Telescope ultraviolet spectroscopy of the supersoft X - ray binaries CAL 83 and RX J 0513 . 9 − 6951 ?

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 Lyα profile are NHI = (6.5 ± 1.0) × 1020 cm−2 for CAL 83 and NHI = (5.5 ± 1.0) × 1020 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 NHI, we find that the X-ray source in CAL 83 has a luminosity of (0.7− 2)× 1037 ergs s−1. 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 (2.5 − 9) × 1037 ergs s−1. The long-term ultraviolet light curve of CAL 83 shows a rare bright state during which the ultraviolet flux has doubled.