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

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.Fil: Orio, Marina. Osservatorio Astronomico Di Padova; Italia. University of Wisconsin; Estados UnidosFil: Luna, Gerardo Juan Manuel. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina. Universidad de Buenos Aires; ArgentinaFil: Kotulla, R.. University of Wisconsin; Estados UnidosFil: Gallager, J. S.. University of Wisconsin; Estados UnidosFil: Zampieri, L.. Osservatorio Astronomico Di Padova; ItaliaFil: Mikolajewska, J.. Nicolaus Copernicus Astronomical Center Of The Polish Academy Of Sciences; PoloniaFil: Harbeck, D.. Wiyn Observatory; Estados UnidosFil: Bianchini, A.. Università di Padova; ItaliaFil: Chiosi, E.. Università di Padova; ItaliaFil: Della Valle, M.. Osservatorio Astronomico Di Capodimonte; ItaliaFil: de Martino, D.. Osservatorio Astronomico Di Capodimonte; ItaliaFil: Kaur, A.. Clemson University; Estados UnidosFil: Mapelli, M.. Osservatorio di Padova; ItaliaFil: Munari, U.. Osservatorio di Padova; ItaliaFil: Odendaal, A.. University of the Free State; SudáfricaFil: Trinchieri, G.. Osservatorio Astronomico di Brera; ItaliaFil: Wade, J.. University of Wisconsin; Estados UnidosFil: Zemko, P.. Università di Padova; Itali