Massive star winds and HMXB donors

Abstract

Understanding the complex behavior of High Mass X-ray binaries (HMXBs) is not possible without detailed information about their donor stars. While crucial, this turns out to be a challenge on multiple fronts. First, multi-wavelength spectroscopy is vital. As such systems can be highly absorbed, this is often already hard to accomplish. Secondly, even if the spectroscopic data is available, the determination of reliable stellar parameters requires sophisticated model atmospheres that accurately describe the outermost layers and the wind of the donor star. For early-type donors, the stellar wind is radiatively driven and there is a smooth transition between the outermost layers of the star and the wind. The intricate non-LTE conditions in the winds of hot stars complicate the situation even further, as proper model atmospheres need to account for a multitude of physics to accurately provide stellar and wind parameters. The latter are especially crucial for the so-called "wind-fed" HXMBs, where the captured wind of the supergiant donor is the only source for the material accreted by the compact object. In this review I will address the different approaches for treating stellar winds in the analysis of HMXBs. Fundamentals of stellar atmosphere modeling are discussed, also addressing the limitations of modern models and examples from recent analysis results for particular HMXBs. Moreover, the path for the next generation of stellar atmosphere models will be outlined, where models can be used not only for measurements, but also to make predictions and provide a laboratory for theoretical conclusions. Stellar atmospheres are a key tool in understanding HMXBs, e.g. by providing insights about the accretion of stellar winds onto the compact object, or by placing the studied systems in the correct evolutionary context in order to identify potential gravitational wave (GW) progenitors.