Multicolored radio rules out more suspects in fast radio burst mystery
Fast radio bursts (FRBs) are among the most intriguing cosmic mysteries of our time, and now we might be one step closer to an answer for what creates them. By examining multiple “radio colors” simultaneously in a batch of repeating signals, astronomers have ruled out a leading model for their origin.
The name for this phenomenon doesn’t leave much to the imagination – fast radio bursts are sharp bursts of radio noise that last mere milliseconds. Ever since they were discovered a few years ago, FRBs have been detected flooding in from all corners of the sky. Some are one-hit wonders, while others repeat either periodically or seemingly at random. Exactly what produces them remains unknown, but evidence is increasingly pointing towards highly magnetized neutron stars called magnetars.
One intriguing signal is called FRB 180916B, which repeats on a precise 16-day cycle, firing off a flurry of activity for four days before going dormant for 12 days. It’s been traced to a galaxy about 500 million light-years away, but it’s not clear what the local environment is like at its point of origin.
For the new study, a team of astronomers was testing one particular hypothesis for how the signals repeat so regularly. The story goes that two neutron stars are locked in a close orbit, each emitting gas called stellar winds. As these winds collide, the shock front can produce emissions such as radio waves, and the periodicity could be explained as a product of their orbit around each other. This is known as the binary wind model.
To check if that’s what was going on here, the astronomers examined signals from FRB 180916B at two different wavelengths at the same time.
"Strong stellar winds from the companion of the fast radio burst source were expected to let most blue, short-wavelength radio light escape the system,” says Inés Pastor-Marazuela, first author of the study. “But the redder long-wavelength radio should be blocked more, or even completely.”
To check if this was the case, the team connected two telescopes, LOFAR and the Westerbork Synthesis Radio Telescope (WSRT), both in the Netherlands. They each observed the same FRBs at different wavelengths – WSRT at the shorter, “bluer” wavelength of 21 cm (8.3 in), and LOFAR at a very red 3 m (9.8 ft). But the results were unexpected.
"Once we analyzed the data, and compared the two radio colors, we were very surprised," says Pastor-Marazuela. "Existing binary-wind models predicted the bursts should shine only in blue, or at least last much longer there. But we saw two days of bluer, radio bursts, followed by three days of redder radio bursts. We rule out the original models now – something else must be going on.”
The team says that the finding shows that the FRB sources must reside in fairly clean environments, with no obstruction by clouds of electrons as has been proposed. Instead, it points towards an isolated magnetar being the culprit, which fits in with many other observations.
Astronomers will no doubt continue to investigate fast radio bursts, slowly piecing together the puzzle until we have our answer.
The research was published in the journal Nature.