CXO J004318.8+412016*A steady supersoft X-ray source in M 31

Marina Orio, G. J.M. Luna, R. Kotulla, J. S. Gallager, L. Zampieri, J. Mikolajewska, D. Harbeck, A. Bianchini, E. Chiosi, M. Della Valle, D. de Martino, A. Kaur, M. Mapelli, U. Munari, A. Odendaal, G. Trinchieri, J. Wade, P. Zemko

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We obtained an optical spectrum of a star we identify as the optical counterpart of the M31 Chandra source CXO J004318.8+412016, because of prominent emission lines of the Balmer series, of neutral helium, and a He II line at 4686 Å. The continuum energy distribution and the spectral characteristics demonstrate the presence of a red giant of K or earlier spectral type, so we concluded that the binary is likely to be a symbiotic system. CXO J004318.8+412016 has been observed in X-rays as a luminous supersoft source (SSS) since 1979, with effective temperature exceeding 40 eV and variable X-ray luminosity, oscillating between a few times 1035 erg s-1 and a few times 1037 erg s-1 in the space of a few weeks. The optical, infrared and ultraviolet colours of the optical object are consistent with an an accretion disc around a compact object companion, which may be either a white dwarf or a black hole, depending on the system parameters. If the origin of the luminous supersoft X-rays is the atmosphere of a white dwarf that is burning hydrogen in shell, it is as hot and luminous as post-thermonuclear flash novae, yet no major optical outburst has ever been observed, suggesting that the white dwarf is very massive (m ≥ 1.2 M) and it is accreting and burning at the high rate m > 10-8 M yr-1 expected for Type Ia supernovae progenitors. In this case, the X-ray variability may be due to a very short recurrence time of only mildly degenerate thermonuclear flashes.

Original languageEnglish (US)
Pages (from-to)2212-2224
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Sep 11 2017

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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