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.
Kahabka, P. & van den Heuvel, E. P. J. Luminous supersoft X-ray sources. Annu. Rev. Astron. Astrophys. 35, 69–100 (1997).
Piro, A. L. & Bildsten, L. Spreading of accreted material on white dwarfs. Astrophys. J. 610, 977–990 (2004).
Jha, S. W. et al. ASASSN-16oh: an unusual transient in the vicinity of the SMC. The Astronomer’s Telegram 9859 (2016).
Maccarone, T. J., Brown, P. & Mukai, K. Swift observations of ASASSN-16oh. The Astronomer’s Telegram 9866 (2016).
Mroz, P. et al. OGLE-IV observations of ASASSN-16oh. The Astronomer’s Telegram 9867 (2016).
Graczyk, D. et al. The Araucaria Project. The distance to the Small Magellanic Cloud from late-type eclipsing binaries. Astrophys. J. 780, 59–71 (2014).
Rauch, T. et al. Non-local thermal equilibrium model atmospheres for the hottest white dwarfs: spectral analysis of the compact component in nova V4743 Sgr. Astron. J. 717, 363–371 (2010).
Darnley, M. J. et al. M31N 2008-12a—the remarkable recurrent nova in M31: panchromatic observations of the 2015 eruption. Astrophys. J. 833, 149 (2016).
Pringle, J. E. Soft X-ray emission from dwarf novae. Mon. Not. R. Astron. Soc. 178, 195–202 (1977).
Inogamov, N. A. & Sunyaev, R. A.Spread of matter over a neutron-star surface during disk accretion. Astron. Lett. 25, 269–293 (1999).
Kippenhahn, R. & Thomas, H.-C.Accretion belts on white dwarfs. Astron. Astrophys. 63, 265–272 (1978).
Mauche, C. W., Raymond, J. C. & Mattei, J. A. EUVE observations of the anomalous 1993 August outburst of SS Cygni. Astrophys. J. 446, 842 (1995).
Warner, B. Absolute magnitudes of cataclysmic variables. Mon. Not. R. Astron. Soc. 227, 23–73 (1987).
Salazar, I. V., LeBleu, A., Schaefer, B. E., Landolt, A. U. & Dvorak, S. Accurate pre- and post-eruption orbital periods for the dwarf/classical nova V1017 Sgr. Mon. Not. R. Astron. Soc. 469, 4116–4132 (2017).
Shears, J. & Poyner, G. The 2009 outburst of V630 Cassiopeiae. J. British Astron. Association 120, 169 (2010).
Orosz, J., Thorstensen, J. R. & Honeycutt, R. K. The long-period orbit of the dwarf nova V630 Cassiopeiae. Mon. Not. R. Astron. Soc. 326, 1134–1140 (2001).
Smak, J. Outbursts of dwarf novae. Publ. Astron. Soc. Pac. 96, 5–18 (1984).
Di Stefano, R. & Kong, A. K. H. The discovery of quasi-soft and supersoft sources in external galaxies. Astrophys. J. 609, 710–727 (2004).
Brown, P. J. et al. Ultraviolet light curves of supernovae with the Swift Ultraviolet/Optical Telescope. Astron. J. 137, 4517–4525 (2009).
Gordon, K., Clayton, G. C., Misselt, K. A., Landolt, A. U. & Wolff, M. J. Astrophys. J. 594, 279–293 (2003).
Cannizzo, J. K. The accretion disk limit cycle model: toward an understanding of the long-term behavior of SS Cygni. Astrophys. J. 419, 318–336 (1993).
Gordon, K. D. et al. Surveying the Agents of Galaxy Evolution in the tidally stripped, low metallicity Small Magellanic Cloud (SAGE-SMC). I. Overview.Astron. J. 142, 102–116 (2011).
Casares, J. A FWHM-K2 correlation in black hole transients. Astrophys. J. 808, 80–90 (2015).
Wolf, W. M., Bildsten, L., Brooks, J. & Paxton, B. Hydrogen burning on accreting white dwarfs: stability, recurrent novae, and the post-nova supersoft phase. Astrophys. J. 777, 136–150 (2013).
Kato, M., Saio, H., Hachisu, I. & Nomoto, K. Shortest recurrence periods of novae. Astrophys. J. 793, 136 (2014).
Udalski, A. et al. OGLE-IV: fourth phase of the Optical Gravitational Lensing Experiment. Acta Astron. 65, 1–38 (2015).
Hynes, R. I. et al. Correlated X-ray and optical variability in V404 Cygni in quiescence. Astrophys. J. Lett. 611, L125–L128 (2004).
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.
The authors declare no competing interests.
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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) doi:10.1038/s41550-018-0639-1
The Astrophysical Journal (2019)