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A radio-pulsing white dwarf binary star

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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Figure 1: AR Sco’s optical brightness and radial velocity curve.
Figure 2: Ultraviolet, optical, infrared and radio fluxes of AR Sco.
Figure 3: Fourier amplitudes of the ultraviolet, optical, infrared and radio fluxes of AR Sco versus temporal frequency.
Figure 4: The wide band SED of AR Sco.

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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.

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Authors

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.

Corresponding author

Correspondence to T. R. Marsh.

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The authors declare no competing financial interests.

Additional information

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

Extended data figures and tables

Extended Data Figure 1 The optical spectrum of the white dwarf’s M star companion.

A 10 min exposure of AR Sco taken with FORS on the VLT between orbital phases 0.848 and 0.895 (black). Other spectra: an optimally scaled M5 template (green); the sum of the template plus a fitted smooth spectrum (red); AR Sco minus the template, that is, the extra light (magenta); a white dwarf model atmosphere of T = 9,750 K, log[g] = 8.0, the maximum possible consistent with the HST data (blue). A slit-loss factor of 0.61 has been applied to the models. The strong emission lines come from the irradiated face of the M star.

Extended Data Figure 2 HST ultraviolet spectrum of AR Sco.

This shows the mean HST spectrum with geocoronal emission plotted in grey. The blue line close to the x axis is a white dwarf model atmosphere of T = 9,750 K, log[g] = 8.0, representing the maximal contribution of the white dwarf consistent with light curves. The radial velocities of the emission lines (Extended Data Fig. 4) show that, similar to the optical lines, the ultraviolet lines mainly come from the irradiated face of the M star.

Extended Data Figure 3 Velocity variations of atomic emission lines compared with those of the M star.

ad, Emission lines from a sequence of spectra from the VLT+X-SHOOTER data (a, b, d) and the Na I 8,200 absorption doublet from the M star (d). The dashed line shows the motion of the centre of mass of the M star deduced from the NaI measurements, while the dotted lines show the maximum range of radial velocities from the M star for q = M2/M1 = 0.35. The emission lines move in phase with the Na I doublet but at lower amplitude, showing that they come from the inner face of the M star.

Extended Data Figure 4 The origin of the emission lines relative to the M star.

Velocities of the lines were fitted with VR = −VX cos(2πφ) + VYsin(2πφ). The points show the values of (VX, VY). The M star from Na I is shown by the red dot (by definition this lies at VX = 0). Si IV and He II lines from the HST FUV data are shown by the blue dots. Hα, Hβ and Hγ from optical spectroscopy are shown by the green dots. The black and green plus signs mark the centres of mass of the binary and white dwarf, respectively. Error bars are ±1σ, calculated from fits to the radial velocities with uncertainties on the velocities scaled to result in χ2 = 1 per degree of freedom, and the uncertainties on the fit parameters calculated from the covariance matrix of the linear least-squares fit. The red line shows the Roche lobe of the M star for a mass ratio q = 0.35.

Extended Data Figure 5 Amplitude spectra from nine days of monitoring with a small telescope.

a, Amplitude as a function of frequency around the 1.97 min signal from data taken with a 40 cm telescope. b, The same at the second harmonic. c, d, The same as a and b after subtracting the beat frequency signals at νB (c) and 2νB (d). Signals at νS + νO and 2νSνO are also apparent. e, The window function, computed from a pure sinusoid of frequency νB and amplitude 0.18 magnitudes (see a).

Extended Data Figure 6 Amplitude spectra from seven years of sparsely sampled CSS data.

ac, The amplitude as a function of frequency relative to the mean orbital (a), beat (b) and spin (c) frequencies listed in Extended Data Table 2. The grey line is the spectrum without any processing; the blue line is the spectrum after subtraction of the orbital signal.

Extended Data Table 1 Observation log
Extended Data Table 2 Statistics of the orbital, beat and spin frequencies from bootstrap fits
Extended Data Table 3 Archival data sources and flux values

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Marsh, T., Gänsicke, B., Hümmerich, S. et al. A radio-pulsing white dwarf binary star. Nature 537, 374–377 (2016). https://doi.org/10.1038/nature18620

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