A study of the low-mass X-ray binary HZ HER/HER X-1 using IUE and optical data.

Abstract

An extensive body of uniformly extracted IUE spectrophotometry of HZ Her/Her X-l is presented. To interpret these data a light-curve synthesis model comprising a distorted, X-ray heated primary accompanied by an accretion disc is developed. Roche geometry is used for the star, and the X-ray shadow thrown by the disc accounted for. The model's parameters are estimated by fitting to optical photometry compiled by Gerend & Boynton. We conclude that the intrinsic hard X-ray luminosity of Her X-1 is ∼⅓L_Edd, that a fan pattern is favoured over pencil-beamed X-ray emission, and that ∼½ the hard X-rays incident on HZ Her are reprocessed to optical radiation. It is argued that non-local reradiation of incident X-ray energy is unimportant. In addition, a large accretion disc is required (R_disc O.8R_lobe). The model successfully reproduces the lUE data near maximum light, and allows the UV spectrum of the accretion disc to be isolated. This spectrum can be approximated by a power law, f_λ ∝ λ^-5/4, which varies in level by a factor ≳ 3. The disc's luminosity is provided by reprocessed X-rays.