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 \times 10 ^ { 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 } ) .