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Astron. Astrophys. 333, 163-171 (1998)

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Hubble Space Telescope ultraviolet spectroscopy of the supersoft X-ray binaries CAL 83 and RX J0513.9-6951 *

B.T. Gänsicke 1, A. van Teeseling 1, K. Beuermann 1 and D. de Martino 2

1 Universitäts-Sternwarte, Geismarlandstrasse 11, D-37083 Göttingen, Germany
2 Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Naples, Italy

Received 13 June 1997 / Accepted 23 January 1998

Abstract

We present Hubble Space Telescope ultraviolet observations with the Goddard High Resolution Spectrograph of the supersoft X-ray binaries CAL 83 and RX J0513.9-6951 in the Large Magellanic Cloud. Both sources show a remarkably similar spectrum with an almost flat continuum and weak N V and O V emission lines. The neutral hydrogen column densities derived from the broad [FORMULA] profile are [FORMULA] cm-2 for CAL 83 and [FORMULA] cm-2 for RX J0513.9-6951. These column densities are very similar to the galactic foreground column density, indicating that the two sources are located on the near side of the LMC. With this independent estimate of [FORMULA], we find that the X-ray source in CAL 83 has a luminosity of [FORMULA] [FORMULA]. The inferred radius for the supersoft source in CAL 83 is consistent with the radius of a white dwarf which has not significantly expanded due to the shell burning. For RX J0513.9-6951, we find a luminosity of [FORMULA] [FORMULA]. The long-term ultraviolet light curve of CAL 83 shows a rare bright state during which the ultraviolet flux has doubled.

Key words: accretion, accretion disks – binaries: close – stars: atmospheres – stars, individual: CAL 83, RX J0513.9 – 6951 – ultraviolet: stars

* Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555 and on observations made with the International Ultraviolet Explorer, retrieved from the IUE Final Archive

Send offprint requests to: B. Gänsicke (boris@uni-sw.gwdg.de)

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© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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