Tuesday, August 10, 2021

AR Scorpii: White dwarf punishes red dwarf with electron beams at relativistic speeds

 Astronomers used ESO's Very Large Telescope and other telescopes both on the ground and in space, and discovered a new type of exotic binary star.  In the AR Scorpii system, a rapidly rotating white dwarf accelerates electrons to almost the speed of light.  These high-energy particles release amounts of radiation that blast the companion star, a red dwarf, causing the entire system to pulse drastically every 1.97 minutes and release radiation ranging from the ultraviolet to radio waves.  This work will be published in the journal Nature on July 28, 2016.

 The AR Scorpii  (AR Sco) star system lies in the direction of the constellation Scorpio and is 380 light-years away from Earth.  It is composed of a rapidly rotating white dwarf [2], the size of Earth but about 200,000 times more massive, and a cold red dwarf a third the mass of the Sun [3], which orbit each other with a period. 3.6 hours, performing a cosmic dance as regular as clockwork.

 This binary star system exhibits very violent behavior.  Highly magnetized and spinning very quickly, the white dwarf accelerates electrons to almost the speed of light.  As these high-energy particles move through space, they release radiation in a beam similar to a lighthouse, which shoots out the cold red dwarf, causing the entire system to glow and go out every 1.97 minutes.  These powerful pulses include radiation at radio frequencies, something that has never before been detected in a system with a white dwarf.

 AR Scorpii was discovered more than 40 years ago, but we didn't suspect its true nature until we started looking at it in 2015. We realized we were seeing something extraordinary within minutes of starting the observations.

 The observed properties of AR Sco are unique and mysterious.  Radiation emitted over a wide range of frequencies indicates emission of accelerated electrons in magnetic fields, which can be explained by the rotating white dwarf.  The electron source itself remains, however, a mystery—it is not clear whether it will be associated with the white dwarf itself or its cooler companion.

 AR Scorpii was first observed in the early 1970s and its regular brightness fluctuations every 3.6 hours caused it to be erroneously classified as an isolated variable star [4].  The true nature of AR Scorpii's variation in luminosity was revealed through the joint efforts of professional and amateur astronomers.  A similar pulse has been observed before, but it comes from neutron stars—some of the densest celestial objects known in the Universe—rather than from white dwarfs.

 We've known pulsating neutron stars for nearly 50 years, and some theories predicted that white dwarfs might also exhibit similar behavior.  It is very exciting that we have discovered such a system and it is also a fantastic example of collaboration between amateur and professional astronomers.

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 This artistic conception shows the strange object AR Scorpii.  In this unique double star, a rapidly rotating white dwarf (right) accelerates electrons to almost the speed of light.  These high-energy particles release amounts of radiation that lash the companion star, a red dwarf (left), causing the entire system to pulse drastically every 1.97 minutes and release radiation ranging from the ultraviolet to radio waves. .  Credits: M. Garlick/University of Warwick/ESO

 

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