On possible photometric manifestation of magnetic field in Cyg X-1
The influence of recently detected magnetic field of O9.7 Iab supergiant component of Cyg X-1 X-ray binary system on its atmosphere was considered. In the frame of the simplest model (the unipolar cylindrically symmetric circum-polar magnetic spot model in static approximation with neglect of the Lorentz force component related to the force line curvature) the magnetic pressure is found to be comparable with model atmosphere gas and radiative pressure, exceeding them in the area surrounding the magnetic poles. It should lead to formation of bright spots on the stellar surface. We obtained the upper estimation of the relative brightness ΔI∕I ≃ +0.25 of magnetic spot near 5000 Å on the Cyg X-1 optical component. The dipolar or quadrupolar magnetic field can create large size bright spots which can be studied by the ground-based optical photometry. In the case of magnetic field, inclined to the stellar rotation axis the suspected variability may achieve about 1 %. The field of higher multipolar configuration (produced by dynamo) can form the spots of less size and may be revealed only by space telescopes. Besides, the spots may be revealed in spectral line profile variability. The spot observation can be considered as an independent instrument of magnetic field analysis for O-supergiants such as in Cyg X-1.
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