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Shadows of black holes surrounded by an optically thin emitting medium havebeen extensively discussed in the literature. The Hioki-Maeda algorithm is asimple recipe to characterize the shape of these shadows and determine theparameters of the system. Here we extend their idea to the case of a dressedblack hole, namely a black hole surrounded by a geometrically thin andoptically thick accretion disk. While the boundary of the shadow of black holessurrounded by an optically thin emitting medium corresponds to the apparentphoton capture sphere, that of dressed black holes corresponds to the apparentimage of the innermost stable circular orbit. Even in this case, we cancharacterize the shape of the shadow and infer the black hole spin and viewingangle. The shape and the size of the shadow of a dressed black hole arestrongly affected by the black hole spin and inclination angle. Despite that,it seems that we cannot extract any additional information from it. Here westudy the possibility of testing the Kerr metric. Even with the full knowledgeof the boundary of the shadow, those of Kerr and non-Kerr black holes are verysimilar and it is eventually very difficult to distinguish the two cases.

Shadow of a dressed black hole and determination of spin and viewing angle