The mystery of fast radio bursts (FRBs) from space may be a step closer to being solved. Astronomers studying a repeating signal from a nearby galaxy have detected radiation at the lowest frequency of any FRB found so far, providing new potential hints about their origin.
FRBs are exactly what they sound like – bursts of radio signals that only last milliseconds. Ever since they were first detected over a decade ago, they’ve poured in from all corners of the sky, with each detection either deepening the mystery or bringing new clues about what might be causing them – or sometimes both at once.
Some of them are one-off events, while others appear to repeat either randomly or on a predictable schedule. Studying the radio waves they give off provides other hints about the environment they’re being produced in – some appear to come from calm settings, while other signals are being twisted and polarized in a way that suggests interference by powerful magnetic fields.
Now, in a pair 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 traced back to a galaxy just 500 million light-years away. It repeats like clockwork on a 16-day cycle, chirping actively for four days before falling quiet for the next 12.
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, far lower than any seen from FRBs before.
“We detected fast radio bursts down to 110 MHz where before these bursts were only known to exist down to 300 MHz,” says Ziggy Pleunis, lead author of the study. “This tells us that the region around the source of the bursts must be transparent to low-frequency emission, whereas some theories suggested that all low-frequency emission would be absorbed right away and could never be detected.”
Intriguingly, the team also noticed a significant delay between frequencies. The higher frequencies consistently arrived at CHIME three days before the lower ones were detected by LOFAR.
"At different times we see radio bursts with different radio frequencies,” says Jason Hessels, co-author of the study. “Possibly the FRB is part of a binary star. If so, we would have a different view at different times of where these enormously powerful bursts are generated.”
In the second study, another team of astronomers examined FRB 180916 in higher “time resolution” than ever before, taking measurements more regularly than other studies. They found that the polarization of the bursts varied from one microsecond to the next, which they hypothesize could be the influence of a “dancing” magnetosphere, such as that around a neutron star.
That adds weight to the leading theory about where FRBs come from: magnetars, a type of neutron star with an extremely strong magnetic field. The clearest smoking gun came last year when FRB-like signals were detected coming from a magnetar in our own galaxy.
The more we study these strange signals, the more likely it is that we’ll stumble onto a clue that unravels the whole mystery. The researchers say that it's possible that FRBs transmit at even lower frequencies at which they haven't been studied yet, and future work will try to detect these.
The LOFAR study was published in the Astrophysical Journal Letters, while the time resolution study appeared in Nature Astronomy.
Sources: McGill University, JIVE