  Magnetic fields in an accretion disk around the central black hole can modify
the position of the innermost stable circular orbit (ISCO) radius and produces
the difference for the classical Novikov-Thorne radius. We estimated the ISCO
magnetic field strength on the base of polarimetric observations of the
accretion disk radiation. This estimate can be obtained with taking into
account the effect of Faraday rotation of the polarization plane at the length
of the free path of photon between successive electron scattering events. In a
result we presented the new method for real estimation of the ISCO radius in
the accretion disk, i.e. in the nearest vicinity of a central black hole. Our
estimations confirmed the V.P. Frolov, A.A. Shoom and C. Tzounis (2014)
conclusion that magnetic field produces the effect in a result of which the
innermost stable circular orbit becomes closer to the horizon of a black hole.
