial bodies may produce both repeating and non-repeating FRBs.
Scientists from the University of Toronto used the Canadian Hydrogen Intensity Mapping Experiment (CHIME) to focus on the properties of polarized light associated with 128 non-repetitive FRBs. This revealed that single-shot FRBs originate from distant galaxies much like the Milky Way, as opposed to the extreme environments that eject their repeating cousins. The results may bring scientists closer to finally solving the unsolvable celestial riddle of FRBs.
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“So far, when we’ve thought about FRBs, we’ve just looked at them the same way we’d look at a star in the sky, figured out how bright it is, maybe figured out how far away it is, things like that,” said study lead author Ayush Pandhi, Ph.D. David A. Dunlap, a student at the Dunlap Institute for Astronomy and Astrophysics and the Department of Astronomy and Astrophysics at the University of Toronto, told Space.com. “But FRBs are special because they also emit polarized light, which means the light from these sources is all directed in a single direction.”
The main difference of this research is that it really digs into the investigation of 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 may explain the mechanism that initiates FRB and therefore reveal what its source is. Polarization can also reveal details about what environments the FRB must pass through before reaching our detectors on Earth. This study represented the first large-scale look at the nonrepeatable 97% of FRBs in polarized light.
There is a gap in non-repeating FRB searches because repeating FRBs are much easier to observe because astronomers already know where they will occur, meaning it is possible to point any radio telescope at that part of the sky and wait. Because of non-repeating FRBs, astronomers need to have a telescope that can look at a large area of the sky at once because they don’t really know where the signal will come from.
“They can appear anywhere in the sky. CHIME is unique in this sense because it looks at such a large portion of the sky at once,” Pandhi said. “Also people haven’t really studied this polarization yet because it’s much harder to detect just on a technical level.
“Other studies have looked at the polarization of maybe 10 non-repetitive FRBs, but this is the first time we’ve looked at more than 100. This allows us to reconsider what FRBs are and see how repetitive and non-repeating there are.” FRBs may differ.”
To repeat or not to repeat?
In 2007, astronomers Duncan Lorimer and David Narkevich, then students of Lorimer, discovered the first FRB. This was a non-recurring burst of energy now commonly referred to as the “Lorimer Burst”. Five years later, in 2012, astronomers discovered the first repeating FRB: FRB 121102. Then, more repeating bursts gradually revealed themselves.
Astronomers naturally wonder whether there is a different phenomenon behind these two types of FRBs. And Pandhi’s team indeed found that non-repeating FRBs look slightly different from repeating FRBs, since most of the former appear to come from galaxies like the Milky Way.
Although the origins of FRBs are shrouded in mystery, these bursts of radio waves can act as harbingers of the environments they pass through as they race towards Earth. This information is encoded in their polarization.
“If polarized light passes through electrons and magnetic fields, the angle at which it is polarized rotates, and we can measure this rotation,” Pandhi said. “So if an FRB goes through more material, it will spin more. If it goes through,” Pandhi said. The less it is, the less it will return.”
The polarization of non-repeating FRBs is less than repeating FRBs, indicating that the former passes through less material or a weaker magnetic field than the latter. Pandhi added that while recurring bursts of radiation appear to come from more extreme environments (such as the remnants of stars that die in supernova explosions), their non-repeating siblings appear in slightly less violent environments.
“Non-repeating FRBs tend to come from environments with weaker magnetic fields or less matter around them than repeating FRBs,” Pandhi continued. “So repeating FRBs seems a little bit more extreme in that sense.”
Are neutron stars out of the picture?
One of the biggest surprises this research provided for Pandhi was that polarization in non-repeating FRBs appeared to clear one of the biggest suspects behind their ejection: 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 much similarity between FRBs and pulsar light,” Pandhi said. “If you have the same type of objects you might expect some similarities, but it turns out they’re actually quite different.”
In terms of understanding which objects initiate FRBs, Pandhi thinks expanding our understanding of the polarization of these radio wave bursts could help narrow down theoretical predictions.
“If we get confused between multiple different theories, we can now look at the polarized light and say: ‘Well, does this rule out theories that we didn’t rule out before?'” he said. “This gives us another parameter, maybe even a few extra parameters, to help us eliminate theories about what these might be until we get a valid one.”
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Pandhi continued by explaining that this study paves the way for future FRB research; he is also working on a way to distinguish the polarization of FRBs occurring in the Milky Way from that occurring in other galaxies and those closer to their source of emission.
This should help us better understand the mechanisms behind the ejection of FRBs, but according to Pandhi, the mysterious nature of these cosmic energy bursts warrants studying them for some time to come.
“I mean, what could be more mysterious than explosions in the sky that occur thousands of times a day, but you have no idea what causes them?” Pandhi said. “If you’re a detective who likes to solve some mysteries, FRBs are a mystery just begging to be solved.”
The team’s research was published Tuesday, June 11, in the Astrophysical Journal.