Shadow of a circular disformal Kerr black hole beyond GR

Abstract: We have studied the shadows of a circular disformal Kerr black hole with a deformation parameter, which represents a rotating solution within a generalized scalar-tensor framework of Horndeski gravity that characterized by second-order field equations. Our result show that for the non-rotating case, the shadow remains perfectly circular, with its radius independent of the deformation parameter. For the rotating case, the size of the shadow decreases as the deformation parameter decreases, and the shape of the shadow gradually becomes more flattened. It is worth noting that while the shadow of a rotating black hole remains north-south symmetric for equatorial observers, this symmetry is broken once the observer moves away from the equatorial plane. For an observer in the northern hemisphere, the geometric center of the shadow shifts northward when $D_0<0$ and southward when $D_0>0$, while the opposite behavior occurs for observers in the southern hemisphere. These features in the black hole shadow originating from the scalar field could help us to understand the circular disformal Kerr black hole and generalized scalar-tensor framework of Horndeski gravity.