The Mass of the Compact Object in the X-Ray Binary Her X-1/HZ Her

TL;DR

This study estimates the masses of the components in the Her X-1/HZ Her X-ray binary system using non-LTE effects, providing initial values that differ from LTE models and highlighting the need for further precise measurements.

Contribution

First estimates of component masses in Her X-1/HZ Her considering non-LTE effects, improving upon previous LTE-based estimates.

Findings

Mass of X-ray pulsar: 1.8 solar masses (non-LTE)

Mass of optical star: 2.5 solar masses (non-LTE)

LTE model estimates differ significantly from non-LTE estimates

Abstract

We have obtained the first estimates of the masses of the components of the Her X-1/HZ Her X-ray binary system taking into account non-LTE effects in the formation of the H_gamma absorption line: mx=1.8Msun and mv=2.5Msun. These mass estimates were made in a Roche model based on the observed radial-velocity curve of the optical star, HZ Her. The masses for the X-ray pulsar and optical star obtained for an LTE model lie are mx=0.85\pm0.15Msun and mv=1.87\pm0.13Msun. These mass estimates for the components of Her X-1/HZ Her derived from the radial-velocity curve should be considered tentative. Further mass estimates from high-precision observations of the orbital variability of the absorption profiles in a non-LTE model for the atmosphere of the optical component should be made.