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Astron. Astrophys. 347, L43-L46 (1999)

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Letter to the Editor

Luminous supersoft X-ray emission from the recurrent nova U Scorpii

P. Kahabka 1, H.W. Hartmann 2, A.N. Parmar 3 and I. Negueruela 4

1 Astronomical Institute and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3 Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands
4 SAX Science Data Center, ASI, c/o Telespazio, via Corcolle 19, I-00131 Roma, Italy

Received 7 May 1999 / Accepted 15 June 1999

Abstract

BeppoSAX detected luminous 0.2-2.0 keV supersoft X-ray emission from the recurrent nova U Sco [FORMULA]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 [FORMULA]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 [FORMULA] and the bolometric luminosity [FORMULA]. These values are in agreement with those predicted for steady nuclear burning on a WD close to the Chandrasekhar 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.

Key words: stars: binaries: close – X-rays: stars – stars: evolution – stars: white dwarfs – stars: individual: U Scorpii

Send offprint requests to: ptk@astro.uva.nl

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999
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