Origin of mysterious fast radio bursts unvewiled

New detections of radio waves from a repeating fast radio burst have revealed an astonishingly potent magnetic field in the source's environment. A year ago, the astronomers pinpointed the location of the enigmatic fast radio burst (FRB) source named FRB 121102 and reported that it lies in a star-forming region of a dwarf galaxy more than three billion light years from Earth. The vast distance to the source implies that it releases an enormous amount of energy in each burst - roughly as much energy in a single millisecond as the Sun releases in an entire day.

Using data from the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia, the researchers including Shriharsh Tendulkar of McGill University in Canada have shown that the radio bursts from FRB121102 are highly polarised.

The behaviour of this polarised emission enables scientists to probe the source's environment in a new way. When polarised radio waves pass through a region with a magnetic field, the polarisation gets "twisted" by an effect known as Faraday rotation: the stronger the magnetic field, the greater the twisting. The amount of twisting observed in FRB 121102's radio bursts is among the largest ever measured in a radio source, leading the researchers to conclude that the bursts are passing through an extraordinarily strong magnetic field in a dense plasma.

"This sort of enormous Faraday rotation is extremely rare. Once we digested it, we realised it was a huge clue about where this bizarre source resides," said Kaspi, professor at McGill.

One possible explanation for the hugely magnetised environment is that FRB 121102 is located close to a massive black hole in its host galaxy. Such highly magnetised plasmas have so far been seen only near the centre of the Milky Way, which has its own massive black hole. However, researchers also speculate that the twisting of the radio bursts could be explained if FRB 121102 is located in a powerful nebula - an interstellar cloud of gas and dust - or amid the remains of a dead star.

FRBs are a recently discovered class of transient astrophysical events, originating from deep in extragalactic space. Their physical nature remains a mystery. FRB 121102 is the only known repeating FRB, and this has also raised the question of whether it has a different origin compared to the apparently non-repeating FRBs. "FRB 121102 was already unique because of its repetition; now the huge Faraday rotation we have observed singles it out yet again. We're curious as to whether these two unique aspects are linked," said

Daniele Michilli, PhD candidate at the University of Amsterdam.