In 2017, astronomers picked up a mysterious signal from deep space. Within a few milliseconds, it flashed brightly in the RF spectrum and disappeared. But it turned out to be not so.
Subsequent observations showed that the signal was repeating, but it became 600 times weaker than the first burst. Repetition suggests that the strange radio bursts we continue to detect from space may be more active and more complex than we thought.
Flares are called fast radio bursts (FRBs), one of the most amazing phenomena in deep space. They show huge bursts of electromagnetic energy, as powerful as the radiation from hundreds of millions of Suns, in just milliseconds.
We don't yet know what causes them, or even where most of them come from; of the more than 150 detected bursts, only a few have been traced back to their galaxies of origin.
Not all radio bursts are the same. There are differences such as signal strength, the way it is polarized, and slight differences in duration. But one of the biggest differences is whether the signal repeats. Some yes. Most don't.
Or at least no repetitions were found.
“One of the main questions is whether they all repeat themselves or not. Although there are more than a hundred known bursts, until recently only one was found to be recurring, 'said astronomer Pravir Kumar of Swinburne University of Technology in Australia.
To find radio bursts and their repetitions, Kumar and colleagues used the Australian Array Pathfinder (ASKAP).
“We found 20 radio bursts and looked for replays with ASKAP for two years,” he said. 'In 12,000 hours we have not found a single one! However, is it possible that the repeats were too weak for ASKAP to detect? '
The focus of the study was a particularly bright single burst named FRB 171019. Although it was not traced back to its point of origin, the region of the sky where it came from was known; therefore, Kumar and his colleagues used some of the world's most powerful radio telescopes for follow-up observations.
Finding nothing with ASKAP, the researchers pointed the Green Bank telescope in the United States and the Parks Observatory radio telescope in Australia at that region of the sky.
Parks Observatory didn't find anything either. But in the data from the Green Bank Telescope, two weak signals appeared.
“When I first saw the signal on my computer screen, I just couldn't believe it,” Kumar said. 'It was really a very exciting moment!'
Last August, after Kumar's team detected a repeat signal and prepared their study, CHIME in Canada – an experiment scanning a lower frequency range than telescopes such as ASKAP – detected a third repeat signal from FRB 171019.
These results add to the total number of known repetitive fast radio signals and hint that some of the many other single signals detected so far may also have flashed repeatedly outside the detection range of the instruments that were used to track them.
But the great thing about FRB 171019 replay signals is that they are incredibly weak – about 590 times weaker than the surge detected by ASKAP. This is the largest difference in energy emission found in a repetitive, fast radio burst.
We still cannot say what caused this signal. A study last year found similarities to magnetar flares, with one major problem: signal sources must be 100 billion times brighter than magnetars.
“The fact that the signal [FRB 171019] is repeating gives us an excellent opportunity to pinpoint the location of its source using a radio interferometer,” said Pravir Kumar.
'The problem is that the repeats are very weak, so we need to use the most sensitive interferometer. We are fortunate to have been given time this year to verify the source on a very large array of radio telescopes, so stay tuned. '
The study was published in the Astrophysical Journal Letters.
