Free Astronomy Magazine September-October 2016

17 SEPTEMBER-OCTOBER 2016 SPACE CHRONICLES spinning rapidly, AR Sco’s white dwarf accelerates electrons up to al- most 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 dra- matically every 1.97 minutes. These power- ful pulses in- clude radiation at radio frequencies, which has never been detected be- fore from a white dwarf system. Lead researcher Tom Marsh of the Uni- versity of Warwick’s Astrophysics Group commented: “AR Sco was discovered over 40 years ago, but its true nature was not suspected until we started observing it in June 2015. We realised we were seeing some- thing extraordi- nary the more we progressed with our observations.” The observed pro- perties of AR Sco are unique. And they are also mysterious. The radiation across a broad range of frequen- cies is indicative of emis- sion from electrons accel- erated in magnetic fields, which can be explained by AR Sco’s spinning white dwarf. The source of the electrons them- selves, however, is a major mystery — it is not clear whether it is associated with the white dwarf, or its cooler com- panion. AR Scor- pii was first ob- served in the early 1970s and regular fluctua- tions in brightness every 3.6 hours led it to be incorrectly classi- fied as a lone variable star. A vari- able star is one whose brightness fluctuates as seen from Earth. The fluctuations may be due to the in- trinsic properties of the star itself changing. For instance some stars noticeably expand and contract. It could also be due to another object regularly eclipsing the star. AR Scorpii was mistaken for a single variable star since regular fluctua- tions in observed brightness occur as the two stars orbit each other and one blocks some of the light from the other. The true source of back- ground ar- tist’s impression shows the strange ob- ject AR Scorpii. In this unique double star a rapidly spinning white dwarf star (right) powers electrons up to almost the speed of light. These high energy particles release blasts of radiation that lash the companion red dwarf star (left) and cause the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ul- traviolet to radio. [M. Garlick/Uni- versity of Warwick, ESA/Hubble] T he AR Scorpii’s varying luminosity was revealed thanks to the combined efforts of amateur and profession- al astronomers. Similar pulsing be- haviour has been observed before, but from neu- tron stars — some of the densest celes- tial objects known in the Universe — ra- ther than white dwarfs. Boris Gänsicke, co- author of the new study, also at the Univer- sity of Warwick, c o n c l u d e s : "We've known about pulsing neutron stars for nearly fif- ty years, and some theories predicted white dwarfs could show similar behaviour. It's very exciting that we have discover- ed such a system, and it has been a fantastic example of amateur astronomers and academics work- ing together." n C losing in the exotic AR Scorpii system, this video shows how the lighthouse-like beam of radiation due to ac- celerated electrons orbiting the white dwarf lashes its red companion, making it flash and fade dramatically every 1.97 minutes. [ESA/Hubble, L. Calçada, University of Warwick]