CXO J004318.8+412016, a steady supersoft X-ray source in M 31

ORIO, MarinaMarinaORIOLuna, G. J. M.G. J. M.LunaKotulla, R.R.KotullaGallager, J. S.J. S.GallagerZAMPIERI, LucaLucaZAMPIERIMikolajewska, J.J.MikolajewskaHarbeck, D.D.HarbeckBianchini, A.A.BianchiniChiosi, E.E.ChiosiDELLA VALLE, MassimoMassimoDELLA VALLEDE MARTINO, DomitillaDomitillaDE MARTINOKaur, A.A.KaurMapelli, M.M.MapelliMUNARI, UlisseUlisseMUNARIOdendaal, A.A.OdendaalTRINCHIERI, GinevraGinevraTRINCHIERIWade, J.J.WadeZemko, P.P.Zemko2020-09-102020-09-1020170035-8711http://hdl.handle.net/20.500.12386/27286We 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 \dot{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.STAMPAenCXO J004318.8+412016, a steady supersoft X-ray source in M 31Article10.1093/mnras/stx13552-s2.0-85023748094000406839900069http://arxiv.org/abs/1705.10875v1https://academic.oup.com/mnras/article/470/2/2212/38618542017MNRAS.470.2212OFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering