Periodic Fast Radio Burst found bare, unobscured by strong binary wind

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

The team of astronomers have now studied a Fast Radio Burst at two radio wavelengths – one bluer, one much redder – at the same time. Fast Radio Bursts are some of the brightest flashes in the radio sky, but they emit outside of our human vision. They only last about 1/1000^th of a second. The energy required to form Fast Radio Bursts must be exceedingly high. Still, their exact nature is unknown. Some Fast Radio Bursts repeat, and in the case of FRB 20180916B, that repetition is periodic. This periodicity led to a series of models in which Fast Radio Bursts come from a pair of stars orbiting each other. The binary orbit and stellar wind then create the periodicity. “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. But the redder long-wavelength radio should be blocked more, or even completely,” says Inés Pastor-Marazuela (University of Amsterdam and ASTRON), the first author of the publication.

We report how the Westerbork dishes (left) detected a periodic, short Fast Radio Burst in the blue, high-frequency radio sky. Time passed, the steady background stars turned into trails. Only much later did the same source emit in the red, low-frequency radio sky. The LOFAR telescope (right) now detected these for the first time. This chromatic behaviour shows the bursts are not periodically blocked by binary-star winds. (Image credit: Joeri van Leeuwen)

To test this model, the astronomer team combined the LOFAR and renewed Westerbork telescopes. They could thus simultaneously study FRB 20180916B at two radio colours. Westerbork looked at the bluer wavelength of 21 centimetre, LOFAR observed the much redder, 3-meter wavelength. Both telescopes recorded radio movies with thousands of frames per second. A very fast machine-learning supercomputer quickly detected bursts. “Once we analysed the data, and compared the two radio colours, 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 2 days of bluer, radio bursts, followed by 3 days of redder radio bursts. We rule out the original models now – something else must be going on”.

The Fast Radio Burst detections were the first ever with LOFAR. None had been seen at any wavelengths longer than 1 meter, up to then. Dr Yogesh Maan from ASTRON first laid eyes on the LOFAR bursts: “It was thrilling to discover that Fast Radio Burst shine at such long wavelengths. After going through immense amounts of data, I had a hard time believing it at first, even though the detection was convincing. Soon, even more bursts came in.” This discovery is important because it means the redder, long-wavelength radio emission can escape the environment around the source of the Fast Radio Burst. “The fact that some Fast Radio Bursts live in clean environments, relatively unobscured by any dense electron mist in the host galaxy, is very exciting”, says co-author Dr Liam Connor (U. Amsterdam/ASTRON). “Such bare Fast Radio Bursts will allow us to hunt down the elusive baryonic matter that remains unaccounted for in the Universe”.

Fast Radio Bursts were never seen at long radio wavelengths before, fuelling theories that they were surrounded by dense electron mists. We report the first-ever long wavelength radio detection, using the LOFAR telescope (squares on the right); working in tandem with the Westerbork dishes (left). We conclude there is, in fact, no mist around the Fast Radio Burst. That clear and transparent view is important for cosmology. (Image credit: Joeri van Leeuwen)

The LOFAR telescope and the Apertif system on Westerbork each are formidable in their own right, 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 came in”, says principal investigator Dr Joeri van Leeuwen (ASTRON/U. Amsterdam), “But no simultaneous LOFAR bursts were seen. First, we thought a haze around the Fast Radio Bursts was blocking all redder bursts – but surprisingly, once the bluer bursts had stopped, redder bursts appeared after all. That’s when we realised simple binary wind models were ruled out. Fast Radio Bursts are bare, and could be made by magnetars.”

Such magnetars are neutron stars, of a much higher density than lead, that are also highly magnetic. Their magnetic fields are many times stronger than the strongest magnet in any Earth lab. “An isolated, slowly rotating magnetar best explains the behaviour we discovered”, says Pastor-Marazuela. “It feels a lot like being a detective – our observations have considerably narrowed down which Fast Radio Burst models can work.”

High resolution images can be downloaded here

Paper

DOI: 10.1038/s41586-021-03724-8.
Free preprint: https://arxiv.org/abs/2012.08348

The International LOFAR Telescope is a trans-European network of radio antennas, with a core located in Exloo in the Netherlands. LOFAR works by combining the signals from nearly 110,000 individual antenna dipoles, located in ‘antenna stations’ across the Netherlands and in partner European countries. The stations are connected by a high-speed fibre optic network, with powerful computers used to process the radio signals in order to simulate a trans-European radio antenna that stretches over 2000 kilometres. The International LOFAR Telescope is unique, given its sensitivity, wide field-of-view, and image resolution or clarity. The LOFAR data archive is the largest astronomical data collection in the world.

LOFAR was designed, built and is presently operated by ASTRON, the Netherlands Institute for Radio Astronomy. France, Germany, Ireland, Italy, Latvia, the Netherlands, Poland, Sweden and the UK are all partner countries in the International LOFAR Telescope.

The Westerbork Synthesis Radio Telescope (WSRT-Apertif) was built in 1970. It consists of fourteen separate dish telescopes aligned over a length of 2.7 kilometers. The dishes contain special receivers for different radio wavelengths. Thanks to the newest receiver, called Apertif (APERture Tile In Focus), the area of the sky that can be measured in one observation is forty times larger than before.

Apertif is well suited for mapping the entire sky with great sharpness and sensitivity. Apertif is linked to a dedicated supercomputer that continuously maps the sky, searching for explosive events in the distant universe.

WSRT-Apertif was developed and is operated by ASTRON in the Netherlands.

With the support of the Netherlands eScience Center’s Research Software Engineers, the team managed to use algorithms to find the mystery radio bursts. The telescope produced vast amounts data. The data rate was so high that the data could not be kept for long, so the search for FRBs had to be done in real-time. The eScience Center’s team developed and coded tailored software that could make the relevant data analysis in real time.  Through this collaboration, ASTRON was able to discover that a simple binary wind cannot cause the FRB. Link to full article.