Article

Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii

Received:
Accepted:
Published online:

Abstract

The variable star AR Scorpii (AR Sco) was recently discovered to pulse in brightness every 1.97 min from ultraviolet wavelengths into the radio regime. The system is composed of a cool, low-mass star in a tight, 3.55-hour orbit with a more massive white dwarf. Here we report new optical observations of AR Sco that show strong linear polarization (up to 40%) that varies strongly and periodically on both the spin period of the white dwarf and the beat period between the spin and orbital period, as well as low-level (up to a few per cent) circular polarization. These observations support the notion that, similar to neutron-star pulsars, the pulsed luminosity of AR Sco is powered by the spin-down of the rapidly rotating white dwarf that is highly magnetized (up to 500 MG). The morphology of the modulated linear polarization is similar to that seen in the Crab pulsar, albeit with a more complex waveform owing to the presence of two periodic signals of similar frequency. Magnetic interactions between the two component stars, coupled with synchrotron radiation from the white dwarf, power the observed polarized and non-polarized emission. AR Sco is therefore the first example of a white dwarf pulsar.

  • Subscribe to Nature Astronomy for full access:

    $99

    Subscribe
  • Purchase article full text and PDF:

    $32

    Buy now

Additional access options:

Already a subscriber? Log in now or Register for online access.

References

  1. 1.

    Rotating neutron stars as the origin of the pulsating radio sources. Nature 218, 731–732 (1968).

  2. 2.

    et al. A radio-pulsing white dwarf binary star. Nature 537, 374–377 (2016).

  3. 3.

    , , & A magnetic propeller in the cataclysmic variable AE Aquarii. Mon. Not. R. Astron. Soc. 286, 436–446 (1997).

  4. 4.

    The pulsar-like white dwarf in AE Aqarii. Astron. Astrophys. 338, 521–526 (1998).

  5. 5.

    & The diamagnetic blob propeller in AE Aquarii and non-thermal radio to mid-infrared emission. Mon. Not. R. Astron. Soc. 360, 573–582 (2005).

  6. 6.

    & X-ray characteristics and the spectral energy distribution of AE Aquarii. Mon. Not. R. Astron. Soc. 421, 1557–1586 (2012).

  7. 7.

    et al. Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524). Mon. Not. R. Astron. Soc. 402, 1161–1170 (2010).

  8. 8.

    , , & Optical polarization of the Crab pulsar: Precision measurements and comparison to the radio emission. Mon. Not. R. Astron. Soc. 397, 103–123 (2009).

  9. 9.

    & Absolute polarization position angle profiles of southern pulsars at 1.4 and 3.1 GHz. Mon. Not. R. Astron. Soc. 365, 353–366 (2006).

  10. 10.

    & Magnetic poles and the polarization structure of pulsar radiation. Astrophys. J. Lett. 3, 225–229 (1969).

  11. 11.

    , , & Linear polarimetry of Ap stars. I. A simple canonical model. Astron. Astrophys. 272, 285–298 (1993).

  12. 12.

    et al. Circular polarization survey of intermediate polars. I. Northern targets in the range 17 h < RA < 23. Astron. Astrophys. 496, 891–902 (2009).

  13. 13.

    et al. On the spin modulated circular polarization from the intermediate polars NY Lup and IGR J150946649. Mon. Not. R. Astron. Soc. 420, 2596–2602 (2012).

  14. 14.

    & Circular polarization of synchrotron radiation in high magnetic fields. Mon. Not. R. Astron. Soc. 450, 533–540 (2015).

  15. 15.

    , & Magnetic white dwarfs. Space Sci. Rev. 191, 111–169 (2015).

  16. 16.

    AR Sco: A white dwarf synchronar. Astrophys. J. Preprint at (2017).

  17. 17.

    & Secondary star magnetic fields in close binaries. Mon. Not. R. Astron. Soc. 372, 1279–1288 (2006).

  18. 18.

    & The X-ray luminosity of rotation-powered pulsars. Astron. Astrophys. 326, 682–691 (1997).

  19. 19.

    in Neutron Stars and Pulsars (ed. Becker, W. ) 91–140 (Springer, 2009).

  20. 20.

    et al. The first Fermi Large Area Telescope catalog of gamma-ray pulsars. Astrophys. J. Suppl. Ser. 187, 460–494 (2010).

  21. 21.

    , & A model of white dwarf pulsar AR Scorpii. Astrophys. J. 831, L10 (2016).

  22. 22.

    & Magnetic fields of nondegenerate stars. Annu. Rev. Astron. Astrophys. 47, 333–370 (2009).

  23. 23.

    Magnetic braking by a stellar wind I. Mon. Not. R. Astron. Soc. 138, 359–391 (1968).

  24. 24.

    & On magnetic braking of late-type stars. Mon. Not. R. Astron. Soc. 226, 57–66 (1987).

  25. 25.

    Acceleration from field-aligned potential drops. Astrophys. J. Suppl. S. 90, 765–774 (1994).

  26. 26.

    & The tenuous X-ray corona enveloping AE Aquarii. Mon. Not. R. Astron. Soc. 378, 681–690 (2007).

  27. 27.

    A model for variable extragalactic radio sources. Nature 211, 1131–1133 (1966).

  28. 28.

    , & Radio flares from AE Aquarii: A low-power analog to Cygnus X-3? Astrophys. J. 324, 431–440 (1988).

  29. 29.

    & Pair formation above polar caps — structure of the low altitude acceleration zone. Astrophys. J. 231, 854–879 (1979).

  30. 30.

    Coherent neutral sheet radiation from pulsars. Comments Astrophys. Space Phys. 3, 80–86 (1971).

  31. 31.

    Magnetically striped relativistic magnetohydrodynamic winds: The Crab nebula revisited Astrophys. J. 349, 538–545 (1990).

  32. 32.

    , & in Neutron Stars and Pulsars (ed. Becker, W. ) 421–450 (Springer, 2009).

  33. 33.

    The DQ Herculis stars. Publ. Astron. Soc. Pacif. 106, 209–238 (1994).

  34. 34.

    On the evolution of the nova-like variable AE Aquarii. Mon. Not. R. Astron. Soc. 336, 265–275 (2002).

  35. 35.

    , , , & AE Aquarii: How cataclysmic variables descend from supersoft binaries. Mon. Not. R. Astron. Soc. 337, 1105–1112 (2002).

  36. 36.

    et al. Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524). Mon. Not. R. Astron. Soc. 402, 1161–1170 (2010).

  37. 37.

    & On standard polarized stars. Astrophys. J. 262, 732–738 (1982).

  38. 38.

    , & Achieving better than 1 minute accuracy in the heliocentric and barycentric Julian dates. Publ. Astron. Soc. Pacif. 122, 935–946 (2010).

  39. 39.

    Evolution of single stars. VI. Model nuclei of planetary nebulae. Acta Astron. 21, 417–435 (1971).

  40. 40.

    & Tables for the Roche model of close binaries. Bull. Astr. Czech. 15, 165–170 (1964).

  41. 41.

    , & Synchronous rotation in magnetic X-ray binaries. Astrophys. J. 230, 176–183 (1979).

  42. 42.

    & A model for VW Hydri. Astron. Astrophys. 70, L65–67 (1978).

  43. 43.

    in Magnetohydrodynamics in Binary Stars 88–89 (Kluwer, 1997).

  44. 44.

    & in Neutron Stars: Theory and Observation (eds Ventura, J. E. & Pines, D. ) 363–444 (Kluwer, 1991).

  45. 45.

    & Propeller spin-down and non-thermal emission from AE Aquarii. Mon. Not. R. Astron. Soc. 311, 611–620 (2000).

  46. 46.

    & Pulsar electrodynamics. Astrophys. J. 157, 869–880 (1969).

Download references

Author information

Affiliations

  1. South African Astronomical Observatory, PO Box 9, Observatory, 7935, Cape Town, South Africa

    • D. A. H. Buckley
    •  & S. B. Potter
  2. Department of Physics, University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa

    • P. J. Meintjes
  3. Department of Physics, Gibbet Hill Road, University of Warwick, Coventry, CV4 7AL, UK

    • T. R. Marsh
    •  & B. T. Gänsicke

Authors

  1. Search for D. A. H. Buckley in:

  2. Search for P. J. Meintjes in:

  3. Search for S. B. Potter in:

  4. Search for T. R. Marsh in:

  5. Search for B. T. Gänsicke in:

Contributions

D.A.H.B. conceived the HIPPO observing programme, organized and undertook the observations, assisted in the analysis and interpretation of the polarimetry, participated in the modelling and was primary author of the paper. P.J.M. undertook the modelling and led most of the interpretation. S.B.P. undertook the reductions of the HIPPO data, produced most of the figures and assisted in interpretation of the results. T.R.M. and B.T.G. provided information on AR Sco, including pre-publication material, and assisted in the interpretation of the results and models.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to D. A. H. Buckley.

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    Supplementary Table 1, Supplementary Figures 1–8.