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A 5.3-min-period pulsing white dwarf in a binary detected from radio to X-rays

Nature Astronomy volume 7, pages 931–942 (2023)Cite this article

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Abstract

White dwarf stars are the most common stellar fossils. When in binaries, they make up the dominant form of compact object binary within the Galaxy and can offer insight into different aspects of binary formation and evolution. One of the most remarkable white dwarf binary systems identified to date is AR Scorpii (AR Sco). AR Sco is composed of an M dwarf star and a rapidly spinning white dwarf in a 3.56 h orbit. It shows pulsed emission with a period of 1.97 min over a broad range of wavelengths, which led to it being known as a white dwarf pulsar. Both the pulse mechanism and the evolutionary origin of AR Sco provide challenges to theoretical models. Here we report the discovery of a sibling of AR Sco, J191213.72-441045.1, which harbours a white dwarf in a 4.03 h orbit with an M dwarf and exhibits pulsed emission with a period of 5.30 min. This discovery establishes binary white dwarf pulsars as a class and provides support for proposed formation models for white dwarf pulsars.

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Fig. 1: J1912-4410’s photometry and radial velocities.
Fig. 2: Radio, optical and X-ray fluxes of J1912-4410.
Fig. 3: Spectral energy distribution of J1912-4410.
Fig. 4: Photopolarimetry of J1912-4410.
Fig. 5: Simultaneous XMM-Newton and ULTRACAM data.

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Data availability

The TESS data used in this work are public and can be accessed via the Barbara A. Mikulski Archive for Space Telescopes (https://mast.stsci.edu/). ULTRACAM and X-shooter data are made available in a Zenodo repository (https://doi.org/10.5281/zenodo.7875811). Other data will become public in the respective telescope repositories after the proprietary time expires, but can be made available for analysis upon request to the corresponding author.

Code availability

Any of the custom data analysis scripts used in this work can be made available upon reasonable request to the corresponding author. This research made extensive use of Astropy (http://www.astropy.org), a community-developed core Python package for astronomy75,76.

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Acknowledgements

I.P. and T.R.M. acknowledge funding by the UK’s Science and Technology Facilities Council (STFC), grant ST/T000406/1. I.P. also acknowledges funding from a Warwick Astrophysics Prize post-doctoral fellowship, made possible thanks to a generous philanthropic donation. I.P. was additionally supported in part by the National Science Foundation under grant NSF PHY-1748958, and thanks the organizers of the KITP Programme ‘White Dwarfs as Probes of the Evolution of Planets, Stars, the Milky Way and the Expanding Universe’. S.G.P. acknowledges the support of an STFC Ernest Rutherford Fellowship. V.S.D. and ULTRACAM are funded by STFC, grant ST/V000853/1.

This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program.

IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation (NSF).

This work is based in part on observations obtained at the SOAR telescope, which is a joint project of the Ministério da Ciência, Tecnologia e Inovações do Brasil (MCTI/LNA), the US National Science Foundation’s NOIRLab, the University of North Carolina at Chapel Hill (UNC) and Michigan State University (MSU).

This work is also based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 109.24EM and 109.234F.

The SALT observations were obtained under the SALT Large Science Programme on transients (2021-2-LSP-001; principal investigator D.A.H.B.). Polish participation in SALT is funded by grant MEiN 2021/WK/01. D.A.H.B. and S.B.P. acknowledge research support by the National Research Foundation.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory (SARAO), which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. We thank SARAO for the award of the MeerKAT Director’s Discretionary Time.

This work is also based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.

This work is also based on data from eROSITA, the soft X-ray instrument aboard SRG, a joint Russian–German science mission supported by the Russian Space Agency (Roscosmos), in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL) and its subcontractors, and is operated by NPOL with support from the Max Planck Institute for Extraterrestrial Physics (MPE). The development and construction of the eROSITA X-ray instrument was led by MPE, with contributions from the Dr. Karl Remeis Observatory Bamberg & ECAP (FAU Erlangen-Nürnberg), the University of Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP) and the Institute for Astronomy and Astrophysics of the University of Tübingen, with the support of DLR and the Max Planck Society. The Argelander Institute for Astronomy of the University of Bonn and the Ludwig-Maximilians-Universität Munich also participated in the science preparation for eROSITA. The eROSITA data shown here were processed using the eSASS/NRTA software system developed by the German eROSITA consortium.

Support of the Deutsche Forschungsgemeinschaft (DFG) under grant number 536/37-1 is gratefully acknowledged.

Author information

Authors and Affiliations

  1. Department of Physics, University of Warwick, Coventry, UK

    Ingrid Pelisoli, T. R. Marsh & James Munday

  2. South African Astronomical Observatory, Cape Town, South Africa

    David A. H. Buckley, Stephen. B. Potter & Jaco Brink

  3. Department of Astronomy, University of Cape Town, Rondebosch, South Africa

    David A. H. Buckley, Jaco Brink & P. A. Woudt

  4. Department of Physics, University of the Free State, Bloemfontein, South Africa

    David A. H. Buckley

  5. Astrophysics, Department of Physics, University of Oxford, Oxford, UK

    I. Heywood

  6. Department of Physics and Electronics, Rhodes University, Makhanda, South Africa

    I. Heywood

  7. South African Radio Astronomy Observatory, Cape Town, South Africa

    I. Heywood

  8. Department of Physics, University of Johannesburg, Auckland Park, South Africa

    Stephen. B. Potter

  9. Leibniz-Institut für Astrophysik Potsdam (AIP), Potsdam, Germany

    Axel Schwope & Annie Standke

  10. Institute for Physics and Astronomy, University of Potsdam, Potsdam, Germany

    Annie Standke

  11. Department of Physics and Astronomy, University of Sheffield, Sheffield, UK

    S. G. Parsons, A. J. Brown, V. S. Dhillon, M. J. Dyer, P. Kerry, S. P. Littlefair, D. I. Sahman & J. F. Wild

  12. School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv, Israel

    M. J. Green

  13. Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil

    S. O. Kepler & A. D. Romero

  14. Isaac Newton Group of Telescopes, Santa Cruz de La Palma, Spain

    James Munday

  15. Institute of Astronomy, University of Cambridge, Cambridge, UK

    E. Breedt

  16. Instituto de Astrofísica de Canarias, La Laguna, Spain

    V. S. Dhillon

Authors
  1. Ingrid Pelisoli
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  2. T. R. Marsh
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Contributions

All authors contributed to the work presented in this paper. I.P. wrote the manuscript and led the follow-up and analysis of the system, with substantial input from T.R.M. D.A.H.B. carried out follow-up observations at SAAO and contributed to analysis of the optical data. I.H. carried out reduction and analysis of the radio data. S.B.P. carried out follow-up observations at SAAO and the analysis and modelling of polarimetric data. A. Schwope and A. Standke obtained and analysed the X-ray data. P.A.W. obtained the radio data. S.G.P. and M.J.G. contributed to initial identification and analysis of the system. S.O.K., J.M. and A.D.R. contributed to observational follow-up. E.B., A.J.B., V.S.D., M.J.D., P.K., S.P.L., D.I.S. and J.F.W. contributed to the maintenance of operations of ULTRACAM.

Corresponding author

Correspondence to Ingrid Pelisoli.

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The authors declare no competing interests.

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Nature Astronomy thanks Taichi Kato and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Optical spectrum of J1912-4410.

The black line shows the SOAR spectrum obtained for J1912-4410, which confirmed its spectral characteristics to be similar to AR Sco. The grey line shows X-Shooter spectra from the UVB and VIS arms obtained around the same orbital phase as the SOAR spectrum (0.85). The flux calibration of the SOAR spectrum is poor towards the blue due to reduced sensitivity.

Extended Data Fig. 2 Constraints from Roche geometry.

In panel (a) the star marks measurements from the NaII line (by our definition the centre of mass of the M dwarf). The cross, diamond, and circle mark respectively Hα, Hβ, and Hγ, which were fitted as \(V=\gamma -{V}_{X}\cos (2\pi \varphi )+{V}_{Y}\sin (2\pi \varphi )\), where φ is the orbital phase and γ, the systemic velocity, was kept fixed to the previously determined value. The one-sigma uncertainties are comparable to the symbol size. Hβ, and Hγ give consistent measurements, whereas Hα seems to trail the leading face of the M dwarf. The red dashed line is the Roche lobe of the M dwarf for q = 0.1. The black cross and blue triangle mark the centre of mass of the system and of the white dwarf, respectively. The black line in panel (b) shows the expected difference between the irradiated face and centre-of mass radial velocity semi-amplitudes as a function of q, assuming the M dwarf fills its Roche lobe. The right-hand y-axis shows the required inclination to explain the detected K difference. The observed value of K difference sets a minimum for q, which would happen if the system was seen at 90∘ inclination, as indicated by the red dashed line. The area shaded in grey corresponds to M1 values consistent with a white dwarf for a Roche lobe-filling companion, and implies the minimum q value of 0.3. This minimum q corresponds to a maximum inclination of 37∘ (blue dashed lines).

Extended Data Fig. 3 Constraints from the binary mass function.

The colour map shows the required inclination to explain the observed K2 for given values of M1 and M2 shown in the x- and y-axis. The red line marks the maximum inclination of 37∘, derived from Roche geometry, and the blue shaded area indicates the M2 mass derived from a mass–radius relationship. Given the high systematic uncertainty on M2, we adopt less strict constraints of M1 = 1.2 ± 0.2 M⊙ and M2 = 0.25 ± 0.05 M⊙.

Extended Data Fig. 4 Pulse shape for different orbital phases and nights.

The thick red line shows all the X-ray data averaged to 20 phase bins (with an average of 93 measurements per bin) and folded on the spin ephemeris. The uncertainty on the mean is shown for spin phase 0 to 1. The thin lines and symbols show ULTRACAM gs data averaged to 20 phase bins (with an average of 16 measurements per bin) and folded on the same ephemeris, but considering data only within the orbital phase ranges shown on the right of the plot. Uncertainties on the mean are shown for spin phase 1 to 2. The green dashed line shows data taken on 2022 June 07, the black symbols are data taken on 2022 September 17 (simultaneously with the X-ray data) with one-sigma uncertainties, and the solid blue line shows data for 2022 September 23. All data were normalised to the strongest peak to facilitate comparison. As also seen in Fig. 2, the peak of the X-ray pulses does not align with the bulk of optical pulses. However, it does align with the optical peaks observed on 2022 September 17. This difference cannot be attributed to uncertainty in the ephemeris, given the agreement between data taken on nights before and after the X-ray observations. Additional simultaneous data is needed to determine the cause of misalignment, which could possibly be due to sporadic changes on pulse profile.

Extended Data Fig. 5 Flux and colour of the possible flare.

Panel (a) shows the flux in the us (blue triangles), gs (green circles), and rs (red crosses) bands, with respective one-sigma uncertainties for each measurement, in the region of the feature that we identify as a flare (marked by the shaded grey area). Panels (b) and (c) show the us-gs and gs-rs colours, again with one-sigma uncertainties. Unlike typical M dwarf flares, there is no evidence of flux increase towards the blue.

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Supplementary Information

Supplementary Tables 1–4 and Figs. 1–13.

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Pelisoli, I., Marsh, T.R., Buckley, D.A.H. et al. A 5.3-min-period pulsing white dwarf in a binary detected from radio to X-rays. Nat Astron 7, 931–942 (2023). https://doi.org/10.1038/s41550-023-01995-x

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