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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The data from Chandra and Swift are available from the NASA HEASARC repository. The first two SALT spectra are available from https://wis-tns.weizmann.ac.il/object/2016irh, while the additional SALT spectra are available by contacting A.R. (firstname.lastname@example.org). The remaining SALT spectra are available from https://cloudcape.saao.ac.za/index.php/s/qeodvvMP1TLIy4H. 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.