Star lashes companion with mystery ray

Artist's impression of the exotic binary star system AR Scorpii. Credit: M. Garlick/University of Warwick, ESA/Hubble
Artist’s concept of exotic binary star system AR Scorpii. Image via M. Garlick/University of Warwick, ESA/Hubble

Both professional and amateur astronomers have been peering toward the star system AR Scorpii, or AR Sco for short, 380 light-years from Earth. Identified in the 1970s as a lone variable star, it’s now believed to be a double system with a rapidly spinning white dwarf (Earth-sized but with 200,000 times more mass), and a cool red dwarf companion (about a third the mass of our sun). These two dwarf stars are orbiting each other every 3.6 hours. Astronomers said on July 27, 2016 that the system is:

… a new type of exotic binary star.

They say high energy particles from the white dwarf are lashing the companion red dwarf star, causing the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ultraviolet to radio. They’ve never seen this sort of behavior in a white dwarf system before.

A group of amateur astronomers from Germany, Belgium and the UK began observing this system in May 2015. They noticed its unusual behavior. Follow-up observations have been led by Tom Marsh of the University of Warwick, using telescopes on the ground and in space including the Hubble Space Telescope.

ESA explained in a statement:

Highly magnetic and spinning rapidly, AR Sco’s white dwarf accelerates electrons up to almost the speed of light. As these high energy particles whip through space, they release radiation in a lighthouse-like beam which lashes across the face of the cool red dwarf star, causing the entire system to brighten and fade dramatically every 1.97 minutes. These powerful pulses include radiation at radio frequencies, which has never been detected before from a white dwarf system.

The radiation across a broad range of frequencies isn’t mysterious. It’s a sign of electrons accelerated in magnetic fields, which can be explained by AR Sco’s spinning white dwarf.

But the source of the electrons is a major mystery. Astronomers still aren’t sure whether that source was the white dwarf, or its cooler companion.

The true source of AR Scorpii’s varying luminosity was revealed thanks to the combined efforts of amateur and professional astronomers. Boris Gänsicke, co-author of the new study, commented:

We’ve known about pulsing neutron stars for nearly 50 years, and some theories predicted white dwarfs could show similar behavior. 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 zoom sequence below takes you from a wide-field view of our Milky Way galaxy in the night sky into the bright constellation of Scorpius the Scorpion. The final view is centered on the exotic binary star AR Scorpii.

Bottom line: Astronomers thought the star AR Scorpii was a lone variable star. Now they realize it’s a dwarf star bombarding its companion with relativistic electrons.


August 2, 2016

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