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

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.

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Figure 1: Time-series polarimetry.
Figure 2: Polarimetry periodograms.
Figure 3: Spin-modulated polarization.
Figure 4: Stokes parameters Q, U and polarized flux variations over spin period.

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Acknowledgements

For D.A.H.B., P.J.M. and S.B.P., this work was supported by the National Research Foundation of South Africa. T.R.M. was supported by the Science and Technology Facilities Council (STFC) under grant ST/L000733. B.T.G. is supported through European Research Council grant 320964. This work is based on observations obtained at the SAAO.

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Authors

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.

Corresponding author

Correspondence to D. A. H. Buckley.

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The authors declare no competing financial interests.

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Supplementary Table 1, Supplementary Figures 1–8. (PDF 1209 kb)

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Buckley, D., Meintjes, P., Potter, S. et al. Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii. Nat Astron 1, 0029 (2017). https://doi.org/10.1038/s41550-016-0029

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