HOPKINS ULTRAVIOLET TELESCOPE OBSERVATIONS OF HERCULES-X-1

We have obtained a far-ultraviolet spectrum of the X-ray binary Hercul es X-1/HZ Herculis using the Hopkins Ultraviolet Telescope aboard the Astro-1 Space Shuttle mission in 1990 December. This is the first spec trum of Her X-1 that extends down to the Lyman limit at 912 Angstrom. We observed emission lines of O VI lambda lambda 1031.9, 1037.6, N V l ambda lambda 1238.8, 1242.8, and C IV lambda lambda 1548.2, 1550.8 and the far-UV continuum extending to the Lyman limit. We examine the con ditions of the emitting gas through line strengths, line ratios, and d oublet ratios. The UV flux is lower by about a factor of 2 than expect ed at the orbital phase of the observation. We model the UV continuum with a simple power law and with a detailed model of an X-ray-illumina ted accretion disk and companion star. The power law provides a superi or fit, as the detailed model predicts too little flux below 1200 Angs trom. We note, however, that there are uncertainties in the interstell ar reddening, in the background airglow spectrum, and in the long-term phase of the accretion disk. We have searched the data for UV line an d continuum pulsations near the neutron star's spin period but found n one at a detectable level.