I. Introduction

Shortly after the discovery of quasars it was speculated that an imprint of a massive central Black Hole [BH] could be discovered in their radiation propagating to a distant observer. Nowadays researchers are even thinking to possible ways to discriminate between static and spinning BHs through considering the radiation transfer in Kerr metric - it makes no difference for the geodesic calculus if stellar mass objects (like BHCs) instead of supermassive objects (like AGNs) are considered.
The first studies in this sense were performed in the 70es (Bardeen et al. 1972, Cunningham & Bardeen 1973, Cunningham 1975, ...), and it was concluded that the observed radiation flux and thermal spectrum of a source should be substantially modified by the presence of a spinning BH. This pioneer works where in the framework of the standard theory for accretion discs, just developed in those years (Shakura & Sunyaev 1973, Thorne 1974), and looked for the effects of energy shift and focusing due to the Kerr spacetime.

Such a path of research can now bring to a more effective testing of the BH hypotheses because of the newest developments in X-ray observation techniques. In fact the X-band radiation emitted from AGNs and BHCs, which has been observed by detectors mounted on satellites like RoSat, ASCA, Rossi-XTE and SAX, can in principle contain the signatures of BH fields either in the continuum (from about 1 keV up to some hundreds) or in the line profiles.
General relativistic effects in emission line profiles have been already modelled by many authors (for the Kerr spacetime see for instance: Laor 1991, Kojima 1991, Hameury et al. 1994, Karas et al. 1995, Bromley et al. 1997, Fanton et al. 1997, Dabrowski et al. 1997). Observational evidences have also already been sought, expecially in AGNs, in whose high energy spectra an iron K  feature (around E=6.4 keV) can often be detected: this is in particular the case of the Seyfert 1 galaxy MGC-6-30-15 observed by ASCA (Tanaka et al. 1995), in which the line is seen to be very broad and asymmetric.

Not much attention, however, has been paid on the effects on the reflection continuum which is produced along with the iron line following illumination of the disc by a primary X-ray source, to be most probably identified in a hot corona. We consider particularly important to address this matter, that is to self-consistently calculate the shape of iron lines and continuum together, because this will impose further restrictions on physical properties of the system which will thus be less ambiguous, as remarked in part 4.

Therefore we present here some preliminary results of calculations of the relativistic effects on both the reflection continuum and the line profile, obtained with a fully relativistic code in Kerr metric, and shortly discuss the observability of such effects by present and future detectors.