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

TL;DR

This study uses X-ray eclipse data and stellar atmosphere models to determine the radius, temperature, metallicity, and distance of HZ Her, providing insights into its evolutionary state and neutron star mass.

Contribution

It presents a novel combined analysis of X-ray eclipse observations and optical spectra to precisely measure stellar parameters and estimate the distance to HZ Her/Her X-1.

Findings

Stellar radius estimated at 2.58-3.01×10^{11} cm with high accuracy.

Effective temperature of HZ Her's unheated face is 7720K-7865K.

Distance to HZ Her/Her X-1 is approximately 6.1 kpc.

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 to 3.01 $\times10^{11}$ cm, depending on system inclination and mass ratio(q), with accuracy of $\sim$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 2$\sigma$ range 7720K to 7865K, and metallicity ($log(Z/Z_{\odot})$) in the range -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 radius of HZ Her for two narrow ranges of mass: 2.15 to 2.20 $M_{\odot}$ and 2.35 to 2.45 $M_{\odot}$ This lower mass range implies a low neutron star mass (1.3 $M_{\odot}$), whereas the higher mass range implies a high neutron star mass (1.5-1.7 $M_{\odot}$).