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

Abstract

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 Lyalpha absorption line is consistent with an interstellar column density of neutral hydrogen N_HI=(4.0+/-1.0)*E^20};cm{-2. The light curve obtained from the time-tagged dataset puts a 3sigma 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. 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.