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Unconventional origin of supersoft X-ray emission from a white dwarf binary


Supersoft X-ray sources are stellar objects that emit X-rays with temperatures of about 1 million kelvin and luminosities well in excess of what can be produced by stellar coronae. It has generally been presumed that the objects in this class are binary star systems in which mass transfer leads to nuclear fusion on the surface of a white dwarf1. Classical novae—the runaway fusion events on the surfaces of white dwarfs—generally have supersoft phases, and it is often stated that the bright steady supersoft X-ray sources seen from white dwarfs accreting mass at a high rate are undergoing steady nuclear fusion1. Here, we report the discovery of a transient supersoft source in the Small Magellanic Cloud without any signature of nuclear fusion having taken place. This discovery indicates that the X-ray emission probably comes from a ‘spreading layer’2—a belt on the surface of the white dwarf near the inner edge of the accretion disk in which a large fraction of the total accretion energy is emitted—and (albeit more tentatively) that the accreting white dwarf is relatively massive. We thus establish that the presence of a supersoft source cannot always be used as a tracer of nuclear fusion, in contradiction with decades-old consensus about the nature of supersoft emission.

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Fig. 1: Best-fitting Chandra spectrum of ASASSN-16oh using a non-local thermal equilibrium model for the supersoft source atmosphere7.
Fig. 2: The OGLE I-band light curve for ASASSN-16oh.
Fig. 3: SALT optical spectra taken just after the optical maximum (14 December 2016) with gratings PG2300 and PG3000.
Fig. 4: Ultraviolet spectral energy distribution of the new SMC transient.

Data availability

The data from Chandra and Swift are available from the NASA HEASARC repository. The first two SALT spectra are available from, while the additional SALT spectra are available by contacting A.R. ( The remaining SALT spectra are available from All other data that support the plots within this paper and the other findings of this study are available from the corresponding author upon reasonable request.


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We thank J. Sokoloski, D. Crnojević and C. Sneden for useful discussions. We thank B. Wilkes and the CXC staff for approving and executing a director’s discretionary time observation. The OGLE project has received funding from the National Science Center, Poland (grant MAESTRO 2014/14/A/ST9/00121 to A.U.). P.A.C. acknowledges support of the Leverhulme Trust. Some of these observations were done with the SALT under programme 2016-2-LSP-001. D.A.H.B. acknowledges support from the National Research Foundation.

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Authors and Affiliations



T.J.M. wrote one of the proposals for Swift observations, reduced and analysed the Swift X-ray data, modelled the ultraviolet data, wrote the Chandra proposal, contributed heavily to the interpretation and wrote most of the paper text. T.J.N. analysed the Chandra data and contributed heavily to the interpretation. P.J.B. wrote one of the Swift proposals, reduced the Swift Ultraviolet/Optical Telescope data and contributed to the interpretation. K.M. first proposed the spreading layer hypothesis and contributed heavily to the interpretation beyond that. P.A.C., J.S., A.R., C.T.B., L.C., D.A.H.B., S.W.J., P.M., A.U., M.K.S., I.S., R.P., S.K., P.P., J.S. and K.U. provided supporting optical data. All authors reviewed the paper draft.

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Correspondence to Thomas J. Maccarone.

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Maccarone, T.J., Nelson, T.J., Brown, P.J. et al. Unconventional origin of supersoft X-ray emission from a white dwarf binary. Nat Astron 3, 173–177 (2019).

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