In September of 2017, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) in British Columbia commenced operations, looking for signs of Fast Radio Bursts (FRBs) in our Universe. These rare, brief, and energetic flashes from beyond our galaxy have been a mystery ever since the first was observed a little over a decade ago. Of particular interest are the ones that have been found to repeat, which are even rarer.
Before CHIME began collecting light from the cosmos, astronomers knew of only thirty FRBs. But thanks to CHIME’s sophisticated array of antennas and parabolic mirrors (which are especially sensitive to FRBs) that number has grown to close to 700 (which includes 20 repeaters). According to a new study led by CHIME researchers, this robust number of detections allows for new insights into what causes them.
First detected in 2007, FRBs constitute one of the greatest mysteries facing astronomer today. While this phenomenon is incredibly powerful, temporarily outshining even the brightest galactic pulsars by a factor of about one million, they are also incredibly short-lived (lasting about a millisecond). Even though many have been localized to distant galaxies, astronomers are still not sure what accounts for them.
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That is not to say there aren’t a whole lot of theories, which range from them being the result of rotating neutron stars or the collapse of strange star crusts to evidence of extra-terrestrial activity. This latter theory is entertained in part because of the few cases where FRBs were found to repeat. No known natural phenomena can account for this, hence the speculation that it could be a form of communication.
This is the question that an international team led by Emmanuel Fonseca – a postdoctoral researcher in the Department of Physics at McGill University, and part of the McGill Space Institute – sought to address. For the sake of their study, the team relied on data from 9 new repeating FRB sources that were recently detected by CHIME to see what they could infer.
What they found from examining these repeaters confirmed something that astronomers have been theorizing for some time. Essentially, there are two populations of FRBs – repeating and non-repeating – which are likely to be caused by different phenomena and/or in different environments. This can be observed by measuring the level of dispersion, the pulse widths, and the magnetized environment around the FRB’s source.
In the case of dispersion, which is caused by the matter the FRB signals must pass through in order to reach us, the team found that the distribution was the same for repeaters and non-repeaters alike. What this suggests is that the two populations have similar distributions and originate in similar local environments.
When measuring the pulse widths, however, the team found that the widths are larger for repeaters than non-repeaters. From this, they inferred that the bursts from repeating sources are slightly longer in duration, which could also mean that the two populations have two different emission mechanisms. Last, they measured how light interacts with the magnetic environment (aka. Faraday rotation) around the burst sources.
In the case of two of the new repeaters, they found that their rotations measures were actually lower than the rather high measure obtained from the first known repeater (FRB 121101). This could suggest that both repeaters and non-repeaters originate from not-so-heavily magnetized environments. This would further imply that FBR 121101 was an anomaly, though that remains to be seen.
At this juncture, astronomers are still a long way from determining the causes of FRBs and whether or not they fall into distinct populations. But thanks to the rapid evolution taking place in this field, more and more are being detected all the time, thus increasing the likelihood of a major breakthrough!