Hopkins Ultraviolet Telescope Observations of Hercules X-1

We have obtained a far-ultraviolet spectrum of the X-ray binary Hercules X-1/HZ Herculis using the Hopkins Ultraviolet Telescope aboard the Astro-1 Space Shuttle mission in 1990 December. This is the first spectrum of Her X-1 that extends down to the Lyman limit at 912 Å. We observed emission lines of O VI λλ1031.9, 1037.6, N V λλ1238.8, 1242.8, and C IV λλ1548.2, 1550.8 and the far-UV continuum extending to the Lyman limit. We examine the conditions of the emitting gas through line strengths, line ratios, and doublet ratios. The UV flux is lower by about a factor of 2 than expected 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-illuminated accretion disk and companion star. The power law provides a superior fit, as the detailed model predicts too little flux below 1200 Å. We note, however, that there are uncertainties in the interstellar reddening, in the background airglow spectrum, and in the long-term phase of the accretion disk. We have searched the data for UV line and continuum pulsations near the neutron star's spin period but found none at a detectable level.