title: A radio-pulsing white dwarf binary star.
abstract: White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 107-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: T R
authors.initials: TR
authors.lastname: Marsh
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: B T
authors.initials: BT
authors.lastname: Gänsicke
authors.affiliation: Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V. (BAV), Berlin, Germany.|American Association of Variable Star Observers (AAVSO), Cambridge, Massachusetts, USA.
authors.forename: S
authors.initials: S
authors.lastname: Hümmerich
authors.affiliation: Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V. (BAV), Berlin, Germany.|American Association of Variable Star Observers (AAVSO), Cambridge, Massachusetts, USA.|Vereniging Voor Sterrenkunde (VVS), Brugge, Belgium.
authors.forename: F-J
authors.initials: FJ
authors.lastname: Hambsch
authors.affiliation: Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V. (BAV), Berlin, Germany.|American Association of Variable Star Observers (AAVSO), Cambridge, Massachusetts, USA.
authors.forename: K
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authors.lastname: Bernhard
authors.affiliation: Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK.
authors.forename: C
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authors.lastname: Lloyd
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: E
authors.initials: E
authors.lastname: Breedt
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: E R
authors.initials: ER
authors.lastname: Stanway
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: D T
authors.initials: DT
authors.lastname: Steeghs
authors.affiliation: Instituto de Física y Astronomía, Universidad de Valparaíso, Avenida Gran Bretana 1111, Valparaíso, Chile.
authors.forename: S G
authors.initials: SG
authors.lastname: Parsons
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: O
authors.initials: O
authors.lastname: Toloza
authors.affiliation: Instituto de Física y Astronomía, Universidad de Valparaíso, Avenida Gran Bretana 1111, Valparaíso, Chile.
authors.forename: M R
authors.initials: MR
authors.lastname: Schreiber
authors.affiliation: SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584-CA Utrecht, The Netherlands.|Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands.
authors.forename: P G
authors.initials: PG
authors.lastname: Jonker
authors.affiliation: Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands.
authors.forename: J
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authors.lastname: van Roestel
authors.affiliation: Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, California 91125, USA.
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authors.initials: T
authors.lastname: Kupfer
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: A F
authors.initials: AF
authors.lastname: Pala
authors.affiliation: Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK.|Instituto de Astrofisica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain.|Universidad de La Laguna, Departamento Astrofisica, E-38206 La Laguna, Tenerife, Spain.
authors.forename: V S
authors.initials: VS
authors.lastname: Dhillon
authors.affiliation: Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK.
authors.forename: L K
authors.initials: LK
authors.lastname: Hardy
authors.affiliation: Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK.
authors.forename: S P
authors.initials: SP
authors.lastname: Littlefair
authors.affiliation: Department of Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand.
authors.forename: A
authors.initials: A
authors.lastname: Aungwerojwit
authors.affiliation: Program of Physics, Faculty of Science and Technology, Chiang Rai Rajabhat University, Chiang Rai 57100, Thailand.
authors.forename: S
authors.initials: S
authors.lastname: Arjyotha
authors.affiliation: Institut für Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Koester
authors.affiliation: Department of Physical Sciences, The Open University, Milton Keynes, UK.
authors.forename: J J
authors.initials: JJ
authors.lastname: Bochinski
authors.affiliation: Department of Physical Sciences, The Open University, Milton Keynes, UK.
authors.forename: C A
authors.initials: CA
authors.lastname: Haswell
authors.affiliation: Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V. (BAV), Berlin, Germany.
authors.forename: P
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authors.lastname: Frank
authors.affiliation: Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK.
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authors.lastname: Wheatley