The mystery of fast radio bursts (FRB) from space may be a step closer to solving. Astronomers studying a repeating signal from a nearby galaxy have detected radiation at the lowest frequency of any FRB ever found, providing new possible allusions to its origin.
FRBs are exactly how they sound – bursts of radio signals that only last milliseconds. Ever since they were first detected more than a decade ago, they have flowed in from all corners of the sky, with each detection either deepening the mystery or bringing new clues as to what might be causing them – or sometimes both at the same time.
Some of them are one-off events, while others seem to be repeated either randomly or on a predictable schedule. Studying the radio waves they emit gives other hints of the environment in which they are produced – some seem to come from quiet settings, while other signals are twisted and polarized so as to suggest interference from powerful magnetic fields.
Now, in a couple of studies, astronomers have detected new details that may contribute to solving the mystery. Both focused on a signal called FRB 180916, first detected in 2018 and tracked to a galaxy just 500 million light-years away. It repeats itself like a clockwork on a 16-day cycle, chirping actively for four days before falling silent for the next 12 days.
In the first study, astronomers examined the object with two different radio telescopes – CHIME in Canada, which is regularly used to study FRBs, and the Low Frequency Array (LOFAR) in the Netherlands. With the latter, the team detected 18 bursts at frequencies between 110 and 188 MHz, much lower than any seen by FRBs before.
“We detected fast radio bursts up to 110 MHz, where before these bursts only existed up to 300 MHz,” says Ziggy Pleunis, lead author of the study. “This tells us that the region around the source of the explosions must be transparent to low-frequency emission, while some theories have suggested that all low-frequency emissions would be absorbed immediately and could never be detected.”
Interestingly the team also noticed a significant delay between frequencies. The higher frequencies consistently reached CHIME three days before the lower ones were detected by LOFAR.
“At various times we see radio bursts with different radio frequencies,” says Jason Hessels, co-author of the study. “Perhaps the FRB is part of a binary star. If so, we would have a different view at different times from where these extremely powerful explosions are generated.”
In the second study, another team of astronomers examined FRB 180916 with a higher “time resolution” than ever before, measuring more regularly than other studies. They found that the polarization of the explosions varied from one microsecond to the next, which they hypothesize could be the influence of a “dancing” magnetosphere, like the one around a neutron star.
This adds weight to the main theory of where FRBs come from: magnetarians, a kind of neutron star with an extremely strong magnetic field. The clearest smoking gun came last year when FRB-like signals were discovered by a magnet in our own galaxy.
The more we study these strange signals, the more likely we are to fall on a clue that reveals the whole mystery. The researchers say it is possible for FRBs to transmit even lower frequencies at which they have not yet been studied, and future work will attempt to detect these.
The study LOFAR was published in the Astrophysics Journal Letters, while the time resolution study appeared in Natural Astronomy.
Sources: McGill University, JIVE