On the X-ray Emission of Massive Binary Stars

Abstract

This thesis is dedicated to the theoretical investigation of massive binary systems consisting of a normal star and a neutron star or black hole as a companion. The initial focus lies on the mass transfer from an evolved star to a compact companion, analyzed through numerical simulations using a stellar evolution code. The objective is to examine the influence of the hydrogen/helium gradient in the envelope of the massive star on the mass transfer process. To this end, stellar models are constructed with variations in the H/He gradient and the mass of the hydrogen-rich envelope. These models are then used to simulate behavior under constant mass loss and during mass transfer to a compact object. <br /> The simulations indicate that the presence of a hydrogen/helium gradient in the outer layers of a massive star increases the mass–radius exponent, contributing to the stabilization of the mass transfer. It is demonstrated that mass transfer on a nuclear timescale can remain stable, even in systems with a large mass ratio between the components. Furthermore, it is shown that such stabilized mass transfer may be followed by a common-envelope phase. A comparison with observed X-ray binaries supports the theoretical predictions. <br /> A second major focus concerns the detectability of compact companions to massive stars. The goal is to develop a methodological framework that enables the identification of previously undetected companions. This is achieved by simultaneously analyzing X-ray and radial velocity signatures associated with a potential companion to a given massive star. The result is a set of diagnostic diagrams that can be readily applied to single-lined spectroscopic binaries and apparently single massive stars to infer the presence and nature of a companion. <br /> Various models for X-ray emission in massive binary systems are compiled, including a method to estimate whether an accretion disk can form around a black hole. The findings suggest that significant X-ray emission can be expected only if such a disk is present, providing a basis for observational detectability. <br /> The developed methodology is applied to selected massive stars. The analysis suggests that several apparently single Wolf–Rayet stars may host a black hole companion with X-ray emission too faint to have been detected thus far. Evidence also indicates that the single-lined binary VFTS 234 likely contains such a faint X-ray black hole companion. Similar conclusions are drawn for VFTS 514 and VFTS 779. <br /> The overall findings suggest that massive stars can host faint X-ray black hole companions, provided their wind velocities are sufficiently high. These results lend support to the hypothesis of a large, previously undetected population of stellar black holes as companions to massive stars, as predicted by various theoretical studies.