Last updated January 11, 2018 at 2:39 pm
An international team of astronomers, including a researcher from Western Australia, have used two of the world’s largest radio telescopes to step towards solving one of the great mysteries in astronomy – the potential source of strange, short, intense bursts of radiation called fast radio bursts.
They found that these fast radio bursts begin life in astonishingly extreme and unusual environment, with the discovery suggesting that the strange source is in close proximity to a massive black hole, or within a nebula of unprecedented power.
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 polarised, and the polarised signal gets twisted as a function of radio frequency because there is an extreme region of magnetised plasma between Earth and the source of the bursts. Credit: Image design – Danielle Futselaar; Photo – Brian P. Irwin / Dennis van de Water / Shutterstock.com
Fast Radio Bursts (FRBs) are a recently discovered type of cosmic signal coming from deep in extragalactic space. A short, directed blast of radiation, FRBs usually occur from random locations in the sky. However, one location, called FRB 121102, has been seen to have repeated FRBs emanating from it. The physical origin of these bursts, however, remained a mystery.
Using data from the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia, astronomers found a peculiar characteristic of FRBs coming from FRB 121102 – the bursts were almost 100 per cent polarised.
The 100-metre Green Bank telescope in West Virginia is shown amid a starry night. A flash from the Fast Radio Burst source FRB 121102 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. Credit: Image design – Danielle Futselaar; Photo usage – Shutterstock.com
The behaviour of this polarised light has allowed them to track back and investigate the characteristics of the source, and to “peer into the lair” of the mysterious burster.
As polarised radio waves travel through a region with a magnetic field, the waves get “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, revealing that the bursts are most likely passing through an exceptionally strong magnetic field.
Previously, such a strong magnetic field has been observed only in the vicinity of massive black holes. However, the short duration of the bursts suggests they could come from a neutron star in very close proximity to the black hole.
“The only known sources in our galaxy that are twisted as much as FRB 121102 are in the Galactic Centre, which is a dynamic region near a massive black hole. Maybe FRB 121102 is in a similar environment in its host galaxy,” said Daniele Michilli, a PhD students from the University of Amsterdam, who worked on the project.
“However, the twisting of the radio bursts could also be explained if the source is located in a powerful nebula or supernova remnant.”
Researchers not connected with the study did point out that the results only show the FRB has passed by an area of intense magnetic field somewhere between its source and Earth. However, the chances of this are so minute that they agree it is entirely reasonable, and indeed likely, that the source and magnetic field (such as a black hole or star-forming region) are co-located.
A year ago, the research team pinpointed the location of FRB 121102 and reported that it lies in a star-forming region of a dwarf galaxy, over 3 billion light years from Earth. An enormous amount of energy is needed to power each burst across this massive distance, calculated to be roughly as much energy in a single millisecond as our Sun releases in an entire day.
FRB 121102 is the only known repeating FRB, which also gives hints to its source. The repeating nature means that the event causing the FRBs could not have been a cataclysmic event such as the death of the star, which would have given off one single burst. However, this has raised the question of whether it has a different origin compared to apparently non-repeating FRBs.
“Fast Radio Bursts are usually a one-off flash in the pan, but this one is repeating so it gives us a unique opportunity to understand this type of phenomenon, especially as the amount of twisting seems to vary by at least 10 per cent over several months,” said International Centre for Radio Astronomy Research astronomer Dr Charlotte Sobey from her location in Western Australia.
The telescopes will continue to probe the nature of these FRBs as they occur, hoping to reveal more secrets of these still mysterious blasts and their origins. The astronomers will no doubt continue studying FRB 121102, but may also turn their attention to other FRBs to discover if they have a similar nature to them. If so, we could be one step close to understanding one of the great mysteries of recent times.
The research has been published in Nature.
Videos courtesy of Andrew Seymour (NAIC, Arecibo)
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