An Archival Study of Hubble Space Telescope Observations of Hercules X-1/HZ Herculis

Abstract

We present an archival study of observations of Her X-1/HZ Her obtained by the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST). The optical light curve of HZ Her has previously been modeled as an X-ray heated star (e.g., Bahcall & Bahcall 1972) or an X-ray heated star plus disk (e.g., Gerend & Boynton 1976; Howarth & Wilson 1983a, b). We have utilized the system geometry as modeled by Howarth & Wilson (1983a) and have added blackbody components to our X-ray heating code to allow us to match better the observed slope of the HST/FOS spectrum and the strength of the Balmer jump. By fitting our model to the HST/FOS spectrum obtained at total eclipse (binary phase 0.0), we determine the mean temperature of the unheated star surface to be 8100±240 K consistent with an A7 star. From the best-fit model to the HST/FOS spectrum near the binary phase 0.5, we determine a mass transfer rate M^ṡ = (6.5±0.9) x 10^-9 M_sun yr^-1.