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X-ray emission of massive stars and their winds

Gregor Rauw

arXiv, 2022

Abstract

Most types of massive stars display X-ray emission that is affected by the properties of their stellar winds. Single non-magnetic OB stars have an X-ray luminosity that scales with their bolometric luminosity and their emission is thought to arise from a distribution of wind-embedded shocks. The lack of significant short-term stochastic variability indicates that the winds consist of a large number of independent fragments. Detailed variability studies unveiled a connection between the photosphere and the wind: well-studied O-type stars exhibit a ~ 10% modulation of their emission on timescales consistent with the rotation period, and a few early B-type pulsators display ~ 10% modulations of their X-ray flux with the pulsation period. Unlike OB stars, their evolved descendants (WR and LBV stars) lack a well-defined relation between their X-ray and bolometric luminosities, and several subcategories of objects remain undetected. These properties most likely stem from the combined effects of wind optical depth and wind velocity. Magnetic OB stars display an enhanced X-ray emission frequently modulated by the rotation of the star. These properties are well explained by the magnetically confined wind shock model and an oblique magnetic rotator configuration. Some massive binaries display phase-dependent excess emission arising from the collision between the winds of the binary components. Yet, the majority of the massive binaries do not show such an emission, probably as a…

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