Accreting white dwarfs are often found in close binary systems with orbital periods ranging from tens of minutes to several hours. In most cases, the accretion process is relatively steady, with significant modulations only occurring on timescales of ~days or longer1,2. Here we report the discovery of abrupt drops in the optical luminosity of the accreting white dwarf binary system TW Pictoris by factors up to 3.5 on timescales as short as 30 minutes. The optical light curve of this binary system obtained by the Transiting Exoplanet Survey Satellite (TESS) clearly displays fast switches between two distinct intensity modes that probably track the changing mass accretion rate onto the white dwarf. In the low mode, the system also displays magnetically gated accretion bursts3,4,5, which implies that a weak magnetic field of the white dwarf truncates the inner disc at the co-rotation radius in this mode. The properties of the mode switching observed in TW Pictoris appear analogous to those observed in transitional millisecond pulsars6,7,8,9,10, where similar transitions occur, although on timescales of ~tens of seconds. Our discovery establishes a previously unrecognized phenomenon in accreting white dwarfs and suggests a tight link to the physics governing magnetic accretion onto neutron stars.
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This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the National Aeronautics and Space Administration’s (NASA) Science Mission Directorate. Some of the data presented in this paper were obtained from MAST. The Space Telescope Science Institute (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 (HST) data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. This paper uses data from the ASAS-SN project run by the Ohio State University. D.d.M. and A.P. acknowledge financial support from the Italian Space Agency (ASI) and National Institute for Astrophysics (INAF) under agreements ASI-INAF I/037/12/0 and ASI-INAF n.2017-14-H.0, from INAF ‘Sostegno alla ricerca scientifica main streams dell'INAF’, Presidential Decree 43/2018 and INAF ‘SKA/CTA projects’, Presidential Decree 70/2016, and from PHAROS COST Action N. 16214. D.A.H.B acknowledges support from the National Research Foundation (NRF). P.J.G. is supported by NRF South African Research Chairs Initiative (SARChI) grant 111692. We thank the ASAS-SN team for making their data publicly available. We acknowledge T. Boller and F. Haberl for providing the ROSAT light curve of TW Pic. This work has also made use of data from the European Space Agency 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 authors declare no competing interests.
Peer review information Nature Astronomy thanks Knox Long and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Lomb-Scargle periodogram using TESS data (solid black line) within the 3-4 cycle/day region. a. All Cycle 1 observed in 2 min cadence mode. b. Low mode from Cycle 3 observed at 20 sec cadence. c. High mode from Cycle 3 observed at 20 sec cadence. In all panels the dashed red line represented the 1 in 100,000 significance confidence level.
Cycle 3 TESS 20 sec cadence lightcurves displaying the four recorded high-to-low mode transitions (top row) and the three recorded low-to-high mode transitions (bottom three rows).
Cycle 1 and Cycle 3 TESS data (grey points) overlaid onto the long term ASAS-SN V-band (magenta points) and g-band (green points) photometry. TW Pic shows evidence for slow (≈100 days) changes in its mass transfer rate from V-band and g-band ASAS-SN observations. The rapid mode transitions are only observed in g-band ASAS-SN data (around day 1800) and during Cycle 3 TESS observations.
Cycle 3 TESS 20 sec cadence light curves displaying the low-to-high transition (top panel) as well as enlarged panels (bottom row) showing representative failed mode transitions.
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Scaringi, S., de Martino, D., Buckley, D.A.H. et al. An accreting white dwarf displaying fast transitional mode switching. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01494-x