Pulsating White Dwarf Whips Its Companion Star With Beam Of Particles

A radio pulsing white dwarf binary star. University of Warwick

A new type of stellar binary has been discovered thanks to a group of researchers and amateur astronomers from the University of Warwick.

The object is called AR Scorpii, Ar Sco for short, and it is a never-before-seen pulsating white dwarf orbiting a cool red dwarf star. The white dwarf "whips" its cooler companion with a pulse of high-speed particles every two minutes, which makes the red dwarf four times brighter.

Although AR Sco was discovered in the 1970s and classified then as a common variable star, it was brought back to the attention of researchers last May after observations from Belgian, German, and British astronomers.

“AR Sco was discovered over 40 years ago, but its true nature was unsuspected until we observed it last May with a high-speed astronomical camera called ULTRACAM on the William Herschel Telescope,” Professor Tom Marsh, the lead researcher, said in a statement. “We realized we were seeing something extraordinary within minutes of starting to observe it.”

As reported in Nature, the white dwarf is about the size of Earth, but with 200,000 times more mass and shining in all wavelengths. Its companion is about one-third the mass of the Sun, and they orbit each other every 3.56 hours.

White dwarfs are degenerate stars, the burnt-out cores of giant stars that have lost their outer material towards the end of their lives. They are not known to often pulsate, with there being only one possible example so far, but it is possible if the mass loss is quick enough. In that case, the white dwarf will rotate on itself very quickly (due to conservation of angular momentum) and, if the magnetic field of the white dwarf is strong enough, it will also emit beams of particles.

In the case of AR Sco, which is 380 light-years from Earth, the beam of particles and high-energy photons accelerate the electrons in the red dwarf companion to almost the speed of light, causing the increase in brightness. This has never been observed before in dwarf binaries.

"We've known pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behavior,” said Professor Boris Gänsicke, co-author of the study. “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."


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