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Beginning of the Super-Soft Phase of the Classical Nova V2491 Cygni

We present the results of soft X-ray studies of the classical nova V2491 Cygni using the Suzaku observatory. On day 29 after outburst, a soft X-ray component with a peak at ~0.5 keV has appeared, which is tantalising evidence for the beginning of the super-soft X-ray emission phase. We show that an absorbed blackbody model can describe the observed spectra, yielding a temperature of 57 eV, neutral hydrogen column density of 2x10^{21} cm^{-2}, and a bolometric luminosity of ~10^{36} erg s^{-1}. However, at the same time, we also found a good fit with an absorbed thin-thermal plasma model, yielding a temperature of 0.1 keV, neutral hydrogen column density of 4x10^{21} cm^{-2}, and a volume emission measure of ~10^{58} cm^{-3}. Owing to low spectral resolution and low signal-to-noise ratio below 0.6 keV, the statistical parameter uncertainties are large, but the ambiguity of the two very different models demonstrates that the systematic errors are the main point of concern. The thin-thermal plasma model implies that the soft emission originates from optically thin ejecta, while the blackbody model suggests that we are seeing optically thick emission from the white dwarf.