Abstract

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive1. 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 line2 and X-ray3 emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic4. 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 star5, 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.

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Acknowledgements

T.R.M., E.R.S., D.S., E.B., P.J.W., V.S.D., S.P.L. and ULTRACAM were supported by the Science and Technology Facilities Council (STFC, grant numbers ST/L000733 and ST/M001350/1). B.T.G., A.P. and P.G.J. acknowledge support from the European Research Council (ERC, grant numbers 320964 and 647208). O.T., S.G.P. and M.R.S. acknowledge support from Fondecyt (grant numbers 3140585 and 1141269). M.R.S. also received support from Millenium Nucleus RC130007 (Chilean Ministry of Economy). A.A. acknowledges support from the Thailand Research Fund (grant number MRG5680152) and the National Research Council of Thailand (grant number R2559B034). The analysis in this paper is based on observations collected with telescopes of the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, the European Organisation for Astronomical Research in the Southern Hemisphere (observing programmes 095.D-0489, 095.D-0739, 095.D-0802), the NASA/ESA Hubble Space Telescope (observing programmes 14470) and the Thai National Telescope. Archival data from the Herschel, Spitzer and WISE space observatories, and from the Catalina Sky Survey were used. We thank the Swift mission PI for a target-of-opportunity program on AR Sco with the XRT and UVOT instruments and Jamie Stevens for carrying out the ATCA Director’s Discretionary Time observations. This paper is dedicated to the memory of Sirinipa Arjyotha.

Author information

Author notes

    • S. Arjyotha

    Deceased.

Affiliations

  1. Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK

    • T. R. Marsh
    • , B. T. Gänsicke
    • , E. Breedt
    • , E. R. Stanway
    • , D. T. Steeghs
    • , O. Toloza
    • , A. F. Pala
    •  & P. J. Wheatley
  2. Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V. (BAV), Berlin, Germany

    • S. Hümmerich
    • , F.-J. Hambsch
    • , K. Bernhard
    •  & P. Frank
  3. American Association of Variable Star Observers (AAVSO), Cambridge, Massachusetts, USA

    • S. Hümmerich
    • , F.-J. Hambsch
    •  & K. Bernhard
  4. Vereniging Voor Sterrenkunde (VVS), Brugge, Belgium

    • F.-J. Hambsch
  5. Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK

    • C. Lloyd
  6. Instituto de Física y Astronomía, Universidad de Valparaíso, Avenida Gran Bretana 1111, Valparaíso, Chile

    • S. G. Parsons
    •  & M. R. Schreiber
  7. SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584-CA Utrecht, The Netherlands

    • P. G. Jonker
  8. Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

    • P. G. Jonker
    •  & J. van Roestel
  9. Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, California 91125, USA

    • T. Kupfer
  10. Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK

    • V. S. Dhillon
    • , L. K. Hardy
    •  & S. P. Littlefair
  11. Instituto de Astrofisica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain

    • V. S. Dhillon
  12. Universidad de La Laguna, Departamento Astrofisica, E-38206 La Laguna, Tenerife, Spain

    • V. S. Dhillon
  13. Department of Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand

    • A. Aungwerojwit
  14. Program of Physics, Faculty of Science and Technology, Chiang Rai Rajabhat University, Chiang Rai 57100, Thailand

    • S. Arjyotha
  15. Institut für Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany

    • D. Koester
  16. Department of Physical Sciences, The Open University, Milton Keynes, UK

    • J. J. Bochinski
    •  & C. A. Haswell

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Contributions

T.R.M. organised observations, analysed the data, interpreted the results and was the primary author of the manuscript. B.T.G., A.F.P., E.B., S.G.P., P.G.J., J.v.R., T.K., M.R.S. and O.T. acquired, reduced and analysed optical and ultraviolet spectroscopy. E.R.S. acquired, reduced and analysed the ATCA radio data. S.H., F.-J.H., K.B., C.L. and P.F. first identified the unusual nature of AR Sco and started the optical monitoring campaign. V.S.D., L.K.H., S.P.L., A.A., S.A., J.J.B. and C.A.H. acquired and reduced the high-speed optical photometry. D.T.S. and P.J.W. acquired and analysed Swift and archival X-ray data. D.K. calculated the white dwarf model atmosphere. All authors commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to T. R. Marsh.

Reviewer Information Nature thanks S. Ransom and M. H. van Kerkwijk for their contribution to the peer review of this work.

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DOI

https://doi.org/10.1038/nature18620

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