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A repeating fast radio burst from an extreme environment

New detections of radio waves from a repeating fast radio burst have revealed an astonishingly potent magnetic field in the source's environment, indicating that it is situated near a massive black hole or within a nebula of unprecedented power.

A repeating fast radio burst from an extreme environment
The 100-meter Green Bank Telescope in West Virginia is shown amid a starry night. A flash from the Fast Radio Burst 
source FRB 121102 is seen traveling toward the telescope. The burst shows a complicated structure, with multiple 
bright peaks; these may be created by the burst emission process itself or imparted by the intervening plasma near
 the source. This burst was detected using a new recording system developed by the Breakthrough Listen project 
[Credit: Danielle Futselaar, Shutterstock]
The findings by an international team of astronomers, including Victoria Kaspi and Shriharsh Tendulkar of McGill University, are published in the journal Nature and are highlighted on the cover of the journal.

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 3 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.

Now, using data from the Arecibo Observatory (Puerto Rico) and the Green Bank Telescope (West Virginia), the researchers have shown that the radio bursts from FRB121102 are highly polarized. The behavior of this polarized emission enables scientists to probe the source's environment in a new way.

Twisted polarization

When polarized radio waves pass through a region with a magnetic field, the polarization 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.

One of FRB 121102’s radio bursts, as detected with the Arecibo telescope, and then 
converted to sound so one can hear the drift in the emission frequency with time 
[Credit: Andrew Seymour (NAIC, Arecibo)]

"I could not believe my eyes when my colleagues emailed the results around," says Kaspi, who is a professor of physics at McGill and director of the McGill Space Institute. "This sort of enormous Faraday rotation is extremely rare. Once we digested it, we realized it was a huge clue about where this bizarre source resides."

One possible explanation for the hugely magnetized environment is that FRB 121102 is located close to a massive black hole in its host galaxy. Such highly magnetized plasmas have so far been seen only near the center of the Milky Way, which has its own massive black hole. But the authors 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," says Daniele Michilli, PhD candidate at the University of Amsterdam and ASTRON (Netherlands Institute for Radio Astronomy).

A repeating fast radio burst from an extreme environment
The 305-metre Arecibo telescope, in Puerto Rico, and its suspended support platform of radio
 receivers is shown amid a starry night. A flash from the Fast Radio Burst source FRB 121102 is 
seen: originating beyond the Milky Way, from deep in extragalactic space. This radio burst is 
highly polarized, and the polarized signal gets twisted as a function of radio frequency because
 there is an extreme region of magnetized plasma between us and the source of the bursts 
[Credit: Danielle Futselaar/Shutterstock]
New telescopes could provide answers

With a number of wide-field radio telescopes now coming online, more such sources are expected to be discovered in the coming year, and astronomers are poised to answer more fundamental questions about FRBs.

"The CHIME telescope in Penticton, British Columbia, should be an excellent instrument for detecting fast radio bursts and studying their polarization properties," says Shriharsh Tendulkar, postdoctoral researcher at the McGill Space Institute. "When it comes online in 2018, it should be capable of detecting between a few and a few dozen FRBs every day."

Source: McGill University [January 11, 2018]


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