On possible photometric manifestation of magnetic field in Cyg X-1

We consider the influence of the magnetic field of the O9.7 Iab supergiant component in the Cyg X-1 X-ray binary system on its atmosphere. In the frame of the simplest model (the unipolar cylindrically symmetric circum-polar magnetic spot model in static approximation, neglecting the Lorentz force component related to the force line curvature), the magnetic pressure is found to be comparable to model-atmosphere gas and radiative pressure, exceeding them in the area around the magnetic poles. As a result, bright spots are formed on the star surface. The dipolar or quadrupolar magnetic field can create large-size bright spots which can be studied by ground-based optical photometry. In the case of magnetic field inclined to the stellar rotation axis, resulting variability may be of the order of 1%. The field of higher multipolar configuration (produced by dynamo) can form smaller spots and may be detected only by space telescopes. Besides, the spots may be revealed from spectral-line profile variability. Observations of spots can be used as an instrument of magnetic field analysis.