  We suppose that linear optical polarization is due to multiple scattering in
optically thick magnetized accretion disk around central black hole. The
polarization degree is very sensitive to the spin of black hole - for Kerr
rotating hole the polarization is higher than for Schwarzschild non-rotating
one if both holes have the same luminosities and masses. The reason of this
effect is that the radius of the first stable orbit for non-rotating hole is
equal to three gravitational radiuses, and for fast rotating Kerr hole is
approximately 6 times lesser. Magnetic field, decreasing from first stable
orbits, is much larger in the region of escaping of optical radiation for the
case of Schwarzschild hole than for Kerr one. Large magnetic field gives rise
to large depolarization of radiation due to Faraday rotation effect. This
explains the mentioned result. It seems that the ensemble of objects with
observed polarization mostly consists of Kerr black holes.
