Hubble Space Telescope ultraviolet spectroscopy of the supersoft X-ray binaries CAL83 and RXJ0513.9-6951

Abstract

We present Hubble Space Telescope ultraviolet observations with the Goddard High Resolution Spectrograph of the supersoft X-ray binaries CAL83 and RXJ0513.9-6951 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 Lyalpha profile are {N_H}i=(6.5+/- 1.0)*E(20) cm(-2) for CAL83 and {N_H}i=(5.5+/- 1.0)*E(20) cm(-2) for RXJ0513.9-6951. 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 N_Hi, we find that the X-ray source in CAL83 has a luminosity of (0.7 - 2)x 10(37) ergs;s(-1) . The inferred radius for the supersoft source in CAL83 is consistent with the radius of a white dwarf which has not significantly expanded due to the shell burning. For RXJ0513.9-6951, we find a luminosity of (2.5-9)x 10(37) ergs;s(-1) . The long-term ultraviolet light curve of CAL83 shows a rare bright state during which the ultraviolet flux has doubled. 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