Clarification of the nature of rapid radio bursts

By connecting two of the world’s largest radio telescopes, astronomers have discovered that a single binary wind ultimately cannot cause the confusing periodicity of a rapid radio burst. Bursts can originate from an isolated, highly magnetized neutron star. Radio detections also show that rapid radio bursts, some of the most energetic events in the universe, are devoid of protective material. This transparency further increases their importance for cosmology. The results appear in Nature this week.

Radio colors

The use of “radio colors” led to the breakthrough. In optical light, colors are how the eye distinguishes each wavelength. Our rainbow changes from shorter wavelength blue optical light to longer wavelength red optical light. But the electromagnetic radiation that the human eye cannot see, because the wavelength is too long or too short, is just as real. Astronomers call this “ultraviolet light” or “radio light”. The radio-light extends the rainbow beyond the red edge that we see. The radio rainbow itself also changes from a “bluer” short wavelength radio to a “redder” long wavelength radio. Radio wavelengths are a million times longer than optical blue and red wavelengths, but basically they are just “colors”: radio colors.

The team of astronomers have now studied a rapid radio burst at two radio wavelengths – one bluer, one much redder – at the same time. rapid radio bursts are some of the brightest lightning in the radio sky, but they emit outside of our human vision. They only last about 1 / 1000th of a second. The energy required to form rapid radio bursts must be extremely high. However, their exact nature is unknown. Some fast radio bursts repeat, and in the case of FRB 20180916B, this repetition is periodic. This periodicity has led to a series of models in which rapid radio bursts originate from a pair of stars orbiting each other. The binary orbit and the stellar wind then create the periodicity. “The strong stellar winds from the rapid radio burst source mate would have let most of the short-wavelength blue radio lights escape the system. But the redder long-wavelength radio would have to be blocked further,” or even completely “, explains Inés Pastor-Marazuela (University of Amsterdam and ASTRON), the first author of the publication.

Combine Westerbork and LOFAR

To test this model, the team of astronomers combined the LOFAR and the renewed Westerbork telescopes. They were thus able to simultaneously study FRB 20180916B with two radio colors. Westerbork examined the bluest wavelength of 21 centimeters, LOFAR observed the much redder wavelength of 3 meters. Both telescopes recorded radio films with thousands of frames per second. A very fast machine learning supercomputer quickly detected the bursts. “Once we analyzed the data and compared the two radio colors, we were very surprised,” says Pastor-Marazuela. “Existing binary wind models predicted that the bursts should only glow blue, or at least last there much longer. But we saw two days of bluer radio bursts, followed by three days of redder radio bursts. We are now excluding the original models – something else must be happening. “

Detections of rapid radio bursts were the first with LOFAR. None had been seen at wavelengths greater than 1 meter so far. ASTRON’s Dr Yogesh Maan first laid eyes on LOFAR bursts: “It was exciting to find out that fast radio bursts shine at such long wavelengths. After going through immense amounts of data, I found it hard to believe at first, even though the detection was convincing. Soon even more gusts arrived. “This finding is important because it means that the redder, longer wavelength radio emission can escape from the environment around the source of the rapid radio burst.” The fact that some rapid radio bursts live in clean environments, relatively unmasked by dense electron fog in the host galaxy, is very exciting, “says co-author Dr Liam Connor (U. Amsterdam / ASTRON).” Such fast and naked radio bursts will allow us to hunt down the elusive baryonic matter that remains untraceable in the universe. “

Magnetars

The LOFAR telescope and the Apertif system on Westerbork are each great on their own, but the breakthroughs were made possible because the team directly connected the two, as if they were one. “We built a real-time machine learning system on Westerbork that alerted LOFAR whenever a burst arrived,” says lead researcher Dr Joeri van Leeuwen (ASTRON / U. Amsterdam), “But no LOFAR burst First, we thought that a haze around the fast radio bursts blocked all the redder bursts, but surprisingly, once the bluer bursts stopped, redder bursts did appear after all. It was at this point that we realized that simple binary wind patterns were excluded – being made by magnetars.

Such magnetars are neutron stars, of a much higher density than lead, which are also highly magnetic. Their magnetic fields are several times stronger than the strongest magnet in any Earth laboratory. “An isolated, slowly rotating magnetar best explains the behavior we have discovered,” Pastor-Marazuela explains. “It sounds a lot like being a sleuth – our observations have dramatically reduced the fast radio burst patterns that can work.”

Supplied by ASTRON