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abstract
Nova V1974 Cyg was observed by ROSAT on 18 occasions from 22 April 1992 until 3 Dec. 1993. Over this interval it rose from a count rate of 0.3 $\pm$ 0.09 cts s$^{-1}$ to a peak of 76.52 $\pm$ 0.17 cts s$^{-1}$ in the summer of 1993 and then rapidly declined to a value of 0.22 $\pm$ 0.01 cts s$^{-1}$ on the last observation. Its brightness during the summer of 1993 made it the brightest Super Soft Source (SSS) ever observed in X-rays. Our initial observations showed only a hard component with a peak around 1 keV. During the X-ray rise, however, a much softer component appeared that dominated the emitted energy at maximum. It is also this super soft component that decayed the most rapidly. In the same time interval, it declined by a factor of 350 while the harder component declined by less than a factor of 10. We explain the super soft component as the signature of the energy emitted by the underlying white dwarf and its rise caused by the clearing of the ejected nebulae as it expanded and dispersed. The X-ray turn-off was, most likely, caused by the cessation of nuclear burning on the white dwarf as the accreted hydrogen was exhausted. The time to turn-off, about 18 months, implies that the binary system contains a massive ONeMg white dwarf. Although we are still studying the hard component, we suggest that this feature is a signature of mass loss in the system and is caused by the interaction between the differentially moving filaments and the diffuse ejecta. Both of these features were seen and analyzed in high resolution HST ultraviolet spectra (Shore et al. AJ, 106, 2408, 1993).
