Astronomy and Astrophysics Luminous Supersoft X-ray Emission from the Recurrent Nova U Scorpii

BeppoSAX detected luminous 0.2–2.0 keV su-persoft X-ray emission from the recurrent nova U Sco ∼19-20 days after the peak of the optical outburst in February 1999. U Sco is the first recurrent nova to be observed during a luminous supersoft X-ray phase. Non-LTE white dwarf atmosphere spectral models (together with a ∼0.5 keV optically thin thermal component) were fitted to the BeppoSAX spectrum. We find that the fit is acceptable assuming enriched He and an enhanced N/C ratio. This implies that the CNO cycle was active during the outburst, in agreement with a thermonuclear runaway scenario. The best-fit temperature is ∼ 9 × 10 5 K and the bolometric luminosity (0.16−1.2)×10 36 erg s −1 (d/kpc) 2. These values are in agreement with those predicted for steady nuclear burning on a WD close to the Chan-drasekhar mass. The fact that U Sco was detected as a supersoft X-ray source is consistent with steady nuclear burning continuing for at least one month after the outburst. This means that only a fraction of the previously accreted H and He was ejected during the outburst and that the WD can grow in mass, ultimately reaching the Chandrasekhar limit. This makes U Sco a candidate type Ia supernova progenitor.