Amateurs discover exotic binary star system with mysterious radiation blast


The star system AR Scorpii was discovered 40 years ago, but it took a group of amateur astronomers to work out it was different. Bill Condie reports.


An artist’s impression shows the strange object AR Scorpii – a double star system combining a rapidly spinning white dwarf star (right) powering electrons up to almost the speed of light to blast radiation on to its companion red dwarf star (left).
M. Garlick/University of Warwick/ESO

Amateur astronomers have discovered an exotic star grouping unlike anything they have ever encountered.

The binary system consists of a white dwarf star rapidly spinning, releasing blasts of radiation that whip the face of a companion red dwarf, causing the entire system to pulse dramatically every 1.97 minutes.

A mysterious stream of electrons, probably emanating from the white dwarf, are accelerated to almost the speed of light as the two stars orbit each other every 3.6 hours.

The radiation blasting from the group ranges from the ultraviolet to radio frequencies, which have never been detected coming from a white dwarf system.

The star system is named AR Scorpii, or AR Sco for short, and lies in the constellation of Scorpius, 380 light-years from Earth.

While scientists discovered the star system in the early 1970s it was originally miscategorised as a single variable star – the brightness of which fluctuates.

It was not until May 2015, when a group of amateur astronomers from Germany, Belgium and the UK found it behaving unlike anything else they had seen, that it warranted a closer look.

The exotic binary star AR Scorpii lies in the bright constellation of Scorpius. The stars visible with the naked eye on a dark clear night are shown and the location of AR Scorpii marked with a red circle.
ESO/IAU and Sky & Telescope

Observations using some of the world’s most powerful telescopes then uncovered the truly unusual nature of the system.

“We realised we were seeing something extraordinary within minutes of starting the observations,” said lead researcher Tom Marsh of the University of Warwick.

The white dwarf star is about the same size as Earth but with a mass 200,000 times as great and highly magnetic. The red dwarf is a cool M-type star a third the mass of the sun.

While the behaviour is unique, it remains mysterious.

The broad range of radiation frequencies is indicative of emission from electrons accelerated in magnetic fields, but scientists cannot work out the source of the electrons themselves. Nor is it even clear if they are associated with the white or red dwarf.

Still astronomers are excited by the puzzle.

“We’ve known pulsing neutron stars for nearly 50 years, and some theories predicted white dwarfs could show similar behaviour,” says co-author Boris Gänsicke, also of the University of Warwick.

“It’s very exciting that we have discovered such a system, and it has been a fantastic example of amateur astronomers and academics working together.”

The team made observations using the NASA/ESA Hubble Space Telescope, ESO’s Very Large Telescope, the William Herschel and Isaac Newton Telescope in the Canaries, the Australia Telescope Compact Array, and NASA’s Swift satellite.

The research was published in the journal Nature.

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