Fast radio bursts (FRBs) are bursts of intense, short-lived radio waves originating from beyond the Milky Way that can emit the same amount of energy in just a few thousandths of a second as the sun takes three days to issue.
However, despite their power and the fact that around 10,000 FRBs could erupt in Earth’s skies every day, these radio wave explosions remain mysterious. One of the biggest puzzles surrounding FRBs is why most flash once and then disappear while a tiny minority (less than 3%) repeat the flash. This led scientists to search for the mechanisms that initiate FRBs. Some even believe that different celestial objects can produce repeating and non-repeating FRBs.
Scientists at the University of Toronto used the Canadian Hydrogen Intensity Mapping Experiment (CHIME) to focus on the properties of polarized light associated with 128 non-repeating FRBs. This revealed that unique FRBs appear to come from distant galaxies that closely resemble our own Milky Way, as opposed to the extreme environments that launch their repeating cousins. The results could allow scientists to finally solve the persistent celestial puzzle of FRBs.
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“So far, when we’ve thought about FRBs, we’ve only looked at them the same way we would look at a star in the sky, thinking about how bright it is, maybe determining how far away it is, things like that,” said lead author of the research, Ayush Pandhi, a Ph.D. student at the Dunlap Institute for Astronomy and Astrophysics and the David A. Dunlap Department of Astronomy and Astrophysics at the University of Toronto, told Space.com. “However, FRBs are special because they also emit polarized light, meaning that the light from these sources is oriented entirely in one direction.”
The main difference with this research is that it really looks at polarized light.
Polarized light is made up of waves oriented in the same way: vertically, horizontally or at an angle between these two directions. Changes in polarization could explain the mechanism that initiated the FRB and thus reveal what its source was. Polarization can also reveal details about the environments the FRB had to pass through before reaching our detectors on Earth. This study represented the first large-scale examination of 97% non-repetitive FRBs in polarized light.
There is a gap in research on non-repeating FRBs because it is much easier to observe repeating FRBs because astronomers already know where they will occur, meaning it is possible to point at any which radio telescope towards this part of the sky and wait. With non-repeating FRBs, astronomers must have a telescope that can observe a large portion of the sky at once, because they don’t really know where the signal will come from.
“They could appear anywhere in the sky. CHIME is unique in that sense because it examines such a large part of the sky at the same time,” Pandhi said. “Also, people haven’t really looked at 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-repeating FRBs, but this is the first time we’ve looked at more than 100. This allows us to reconsider what we think about FRBs and see how much FRBs are repetitive and non-repetitive. FRBs can be different.”
To repeat or not to repeat?
In 2007, astronomers Duncan Lorimer and David Narkevic, who was Lorimer’s student at the time, discovered the first FRB. This was a non-repeating burst of energy that is now commonly referred to as a “Lorimer explosion”. Five years later, in 2012, astronomers discovered the first repeating FRB: FRB 121102. Then, other repeating bursts were gradually revealed.
Astronomers naturally wonder if there is not a different phenomenon behind these two types of FRB. And Pandhi’s team did indeed find that non-repeating FRBs appear to be a bit different from repeating FRBs, because most of the former appear to come from galaxies like our own Milky Way.
Although the origins of FRBs are shrouded in mystery, these bursts of radio waves can act as messengers from the environments they pass through on their way to 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 less, it will spin less.
The fact that the polarization of non-repeating FRBs is lower than that of repeating FRBs indicates that the former appear to pass through less matter or weaker magnetic fields than the latter. Pandhi added that while repeating bursts of radiation seem to come from more extreme environments (like the remains of dead stars in supernova explosions), their non-repeating brethren seem to emerge in slightly less violent environments.
“Non-repeating FRBs tend to come from environments that have weaker magnetic fields or fewer things around them than repeating FRBs,” Pandhi continued. “So the repetition of FRBs seems to be a little more extreme in that sense.”
Are neutron stars safe?
One of the big surprises of this research for Pandhi was that the polarization of non-repeating FRBs seemed to rule out one of the main suspects behind their launch: highly magnetized, rapidly rotating 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 these things come from the same type of object, one might expect them to have some similarities, but it appears that they are actually quite different.
To determine which objects launch FRBs, Pandhi thinks expanding our understanding of the polarization of these radio wave bursts could help refine theoretical predictions.
“If we’re confused between several different theories, we can now look at polarized light and say, ‘Okay, does this rule out theories that we haven’t already ruled out?'” he said declared. “It provides another parameter, or even a few additional parameters, to help us rule out theories about what they might be until we have one that sticks.”
Pandhi went on to explain that this study laid the foundation for future FRB investigations; he himself is working on a way to disentangle the polarization of FRBs occurring in the Milky Way from those occurring 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 bursts of cosmic energy that ensures he’ll be studying 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?” » said Pandhi. “If you’re a bit of a detective who likes to solve mysteries, FRBs are just a mystery waiting to be solved.”
The team’s research was published Tuesday, June 11, in the Astrophysical Journal.