Shadow of rotating non-Kerr black hole

The shadow of a rotating non-Kerr black hole has been studied, and it was shown that in addition to the specific angular momentum alpha, the deformation parameter of non-Kerr spacetime essentially deforms the shape of the black hole shadow. For a given value of the black hole spin parameter alpha, the presence of a deformation parameter epsilon reduces the shadow and enlarges its deformation with respect to the one in the Kerr spacetime. Optical features of the rotating non-Kerr black hole in terms of rotation of the polarization vector along null congruences have been studied. A comparison of the obtained theoretical results on the polarization angle with the observational data on Faraday rotation measurements provides the upper limit for the dimensionless deformation parameter as epsilon <= 19.