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We discovered and studied an ultraluminous X-ray source (CXOUJ203451.1+601043) that appeared in the spiral galaxy NGC 6946 at some pointbetween 2008 February and 2012 May, and has remained at luminosities$\approx$2-4 $\times 10^{39}$ erg s$^{-1}$ in all observations since then. Ourspectral modelling shows that the source is generally soft, but with spectralvariability from epoch to epoch. Using standard empirical categories of theultraluminous regimes, we find that CXOU J203451.1+601043 was consistent with abroadened disk state in 2012, but was in a transitional state approaching thesuper-soft regime in 2016, with substantial down-scattering of the hard photons(similar, for example, to the ultraluminous X-ray source in NGC 55). It hassince hardened again in 2018-2019 without any significant luminosity change.The most outstanding property of CXOU J203451.1+601043 is a strong emissionline at an energy of of $(0.66 \pm 0.01)$ keV, with equivalent width of$\approx$100 eV, and de-absorbed line luminosity of $\approx$2 $\times 10^{38}$erg s$^{-1}$, seen when the continuum spectrum was softest. We identify theline as OVIII Ly$\alpha$ (rest frame energy of 0.654 keV); we interpret it as astrong indicator of a massive outflow. Our finding supports the connectionbetween two independent observational signatures of the wind in super-Eddingtonsources: a lower temperature of the Comptonized component, and the presence ofemission lines in the soft X-ray band. We speculate that the donor star isoxygen-rich: a CO or O-Ne-Mg white dwarf in an ultracompact binary. If that isthe case, the transient behaviour of CXOU J203451.1+601043 raises intriguingtheoretical questions.

A Sparkler in the Fireworks Galaxy: Discovery of an Ultraluminous X-ray Transient with a Strong Oxygen Line in NGC 6946