This behaviour holds for both the static and spinning BH cases. As already emphasized by some authors (e.g. Martocchia & Matt 1996) possible tests for discriminating between the two situations could be based on the fact that the innermost stable orbit can lie much nearer to the event horizon in the case of a rotating hole - up to slightly over one gravitational radius (m) in the extremal Kerr case against 6m in the Schwarzschild case. If one assumes that the disc can efficiently re-radiate only in the stable-orbit regime, then very red-shifted features would be the imprint of photons emitted at extremely internal radii, therefore in an extreme-Kerr spacetime. Anyway, Reynolds and Begelman (1997) pointed out that the difference would be much smaller if efficient line emission is allowed from matter inside the last stable orbit. This can be of course the subject of specific calculations, which should anyway carefully consider the optical thickness and the ionization state of such free-falling matter.

In order to clearly see the above mentioned smearing effects we considered a narrower energy range (E=4-9 keV) which contains the most important iron features, i.e. the K  and K  lines, whose rest energies in the assumed "low" ionization condition lie at 6.4 and 7.07 keV respectively, and the iron edge (at 7.1 keV). The results can be seen in figure 3, where we used a logarithmic vertical scale in order to better see the features. Diagrams obtained in pseudonewtonian approximation [PN], that is neglecting redshift and curvature of spacetime, in which the lines and the edge can be easily recognized, have also been plotted (filled with yellow) to allow a comparison with the "emitted" spectra.

For an observer located pole-on the iron line is broadened and quite red-shifted because of the high potential well. This effect is more pronounced in the extreme Kerr situation, when the the stable (emitting) orbits can extend pretty near the horizon, and would be much clearer if different emissivity laws were used. It seems indeed quite difficult to discriminate between rotating and non-rotating BHs if ordinary emissivity laws are considered. Only spectra corresponding to situations in which a consistent amount of the flux comes from the innermost part of the disc, such those resulting from a very anysotropic illumination (e.g. Martocchia & Matt 1996), would clearly show the effect of BH rotation. This will be the subject of future work.