ROSAT X-ray observations of nova V1974 Cygni: The rise and fall of the brightest supersoft X-ray source

The classical nova V1974 Cygni (1992) was observed by ROSAT on a total of 18 occasions from 1992 April 22 until 1993 December 3. All observations were carried out with the PSPC. Over the observation interval, the count rate rose from 0.03 ± 0.01 counts s^-1 to a peak of 76.5 ± 0.17 counts s^-1 in summer 1993 and then rapidly declined to a value of 0.22 ± 0.01 counts s^-1 on the last observation. Its brightness during the summer of 1993 made it the brightest supersoft source ever observed in X-rays. The initial observations showed only a hard component with a peak around 1 keV. Subsequently, during the X-ray rise, a much softer component appeared that dominated the emitted energy at maximum. It is also this soft component that decayed most rapidly. In the same time interval, it declined by a factor of 350 while the harder component declined by about a factor of 10. The hard component is most likely a signature of the mass loss in the system caused by the interaction between the expanding envelope and density inhomogeneities in the diffuse ejecta. The soft component showed all the characteristics of a supersoft source. In fact, V1974 Cyg could also serve as a paradigm for supersoft sources in general. From blackbody model fits to the measured spectral energy distribution of V1974 Cyg, it is abundantly clear that simple X-ray fits do not give adequate results and that spectral fit parameters obtained from blackbody fits to the supersoft X-rays sources are unreliable and should be used with care. We explain the supersoft component, in terms of the thermonuclear runaway model of the nova outburst, as the signature of the energy emitted by hydrostatic hydrogen burning on the surface of the white dwarf remnant. Our data are best explained by the soft X-rays source being present from near the beginning of the outburst. The initial rise in soft X-rays is, therefore, caused by the clearing of the ejected nebula as it expanded and its density decreased. In this model, the X-ray turnoff is caused by the cessation of nuclear burning on the white dwarf as the accreted hydrogen is exhausted 18 months after the beginning of the out-burst. We used the timescale for the decrease in the X-rays to estimate a mass of ∼10^-5 M_⊙ for the hydrogen-exhausted, remnant envelope on the white dwarf. For the very first time, the X-ray light curve of a classical nova has been observed from the early increase of the X-ray flux through maximum to turnoff.