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Astron. Astrophys. 351, L27-L31 (1999)

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Letter to the Editor

HST/STIS ultraviolet spectroscopy of the supersoft X-ray source RX J0439.8-6809 *

A. van Teeseling 1, B.T. Gänsicke 1, K. Beuermann 1, S. Dreizler 2, T. Rauch 2 and K. Reinsch 1

1 Universitäts-Sternwarte Göttingen, Geismarlandstrasse 11, 37083 Göttingen, Germany
2 Institut für Astronomie und Astrophysik, Waldhäuser Strasse 64, 72076 Tübingen, Germany

Received 5 August 1999 / Accepted 12 October 1999


We present ultraviolet observations of the supersoft X-ray source RX J0439.8-6809 obtained with the Hubble Space Telescope Imaging Spectrograph. The ultraviolet spectrum is a very blue continuum overlayed with interstellar absorption lines. The observed broad [FORMULA] absorption line is consistent with an interstellar column density of neutral hydrogen [FORMULA]. The light curve obtained from the time-tagged dataset puts a [FORMULA] upper limit of 0.04 mag on the ultraviolet variability of RX J0439.8-6809 on time scales between 10 s and 35 min. The long-term X-ray light curve obtained from our three-year ROSAT HRI monitoring of RX J0439.8-6809 shows the source with a constant count rate, and implies that the temperature did not change more than a few 1000 K. If RX J0439.8-6809 is a massive extremely hot pre-white dwarf on the horizontal shell-burning track, opposed to the alternative possibility of a very compact double-degenerate supersoft X-ray binary, its constant temperature and luminosity are a challenge to stellar evolution theory. Interestingly, RX J0439-6809 is found close to the theoretical carbon-burning main-sequence.

Key words: accretion, accretion disks – stars: individual: RX J0439.8-6809 – stars: novae, cataclysmic variables – stars: white dwarfs – X-rays: 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.

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

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999