Shadows and Accretion Disk Images of Compact Objects

Abstract

AbstractThe black hole shadow was predicted theoretically already in the 1960s as a strong gravitational lensing phenomenon. Recently, its observation became feasible and opened a major experimental channel for probing the gravitational interaction in the strong field regime. Although considered historically as a property of the black hole spacetimes, the shadow is not exclusively a black hole effect and does not require the presence of an event horizon. Other compact objects can also cast a shadow if their gravitational field is sufficiently strong bringing up the issue of how we can differentiate between self-gravitating systems based on their shadow images. In this chapter we discuss the analytical or semi-analytical methods for obtaining the observable images produced by compact objects in some basic physical settings such as a uniform spherical distribution of distant light sources or the presence of a thin accretion disk. We review the calculation of the shadow boundary for the Kerr black hole and focus on recent research on the images created by wormholes and naked singularities. These compact objects can look qualitatively very similar to black holes in some cases, but they can also possess clear-cut observational signatures.