  We consider the massive scalar field, the Proca field and the Fierz-Pauli
field in the Schwarzschild spacetime and we focus more particularly on their
long-lived quasinormal modes. We show numerically that the associated
excitation factors have a strong resonant behavior and we confirm this result
analytically from semiclassical considerations based on the properties of the
unstable circular geodesics on which a massive particle can orbit the black
hole. The conspiracy of (i) the long-lived behavior of the quasinormal modes
and (ii) the resonant behavior of their excitation factors induces intrinsic
giant ringings, i.e., ringings of a huge amplitude. Such ringings, which are
moreover slowly decaying, are directly constructed from the retarded Green
function. If we describe the source of the black hole perturbation by an
initial value problem with Gaussian initial data, i.e., if we consider the
excitation of the black hole from an extrinsic point of view, we can show that
these extraordinary ringings are still present. This suggests that physically
realistic sources of perturbations should generate giant and slowly decaying
ringings and that their existence could be used to constrain ultralight bosonic
field theory interacting with black holes.
