HZ Her: Stellar Radius from X-Ray Eclipse Observations, Evolutionary State, and a New Distance

Abstract

Observations of HZ Her/Her X-1 by the Rossi X-Ray Timing Explorer (RXTE) covering high state eclipses of the neutron star are analyzed here. Models of the eclipse are used to measure the radius and atmospheric scale height of HZ Her, the stellar companion to the neutron star. The radius is 2.58-3.01 × 10¹¹ cm, depending on system inclination and mass ratio (q), with an accuracy of ~1 part in 1000 for given inclination and q. We fit Kurucz model stellar atmosphere models to archival optical observations. The resulting effective temperature (T _eff) of the unheated face of HZ Her is determined to be in the 2σ range of 7720 K-7865 K, and metallicity (log (Z/Z _⊙)) in the range of -0.27 to +.03. The model atmosphere surface flux and new radius yield a new distance to HZ Her/Her X-1, depending on system inclination and q: a best-fit value of 6.1 kpc with upper and lower limits of 5.7 kpc and 7.0 kpc. We calculate stellar evolution models for the range of allowed masses (from orbital parameters) and allowed metallicities (from optical spectrum fits). The stellar models agree with T _eff and the radius of HZ Her for two narrow ranges of mass: 2.15-2.20 M _⊙ and 2.35-2.45 M _⊙. This lower mass range implies a low neutron star mass (1.3 M _⊙), whereas the higher mass range implies a high neutron star mass (1.5-1.7 M _⊙).