1998A&A...333..163G

We present Hubble Space Telescope ultraviolet observations with the Goddard High Resolution Spectrograph of the supersoft X-ray binaries CAL83 and RX J0513.9-6951 in the Large Magellanic Cloud. Both sources show a remarkably similar spectrum with an almost flat continuum and weak NV and OV emission lines. The neutral hydrogen column densities derived from the broad Lyα profile are N_HI=(6.5±1.0)x10²⁰cm^–2 for CAL83 and N_HI=(5.5±1.0)x10²⁰cm^–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 N_HI, we find that the X-ray source in CAL83 has a luminosity of (0.7-2)x10³⁷ergs/s. 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 RX J0513.9-6951, we find a luminosity of (2.5-9)x10³⁷ergs/s. The long-term ultraviolet light curve of CAL83 shows a rare bright state during which the ultraviolet flux has doubled.