Magnetically gated accretion in an accreting ‘non-magnetic’ white dwarf

Abstract

White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 106 gauss or more) to channel the accreted matter along field lines onto the magnetic poles1,2. The remaining systems are referred to as ‘non-magnetic’, because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the ‘non-magnetic’ accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion3,4,5, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 104 gauss and 1 × 105 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified6,7,8,9.

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Figure 1: Optical brightness variations in MV Lyr.
Figure 2: Power spectrum and flux distribution of MV Lyr in the deep low state and regular low state.
Figure 3: Schematic depiction of the accretion flow in MV Lyr during phases of magnetically gated accretion cycles.
Figure 4: Magnetically gated accretion instability plane.

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Acknowledgements

This paper includes data collected by the Kepler mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. Some of the data were obtained from the Barbara A. Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-Hubble Space Telescope data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. P.J.G. acknowledges support from the Erskine programme run by the University of Canterbury.

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S.S. analysed the Kepler data, identified the phenomenon, interpreted the results and was the primary author. T.J.M. first proposed that the phenomenon might be magnetically gated accretion and helped to work out the initial parameter space. C.D’A. contributed theoretical analysis of the bursts and created Fig. 4. C.K. carried out AAVSO-based calibration of the Kepler data, created Fig. 1a and estimated the accretion rate in the magnetic gating state. P.J.G. laid out Figs 1 and 2 and provided the literature on nova-likes. All authors shared ideas, interpreted the results, commented and edited the manuscript.

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Correspondence to S. Scaringi.

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Scaringi, S., Maccarone, T., D’Angelo, C. et al. Magnetically gated accretion in an accreting ‘non-magnetic’ white dwarf. Nature 552, 210–213 (2017). https://doi.org/10.1038/nature24653

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