Energy-dependent polarization variability as a black hole signature

A fully relativistic approach is used to calculate the observed polarization properties of sources orbiting a black hole. Unlike the Newtonian case, an irregular matter distribution around a black hole exhibits energy-dependent polarization variability; both the observed degree of polarization and the angle of the polarization plane have larger variability amplitudes at higher frequencies. These effects are dominated by the strong bending of light rays by the central mass. Observational confirmation of this predicted effect may serve as a rigorous test of the presence of a black hole.