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 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 $M_{\odot }$).

21 pages, 5 figures, 9 tables

References in corpus (7)

• Common Envelope Evolution: Where we stand and how we can move forward
• The 35-Day Evolution of the Hercules X-1 Pulse Profile: Evidence For A Resolved Inner Disk Occultation of the Neutron Star
• Updating the orbital ephemeris of Her X-1; rate of decay and eccentricity of the orbit
• Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances
• The Photoionized Accretion Disk in Her X-1
• Continuous monitoring of pulse period variations in Her X-1 using Swift/BAT
• Hopkins Ultraviolet Telescope Observations of Her X-1

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