Shadow of axisymmetric, stationary and asymptotically flat black holes in the presence of plasma

We study the shadow produced by a class of rotating black holes surrounded by plasma. The metric for these black holes arises by applying the Newman-Janis algorithm to a family of spherically symmetric spacetimes, which includes several well known geometries as special cases. We derive a general expression for the shape of the shadow in the case that the plasma frequency leads to a separable Hamilton-Jacobi equation for light. We present two examples in which we obtain the shadow contours and the observables resulting from them. In one, we analyze Kerr-Newman-like geometries, including braneworld and Horndeski gravity black holes, while in the other, we consider scalar-tensor 4D Einstein-Gauss-Bonnet gravity spacetimes. In both cases, we find that the presence of plasma leads to a smaller and less deformed shadow.

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