Abstract
Most types of massive stars display X-ray emission that is strongly affected by the properties of their stellar winds. Single nonmagnetic 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 investigations of temporal variability 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 their rotation period, and a few early B-type pulsators display ∼10% modulations of their X-ray flux with the same period as their photospheric pulsations. Unlike OB stars, their evolved descendants (Wolf-Rayet stars and luminous blue variables) 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 which is 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, a majority of the massive binaries do not show evidence for such an emission, probably as a consequence of radiative cooling of the shock-heated plasma. Finally, a growing subset of the Be stars, the so-called γ Cas stars, features an unusually hard and strong thermal X-ray emission that varies in a complicated manner over a wide range of timescales. Several scenarios have been proposed to explain these properties, but the origin of the γ Cas phenomenon remains currently one of the major unsolved puzzles in stellar X-ray astrophysics.
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Acknowledgements
I’m grateful to Yaël Nazé for discussion. This work was supported by the Fonds National de la Recherche Scientifique under grant PDR T.0192.19 and by the Belgian Federal Science Policy Office (BELSPO) through the HERMeS PRODEX contract.
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Rauw, G. (2024). X-ray Emission of Massive Stars and Their Winds. In: Bambi, C., Santangelo, A. (eds) Handbook of X-ray and Gamma-ray Astrophysics. Springer, Singapore. https://doi.org/10.1007/978-981-19-6960-7_79
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