A novel shadow from the thin-shell wormhole

For dark compact object such as black hole or wormhole, a shadow has long been thought to come from the outermost unstable photon sphere (region). However, by considering the thin-shell wormhole model, we identify a novel shadow which originates from spacetime asymmetry of both sides connected by the throat other than the photon sphere in the observer's spacetime. The novel shadows appear in three cases: (A2) The observer's spacetime contains a photon sphere and the mass parameter is smaller than that of the opposite side; (B1, B2) there' s no photon sphere no matter which mass parameter is bigger. In particular, comparing with black hole, the wormhole shadow size is always smaller and their difference is significant in most cases, which provides a potential way to observe wormholes directly through Event Horizon Telescope with better detection capability in the future.