UV Spectra of the Eclipsing LMC X-Ray Binary CAL 87
Hutchings, J. B.; Crampton, D.; Cowley, A. P.; Schmidtke, P. C.
Abstract
UV spectra of the supersoft x-ray binary CAL 87 have been obtained with the Faint Object Spectrograph of the HST. Eight spectra were obtained in 1995 January in a continuous observation covering the eclipse, from orbital phase 0.65 to 0.02. Interstellar extinction is modeled and removed to measure the system luminosity. The disk bolometric luminosity is estimated at ~10^36^ ergs s^-1^. Continuum light curves are derived for wavelengths from ~1200 to 2500 A, which are broadly similar to the variations seen in the visible. Line emission is present at HeII 1640 A and in a broad, unidentified feature at ~1500 A. All data show pronounced changes which are related to the eclipse and pre-eclipse dip. The He II 1640A velocities are similar to those of 4686 A from 1994 November, but both differ in amplitude and phase from those of 4686 A observed in 1988. The observations are interpreted in terms of an accretion disk in the system, containing a thickened area where the mass-transfer stream joins it.