Constraining the deformation of a rotating black hole mimicker from its shadow

We consider a black hole mimicker given by an exact solution of the stationary and axially-symmetric field equations in vacuum known as the δ-Kerr metric. We study its optical properties based on a ray-tracing code for photon motion and characterize the apparent shape of the shadow of the compact object and compare it with the Kerr black hole. For the purpose of obtaining qualitative estimates related to the observed shadow of the supermassive compact object in the galaxy M87 we focus on values of the object's spin a and inclination angle of observation θ0 close to the measured values. We then apply the model to the shadow of the δ-Kerr metric to obtain constraints on the allowed values of the deformation parameter. We show that based uniquely on one set of observations of the shadow's boundary it is not possible to exclude the δ-Kerr solution as a viable source for the geometry in the exterior of the compact object.