  The nature of the super-massive black hole candidates in galactic nuclei can
be tested by analyzing the profile of the K$\alpha$ iron line observed in their
X-ray spectrum. In this paper, we consider the possibility that the spacetime
in the immediate vicinity of these objects may be described by some non vacuum
exact solutions of Einstein's equations resulting as the end-state of
gravitational collapse. The vacuum far away portion of the spacetime is
described by the Schwarzschild metric, while the interior part may be either
regular or have a naked singularity at the center. The iron line generated
around this class of objects has specific features, which can be used to
distinguish such objects from Kerr black holes. In particular, their iron line
cannot have the characteristic low-energy tail of the line generated from
accretion disks around fast-rotating Kerr black holes. We can thus conclude
that the super-massive black hole candidates whose spin parameter has been
estimated to be close to 1 assuming the Kerr background cannot be this kind of
objects.
