Light shed on mystery space radio pulses

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Arecibo subreflectorImage copyright
DR SETH SHOSTAK/SCIENCE PHOTO LIBRARY

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The analysis used data from the Arecibo Observatory in Puerto Rico

Astronomers have fresh insight on a mysterious source of recurring radio pulses from space.

Fast radio bursts (FRB) are one of the most persistent puzzles in astronomy. While usually short-lived, one source in the sky was sending out repeated flashes.

Now, a team says the strange emission could be caused by a dead star with an exceptionally powerful magnetic field.

Details were reported here at the 231st American Astronomical Society meeting.

The first FRB was discovered in 2007, in archived data from the Parkes Radio Telescope in Australia. Astronomers were searching for new examples of magnetised neutron stars called pulsars, but found a new phenomenon – a radio burst from 2001.

Since then, 18 FRBs – also referred to as “flashes” or “sizzles” – have been found in total.

The mystery surrounding their nature has spawned a variety of different possible explanations, from black holes to extra-terrestrial intelligence.

Only one of these sources of radio energy has erupted more than once – a so-called burster catalogued as FRB 121102. This FRB has sent out around 150 flashes since its discovery in 2012.

Now, in the journal Nature, a team of scientists explains how the emission might come from a neutron star, perhaps one near a black hole or one embedded in a nebula.

Twisted waves

The researchers found something interesting about the polarisation of the radio waves – which describes the direction in which they vibrate. When polarised radio waves pass through a region with a magnetic field, the polarisation gets “twisted” by an effect known as Faraday rotation. And the stronger the magnetic field, the greater the twisting.

“The only sources in the Milky Way that are twisted as much as FRB121102 are in the galactic centre, which is a dynamic region near a massive black hole. Maybe FRB121102 is in a similar environment in its host galaxy,” said Daniele Michilli, a co-author from the University of Amsterdam.

“However, the twisting of the radio bursts could also be explained if the source is located in a powerful nebula or supernova remnant,” he added.

Vishal Gajjar, from the Breakthrough Listen project and the Berkeley SETI Research Center, commented: “At this point, we don’t really know the mechanism. There are many questions, such as, how can a rotating neutron star produce the high amount of energy typical of an FRB?”

The team used the Arecibo radio observatory in Puerto Rico and the Green Bank telescope in West Virginia to probe the source at higher frequencies than ever before.

Andrew Seymour, a staff astronomer at the Arecibo Observatory, said: “The polarisation properties and shapes of these bursts are similar to radio emission from young, energetic neutron stars in our galaxy. This provides support to the models that the radio bursts are produced by a neutron star.”

A year ago, the research team pinpointed the location of FRB121102 and reported that it lies in a star-forming region of a dwarf galaxy at a distance of more than three billion light-years from Earth.

The enormous distance to the source implies that it releases a monstrous amount of energy in each burst – roughly as much energy in a single burst of one millisecond as the Sun releases in an entire day.

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