Abstract:
We discuss the main features of shadow observations with special care to the presence of a sequence of light rings surrounding the central depression in optically-thin accretion disks. Considering the case of infinitely thin accretion disk we use three analytical model to reconstruct the optical appearances of several ultra-compact objects beyond the Schwarzschild solution. We discuss the chances of detecting new gravitational physics via the observation of these light rings using very-long baseline interferometry.