ar scorpii
An artists impression of how a star 'whips' its neighbor with a beam of particles and radiation every 2 minutes. (Image Credit: University of Warwick)

New Type of Star Exhibits Lighthouse Effect

Amateur astronomers from the UK, Belgium and Germany encountered a star they had never witnessed before in May 2015. A multitude of telescopes was deployed from the ground and space by a combined team of professional astronomers from the University of Warwick and amateurs from across the world. This array of telescopes finally revealed the nature of the system.

In the binary star system, a rapidly spinning white dwarf (a burnt-out stellar remnant) sweeps powerful beams of radiation and particles over its nearby companion star, a cool red dwarf. This causes the red dwarf to pulse across almost the entire electromagnetic spectrum from radio to ultraviolet wavelengths.

A close neighbor of Earth in astronomical terms at just 380 light-years away, the constellation Scorpius contains the white dwarf AR Scorpii or AR Sco for short. Although AR Sco is the same size as Earth, it is 200,000 times heavier. The star orbits a cool star one-third the mass of our sun every 3.6 hours.

AR Sco is highly magnetic and spins on a period of just under two minutes. Beams of radiation and particles from AR Sco lash across the face of the red dwarf. As this happens across our line-of-sight, the entire system brightens and fades dramatically twice every two minutes like a lighthouse. In an effect never observed before in similar types of binary stars, electrons in the atmosphere of the red dwarf are accelerated close to the speed of light.

Professor Tom Marsh of the University of Warwick’s Astrophysics Group and lead researcher, notes that although AR Sco was discovered more than 40 years ago, its true nature was only revealed when it was observed with a high-speed astronomical camera. The camera, called ULTRACAM, was mounted on the William Herschel Telescope for the study last May. Marsh comments that within minutes of starting to observe it, they realized they were seeing something extraordinary.

Electrons moving at near the speed of light emits most of the radiation seen from the system. Intense electric fields caused by the rapidly spinning magnetic field of AR Sco accelerate the electrons, but a major mystery of the system remains. The primary location of the electrons within the atmosphere of the cool star is still unknown.

Professor Boris Gänsicke, a co-author of the paper, is thrilled by the new discovery in spite of the fact that there is a history of studying this type of star. Pulsing neutron stars have been known for nearly fifty years, but it was only speculated on that white dwarfs could show similar behavior. To have discovered such a system was very exciting and Gänsicke is especially pleased that it happened with a team of amateur astronomers and academics working together.

The full research is published in the journal Nature.