  Astrophysical black hole candidates are thought to be the Kerr black holes
predicted by General Relativity, but the actual nature of these objects has
still to be proven. The Kerr black hole hypothesis can be tested by observing
strong gravity features and check if they are in agreement with the predictions
of General Relativity. In particular, the study of the properties of the
electromagnetic radiation emitted by the gas of the accretion disk can provide
information on the geometry of the space-time around these objects and
constrain possible deviations from the Kerr background.
